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Reed Technology

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Content

Reed Switch Characteristics

Reed Switch Operational Characteristics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

The Basic Reed Switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Basic Electrical Parameters of Reed Switch Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

How Reed Switches are used with a Permanent Magnet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Reed Sensors vs . Hall Effect Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

Reed Switches vs . Mechanical Switches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

About Magnets

Magnets and their Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Handling Information for Magnets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Magnetization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35

Precautions

Handling and Load Precautions when using Reed Switches in various Sensor and Relay Applications . . 37

Contact Protection – Load Switching and Contact Protection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Contact Protection – Protection Circuitry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43

Ampere-Turns (AT) versus Millitesla (mT)

A Comparison of Ampere-Turns (AT) and Millitesla (mT) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45

Application Examples

Applications for Reed Switches and Reed Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Automotive and Transportation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

Marine and Boat . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56

Smart Home . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57

Security and Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Medical . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Test and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62

Telecommunication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Additional Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64

Reed Relays

The Reed Switch used as a Reed Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Reed Relay Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Reducing Magnetic Interaction in Reed Relay Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Reed Relay in Comparison with Solid-State and Mechanical Relay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

7 GHz RF Reed Relay – Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74

Applications Notes for RF-Relay Measurement in both the Frequency and Time Domain . . . . . . . . . . . . 76

Life Test Data

Life Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Activate Distance

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Glossary

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PRODUCT SOLUTIONS .

AS DIVERSE AS

THE MARKETS

WE SERVE .

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• Industrial / Power

• Lighting

• Medical

• Measuring & Control

Technology

• Metering

• Military

• Off Highway

• Pool / Spa

• Recreational

• Security / Safety

• Space

• Test & Measurement

• Utilities / Smart Grid

• Aerospace

• Alternative Energy

• Automatization

• Automotive /

Transportation

• Communication

Technology

• Fluid Flow

• Food Service

• General Industrial

• Heavy Duty Truck

• Household / Appliances

• HVAC/R

• Hydraulics and

pneumatic atuators

MARKETS WE SERVE

We offer engineered product solutions for a broad spec-

trum of product applications in all major markets, inclu-

ding but not limited to:

OUR COMPANY

Standex Electronics is a worldwide market leader in the

design, development and manufacture of standard and

custom electro-magnetic components, including mag-

netics products and reed switch-based solutions .

Our magnetic offerings include planar, Rogowski,

current, and low- and high-frequency transformers and

inductors . Our reed switch-based solutions include

KENT, MEDER and KOFU brand reed switches, as

well as a complete portfolio of reed relays, and a com-

prehensive array of fluid level, proximity, motion, water

flow, HVAC condensate, hydraulic pressure differential,

capacitive, conductive and inductive sensors .

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COMMITMENT & EXPERTISE

Standex Electronics has a commitment to absolute cus-

tomer satisfaction and customer-driven innovation, with

a global organization that offers premier sales support,

engineering capabilities, and technical resources world-

wide .

Headquartered in Cincinnati, Ohio, USA, Standex Elec-

tronics has eight manufacturing facilities in six countries,

located in the United States, Germany, China, Mexico,

the United Kingdom, and Japan

MANUFACTURING

•  Auto AT Switch Sorting
•  Bobbin and Toroidal Winding
•  Auto Termination

•  Coil Molding & Packaging
•  Insert and Thermoset Molding
•  Low Pressure Molding (Hot Melt)
•  Pick & Place – Vision & Camera System
•  Plasma Surface Treatment
•  Plastic Injection Molding
•  Potting - 2 Component
•  Progressive Stamping
•  Reflow Oven – Multiple Zone Convection
•  Reed Switch Manufacturing
•  Reed Relay Design and Manufacturing - SMD,

Low Thermal, High Insulation, High Voltage, High

Frequency, Latching and Atex

• Selective Soldering
• Sensor Packaging

CUSTOMER DRIVEN INNOVATION .
PREMIER WORLDWIDE
CAPABILITIES .

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•  Stainless Steel Fabrication and Precise Laser Welding
•  Transformer Design And Manufacturing
•  Wave Soldering

ENGINEERING

•  Electronic sensor engineering
•  Circuit Design and PCB Layout
•  Patented Conductivity Sensors
•  Patented Inductive Sensors
•  3-D CAD Modeling
•  3-D Magnetic Sensor Mapping
•  EMS Software
•  PCB Prototyper
•  Quick Turn Samples
•  3-D Printing

TESTING & TOOLING

•  Automated Assembly and Test Systems
•  Environmental and Durability Testing
•  Life Testing
•  Network Analyzers

•  Fluxmeters
•  Nanovoltmeters
•  Picoammeters
•  Destructive Pull Testers
•  Gauss / Teslameters

QUALITY/ LAB CAPABILITIES

•  Certifications: AS9100, ITAR, ISO9000, TS16949
•  SPC Data Collection
•  Fully Equipped Certified Test Labs
•  Burn-in and Life Testing
•  Complete, In-House Machine Shop
•  Corona Discharge Testing Capabilities
•  Microscopic  Investigation / DPA
•  Moisture Resistance and Seal Testing
•  Radiographic
•  Salt Fog and Solderability
•  Scott T Angular Accuracy
•  Terminal Strength
•  Thermal Cycling
•  Mechanical and Thermal Shock, Temperature

Rise and Vibration

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Notes

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Reed Switch Characteristics

Reed Switch Operational Characteristics

The Reed Switch was first invented by Bell Labs in the

late 1930s . However, it was not until the 1940s when it

began to find application widely as a sensor and a Reed

Relay . Here it was used in an assortment of stepping/

switching applications, early electronic equipment and

test equipment . In the late 1940s Western Electric be-

gan using Reed Relays in their central office telephone

switching stations, where they are still used in some

areas today . The Reed Switch greatly contributed to the

development of telecommunications technology . Over the

years several manufacturers have come and gone, some

staying longer than they should have, tainting the market-

place with poor quality, and poor reliability . However, most

of the manufacturers of Reed Switches today produce

very high quality and very reliable switches . This has

given rise to unprecedented growth . Today Reed Switch

technology is used in all market segments including: test

and measurement equipment, medical electronics, Tele-

com, automotive, security, appliances, general purpose,

etc . Its growth rate is stronger than ever, where the world

output cannot stay abreast with demand . As a technology,

the Reed Switch is unique . Being hermetically sealed,

it can exist or be used in almost any environment . Very

simple in its structure, it crosses many technologies in

its manufacture . Critical to its quality and reliability is its

glass to metal hermetic seal, where the glass and metal

used must have exact linear thermal coefficients of ex-

pansion . Otherwise, cracking and poor seals will result .

Whether sputtered or plated, the process of applying the

contact material, usually Rhodium or Ruthenium, must be

carried out precisely in ultra clean environments similar

to semiconductor technology . Like semiconductors, any

foreign particles present in the manufacture will give rise

to losses, quality and reliability problems .

To meet our customer’s needs, Standex Electronics de-

cided to build up their own assembly line . Reed Switches

are produced since 1968 in England and since 2001 in

Germany .

Over the years, the Reed Switch has shrunk in size

from approximately 50 mm (2 inches) to 3 .9 mm (0 .15

inches) . These smaller sizes have opened up many

more applications particularly in RF and fast time domain

requirements .

Reed Switch Features:

1 .

Ability to switch up to 10,000 Volts

2 .

Ability to switch currents up to 5 Amps

3 .

Ability to switch or carry as low as 10 nano-Volts

without signal loss

4 .

Ability to switch or carry as low as 1 femtoAmp

without signal loss

5 .

Ability to switch or carry up to 7 GigaHz with mini-

mal signal loss

6 .

Isolation across the contacts up to 1015 W

7 .

Contact resistance (on resistance) typical 50 mil-

liOhms (mW)

8 .

In its off state it requires no power or circuitry

9 .

Ability to offer a latching feature

10 .

Operate time in the 100 ms to 300 ms range

11 .

Ability to operate over extreme temperature ranges

from –55 °C to +200 °C

12 .

Ability to operate in all types of environments in-

cluding air, water, vacuum, oil, fuels, and dust lad-

en atmospheres

13 .

Ability to withstand shocks up to 200 Gs

14 .

Ability to withstand vibration environments of 50 Hz

to 2000 Hz at up to 30 g

15 .

Long life . With no wearing parts, load switching

under 5 Volts at 10 mA, will operate well into the

billions of operations

16 .

No power consumption, ideal for portable and

battery-powered devices

17 .

No switching noise

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Reed Switch Characteristics

Fig. #1 The basic hermetically sealed Form 1A (normally open)

Reed Switch and its component makeup.

A Reed Switch consists of two ferromagnetic blades

(generally composed of iron and nickel) hermetically

sealpowered in a glass capsule . The blades overlap

internally in the glass capsule with a gap between them,

and make contact with each other when in the presence

of a suitable magnetic field. The contact area on both

blades is plated or sputtered with a very hard metal, usu-

ally Rhodium or Ruthenium . These very hard metals give

rise to the potential of very long life times if the contacts

are not switched with heavy loads . The gas in the capsule

usually consists of Nitrogen or some equivalent inert gas .

Some Reed Switches, to increase their ability to switch

(up to 10 kV) and standoff high voltages, have an internal

vacuum. The reed blades act as magnetic flux conductors

when exposed to an external magnetic field from either

a permanent magnet or an electromagnetic coil . Poles

of opposite polarity are created and the contacts close

when the magnetic force exceeds the spring force of the

reed blades. As the external magnetic field is reduced so

that the force between the reeds is less than the restoring

force of the reed blades, the contacts open .

Fig. #2 The 1 Form C (single pole double throw) three leaded

Reed Switch and its component makeup.

The Reed Switch described above is a 1 Form A (normally

open (N .O .) or Single Pole Single Throw (SPST)) Reed

Switch. Multiple switch usage in a given configuration

is described as 2 Form A (two normally open switches

or Double Pole Single Throw (DPST)), 3 Form A (three

normally open switches), etc . A normally closed (N .C .)

switch is described as a 1 Form B . A switch with a com-

mon blade, a normally open blade and a normally closed

blade (Figure #2) is described as a 1 Form C (single pole

double throw (SPDT)) .

The common blade (or armature blade), the only moving

reed blade, is connected to the normally closed blade in

the absence of a magnetic field. When a magnetic field of

sufficient strength is present, the common blade swings

over to the normally open blade . The normally open and

normally closed blades always remain stationary . All three

reed blades are ferromagnetic; however, the contact area

of the normally closed contact is a non-magnetic metal

which has been welded to the ferromagnetic blade . When

exposed to a magnetic field, both of the fixed reeds as-

sume the same polarity which is opposite to that of the

armature . The paddle then moves over to the normally

open blade .

The Basic Reed Switch

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Reed Switch Characteristics

Figure 3 shows the general function of a Reed Switch

with the us of a permanent magnet .

Fig. #3 The basic operation of a Reed Switch under the influence

of the magnetic field of a permanent magnet. The polarization of

the reed blades occurs in such a manner to offer an attractive

force at the reed contacts.

The use of a coil wound with copper insulated wire . See

Figure 4 .

Fig. #4 A Reed Switch sitting in a solenoid where the magnetic

field is strongest in its center. Here the reed blades become

polarized and an attractive force exists across the contacts.

When a permanent magnet, as shown, is brought into

the proximity of a Reed Switch the individual reeds be-

come magnetized with the attractive magnetic polarity

as shown. When the external magnetic field becomes

strong enough the magnetic force of attraction closes

the blades . The reed blades are annealed and processed

to remove any magnetic retentively . When the magnetic

field is withdrawn the magnetic field on the reed blades

also dissipates . If any residual magnetism existed on the

reed blades, it would affect the behavior of opening and

closing . Proper processing and proper annealing clearly

are important steps in their manufacturing .

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Reed Switch Characteristics

Pull-in/Drop-out Temperature Effects

-40

-30

-20

-10

-60

-10

140

Temperature (°C)

Rate of

change (%)

Pull-In (PI) is described as that point where the con-

tacts close . Using a magnet, it is usually measured as

a distance from the Reed Switch to the magnet in mm

(inches) or in field strength AT, mTesla, or Gauss. In a

coil, the Pull-In is measured in volts across the coil, mA

flowing in the coil, or ampere-turns (AT). Generally, this

parameter is specified as a maximum. No matter how

well the reed blades are annealed, they will still have a

slight amount of retentivity (a slight amount of magnetism

left in the blades after the magnetic field is removed or

eliminated from the Reed Switch) . To obtain consistent

Pull-In and Drop-out results, saturating the Reed Switch

with a strong magnetic field first, before taking the Pull-

In measurement will produce more consistent results .

See Figure #5 .

When measured in a coil, or specifically, a Reed Relay,

the Pull-in is subject to changes at different temperatures,

and is usually specified at 20° C. See Figure #6.

Fig. #6 The Pull-In and Drop-Out points will change with tem-

perature at the rate of 0.4% /°C.

Here, because the copper coil wire expands and con-

tracts with temperature, the Pull-In or operate point will

vary with temperature by 0 .4% oC . Well designed relays

usually take this parametric change into consideration in

the design and specification.

Basic Electrical Parameters of

Reed Switch Products

Fig. #5 For most accurate results, saturate the contrast with a magnetic field, before testing for Pull-In and Drop-Out.

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Reed Switch Characteristics

Drop-out vs. Pull-in

Pull-in (AT)

Drop-out (AT)

Drop-Out

(DO) is described as that point where the

contacts open and has similar characteristics as the

Pull-In above . It is also described as release or reset

voltage current or AT .

Hysteresis

exists between the Pull-In and Drop-Out and is

usually described in the ratio DO/PI expressed in % . The

hysteresis can vary depending upon the Reed Switch

design, (Figure #7), where variations in plating or sput-

tering thickness, blade stiffness, blade overlap, blade

length, gap size, seal length, etc. will all influence this

parameter . See Figure 7 for example of hysteresis when

using a magnet to handle a Reed Switch .

Fig. #7 The Pull-in and Drop-out ranges are shown. Note that

variation in hysteresis is for low ampere turns (AT) is very small

and increases with higher AT.

Contact Resistance

is the DC resistance generated

by the reed blades (bulk resistance) and the resistance

across the contact gap . Most of the contact resistance

resides in the nickle/iron reed blades . Their resistivity is

7 .8 x 10-8 Ohm/m and 10 .0 x 10-8 Ohm/m respectively .

These are relatively high when compared to the resistivity

of copper, which is 1 .7 x 10-8 Ohm/m . Typical contact re-

sistance for a Reed Switch is approximately 70 mOhm, 10

to 25 mOhm of which is the actual resistance across the

contacts . In a Reed Relay, many times the relay pins will

be nickel/iron improving the overall mag-netic efficiency

but adding bulk resistance to the contact resistance . This

increase can be in the order of 25 mOhm to 50 mOhm .

See Figure #8 .

Fig. #8 A representation of the bulk resistance and resistance

across the contacts making up the contact resistance value in

Ohms for a Reed Switch

Dynamic Contact Resistance

(DCR) is a true measure

of the disposition of the contacts . As already described,

the contact resistance is mostly made up of bulk re-

sistance or lead resistance . Measuring the resistance

across the Reed Switch only gives gross indication that

the contacts are functional . To give a better indication of

the contacts functionality, one must look at the contacts

under dynamic conditions .

Opening and closing the contacts at frequencies in the

range of 50 Hz to 200 Hz can reveal much more infor-

mation . Switching 0 .5 Volts or less with approximately

50 mA will allow enough voltage and current to detect

potential problems . This testing can be carried out using

an oscilloscope or may be easily digitized for more auto-

matic testing . One should avoid test voltages greater than

0 .5 Volts to avoid ‘break-over’ (potential non-conductive

films). This extremely thin film will look like an open circuit

if one is switching very low signals or in current less clos-

ing of the Reed Switch (closing the contacts before any

voltage or current is applied across the contacts) . Using

a voltage above 0 .5 V might hide this potential quality

problem . See Figure 9 .

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Reed Switch Characteristics

Fig. #9 A schematic diagram of a typical circuit used for mea-

suring the dynamic contact resistance across the contacts of

a Reed Switch.

Applying the frequency described above to a coil, the

contacts will operate and close in approximately ½ mA .

The contacts may then bounce for about 100ms and

undergo a period of dynamic noise for as much as ½ ms .

This dynamic noise is generated by the contacts con-

tinuing to bounce but not opening, whereby the contact

resistance varies widely where the force or pressure on

the contacts varies harmonically, critically dampening in

about ½ ms or less . See Figure 10 . Once this dynamic

noise dissipates, the contacts will then undergo a ‘‘wa-

vering period’ . Here the contacts have closed, but will

waver while closed for up to 1 ms or more . This wavering

of the contacts in the coil’s magnetic field generates a

current through the contacts . Once this effect dissipates

the contacts enter their static condition .

Fig. #10 A typical dynamic contact resistance portrayal show-

ing the first closure, bouncing, dynamic noise and pattern gen-

erated by waver-ing contacts

Observing the electrical pattern produced by this dy-

namic test can reveal much about the quality of the Reed

Switch . Generally speaking, once the coil voltage has

been applied, the dynamic contact activity should settle

down by 1 ½ ms . If the contacts continue to bounce more

than 250 ms, the closing force may be weak, which may

result in a shortened life, particularly if one is switching

a load of any size . See Figure #11 .

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Reed Switch Characteristics

Fig. #11 A dynamic contact resistance pattern showing exces-

sive contact bounce.

If the dynamic noise or the wavering contacts con-

tinue for periods longer than indicated, it may mean the

Reed Switch seals are weak or perhaps overstressed .

This could result in capsule cracking or breaking . Also,

if the wavering produced has excessive amplitude,

this could represent a condition of capsules having added

stress which could produce leaking seals . In this case,

outside air and moisture may seep into the capsule

producing unwanted contamination on the contacts .

See Figure #12 & #13 .

Fig. #12 A dynamic contact resistance pattern portraying ex-

cessive dynamic noise indicating potential stressed or cracked

glass seal.

Fig. #13 A dynamic contact resistance pattern with indicated

excessive contact wavering often indicates a stressed or

cracked glass seal.

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Reed Switch Characteristics

Also, when the contact resistance varies by a small de-

gree with successive closures, contamination, a leaking

seal, particles, loose or peeling plating may exist, poten-

tially shortening life expectations (Figure #14) . Varying

the frequency applied to the coil sometimes produces

more subtle awareness of resonance related problems .

This will also manifest itself with higher amplitude or

longer times of dynamic noise or contact wavering .

Fig. #14 A dynamic contact resistance pattern showing contact

resistance changing in each successive operation indicating

contact contamination.

Any time long life, stable contact resistance, and fault

free operation are conditions in your application, dynami-

cally testing the contacts and having tight testing limits

are a must .

Switching Voltage,

iusually specified as a maximum in

units of Volts DC or Volts peak, is the maximum allow-

able voltage capable of being switched across the con-

tacts . Switching voltages above the arcing potential can

cause some metal transfer . The arc potential generally

occurs over 5 Volts . Arcing is the chief cause of shorted

life across the contacts . In the 5 V to 12 V range most

contacts are capable of switching well into the tens of

millions of operations depending on the amount of cur-

rent switched . Most pressurized Reed Switches can not

switch more than

500 Volts, principally because they can not break the arc

occurring when one tries to open the contacts . Gener-

ally, switching above 500 Volts requires evacuated Reed

Switches, where up to 10,000 Volts is possible . Switching

below 5 Volts, no arcing occurs and therefore no blade

wear occurs, extending Reed Switch lifetimes well into

the billions of operations . Properly designed Reed Relays

can switch and discern voltages as low as 10 nanoVolts .

Switching Current

refers to that current measured in

Amperes DC (peak AC), switched at the point of closure

of the contacts . The higher the level of current the more

sustained the arcing at opening and closing and therefore

the shorter the life of the switch .

Carry Current,

also measured in Amperes DC (peak

AC), is specified as the maximum current allowed when

the contacts are already closed . Since the contacts are

closed, higher currents are allowed . No contact damage

can occur, since the only time arcing occurs is during

the opening and closing transitions . A Reed Switch is

also able to transport higher currents, when the pulse

duration is very short, since the heating here is minimal .

Conversely, unlike electromechanical armature style

relays, the Reed Relay can switch or carry currents as

low as femptoAmperes (10-15 Amperes) .

Stray Capacitance

measured in microFarads or Pico

Farads is always present for example due to to conduct-

ing paths and cable . When switching voltage and current,

the first 50 nanoSeconds are the most important. This is

where the arcing will occur. If there is a significant amount

(depending on the amount of voltage switched) of stray

capacitance in the switching circuit, a much greater arc

may occur, and thereby reducing life . When switching

any sizable voltage, it is always a smart idea to place

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Reed Switch Characteristics

a fast current probe in the circuit to see exactly what

one is switching in the first 50 nanoSeconds. Generally

speaking, when switching voltages over 50 Volts, 50 pi-

coFarads or more can be very significant to the expected

life of the switch .

Common Mode Voltage

is also another parameter that

can have a significant effect on the life of a Reed Switch.

Depending upon the circuit and the environment, com-

mon mode voltages can in effect, charge stray capaci-

tances in the switching circuit and dramatically reduce

Reed Switch life in an unexpected manner . Again, a fast

current probe can reveal a startling voltage and current

switched in that first 50 nanoSeconds, having no bearing

on one’s actual load . When line voltages are present in or

near sensitive circuits, be cautious . Those voltages can

be coupled into the circuit creating havoc with your life

requirements . Typically, a faulty Reed Switch is blamed

for this reduced life, when in actuality, it is a product of

unforeseen conditions in the circuit .

Switching Load

is the combined voltage and current

switched at the time of closure . Sometimes there is

confusion with this parameter . For a given switch, with a

switching rating of 200 Volts, 0 .5 Amperes and 10 Watts,

any voltage or current switched, when multiplied together,

can not exceed 10 Watts . If you are switching 200 Volts,

then you can only switch 50 milliAmperes . If you are

switching 0 .5 Amperes, then you can only switch 20 Volts .

Breakdown Voltage

(Dielectric Voltage) generally is

higher than the switching voltage . On larger evacuated

Reed Switches, ratings as high as 15,000 Volts DC are

not uncommon . Some smaller evacuated reeds can stand

off up to 4000 Volts DC . Small pressurized reed switches

generally withstand 250 to 600 Volts DC .

Insulation Resistance

is the measure of isolation

across the contacts and is probably one of the most

unique parameters that separate Reed Switches from

all other switching devices . Typically, Reed Switches

have insulation resistances averaging 1 x 1014 ohms .

This isolation allows usage in extreme measurement

conditions where leakage currents in the picoAmpere or

femtoAmpere range would interfere with the measure-

ments being taken . When testing semi-conductors, one

may have several gates in parallel where the switching

devices have combined leakage currents that become

significant in the test measurement circuit.

Dielectric Absorption

describes the effect different

dielectrics have on very small currents . Currents below

1 nanoAmpere are affected by the dielectric’s tendency

to slow or delay these currents . Depending upon how

low a current one is measuring, these delays can be

on the order of several seconds . Standex Electronics

engineers have designed Reed Relays and circuits to

minimize dielectric absorption .

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Reed Switch Characteristics

Operate Time

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0,4

Pull-in (AT)

Operate Time (ms)

Release Time

Drop-out (AT)

Release time (µs)

Operate Time

is the time it takes to close the contacts

and stop bouncing . Except for mercury wetted contacts,

when the reed blades close, they close with enough force

to set them in harmonic motion . This critically damped

motion dissipates rapidly due to the relatively strong

spring force of the reed blades . One generally sees one

or two bounces occurring over a 50 ms to 100 ms period .

Most small Reed Switches operate, including bounce, in

the range of 100 ms to 500 ms . See Figure #15 .

Fig. #15 A typical graph of the operate time for increasing Pull-

In AT values. With higher Pull-in AT the Reed Switch gap in-

creases taking a longer time for the contacts to close.

Release Time

is the time it takes for the contacts to open

after the magnetic field is removed. In a relay, when the

coil turns off, a large negative inductive pulse (‘kick’) oc-

curs causing the reed blades to open very rapidly . This

release time may be in the order of 20 ms to 50 ms . If a

diode is placed across the coil to remove this inductive

voltage spike (which can be 100 Volts to 200 Volts), the

contact opening time will slow to about 300 ms . Some

designers require the fast release time, but cannot have

the high negative pulses potentially being coupled into

sensitive digital circuity . So they add a 12 Volt to 24 Volt

zener diode in series with a diode, all of which is in paral-

lel across the coil . Here, when the coil is turned off, the

voltage is allowed to go negative by the zener voltage

value, which is sufficient to cause the contacts to open

generally under 100 ms . See Figure #16 .

Fig. #16 A graph of the release time for increasing Dropout

AT. With increasing Drop-out AT the restoring force increases

causing even faster release time.

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Reed Switch Characteristics

Resonant Frequency

6500

7000

7500

8000

8500

9000

Resonant frequency (Hz)

Cumulative frequency

percen

Contact Capacitance (gap)

0,1

0,2

0,3

Pull-in (AT)

Electrostatic

capacitance (PF

)

Resonant Frequency

for a Reed Switch is that physical

characteristic where all reed parameters may be affected

at the exact resonance point of the Reed Switch . Reed

capsules 20 mm long will typically resonate in the 1500

to 2000 Hz range; reed capsules on the order of 10 mm

will resonate in the 7000 to 8000 range . Avoiding these

specific resonance areas will insure a fault free environ-

ment for the Reed Switch . Parameters typically affected

are the switching voltage and the breakdown voltage .

See Figure #17 .

Fig. #17. A depiction of a group of 10 mm Reed Switches and

its resonant frequency distribution.

Capacitance

across the contacts is measured in pico-

Farads and ranges from 0 .1 pF to 0 .3 pF . This very low

capacitance allows switching usage, where semiconduc-

tors having 100’s of picoFarads, can not be considered . In

semiconductor testers, this low capacitance is absolutely

critical . See Figure #18 .

Fig. #18 As the Pull-in AT increases its gap increases, there-

fore reducing the capacitance across the Reed Switch.

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Reed Switch Characteristics

Using Reed Switches in a sensing environment, one

generally uses a magnet for actuation . It is important

to understand this interaction clearly for proper sensor

functioning . Sensors may operate in a normally open,

normally closed, change over or a latching mode .

In the normally open mode, when a magnet is brought

toward the Reed Switch the reed blades will close .

When the magnet is withdrawn the reed blades will

open . With the normally closed sensor, bringing a mag-

net to the Reed Switch the reed blades will open, and

withdrawing the magnet, the reed blades will re-close .

In a latching mode the reed blades are in either an open

or closed state . When a magnet is brought close to the

Reed Switch the contacts will change their state . If they

were initially open, the contacts will close . Withdrawing

the magnet the contacts will remain closed . When the

magnet is again brought close to the Reed Switch, with

a changed magnetic polarity, the contacts will now open .

Withdrawing the magnet the contacts will remain open .

Again, reversing the magnetic polarity, and bringing the

magnet again close to the Reed Switch the contacts

will again close and remain closed when the magnet is

withdrawn . In this manner, one has a latching sensor or

a bi-stable state sensor . In the following diagrams, we

will outline the guidelines one must be aware of when

using a magnet. Please keep in mind the magnetic field

is three-dimensional .

A permanent magnet is the most common source for

operating the Reed Switch . The methods used depend

on the actual application . Some of these methods are the

following: front to back motion . See Figure #19 .

Fig. #19 A Reed switch being shown with a magnet being

moved in front to back motion.

Rotary motion (see below Figure #20); ring magnet with

parallel motion (see Figure #21)

Fig. #21 A circular magnet showing a Reed Switch effectively

passing through its centers showing the opening and closing

points.

Fig. #20 A reed switch being used with magnets in rotary motion.

How Reed Switches are used with a

Permanent Magnet

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Reed Switch Characteristics

The use of a magnetic shield to deflect the magnetic flux

flow. See Figure #22.

Fig. #22 The effects of a magnetic shield passing between a

Reed Switch and permanent magnet shunting the magnetic

lines of flux which influences the opening and closing of the

Reed Switch.

Pivoted motion about an axis . See Figure #23 .

Fig. #23 A pivoting magnet is shown influencing the opening

and closing points of a Reed Switch.

Parallel motion (Figure #24, Figure #25, Figure #26,

Figure #27, Figure #28) and combinations of the above

perpendicular motion (Figure #29, Figure #30, Figure

#31 and Figure #32) .

Before we investigate each of these approaches, it is

important to understand the fields associated with the

various Reed Switch vs . magnet positions and their on/off

domain characteristics . The actual closure and opening

points will vary considerably for different Reed Switches

and different sizes and strengths of magnets .

First consider the case where the magnet and Reed

Switch are parallel . In Figure #24, the open and closure

domains are shown in the x and y-axis . These domains

represent the physical positioning of the magnet relative

to the Reed Switch along the x-axis . The closure and

opening points are relative to the movement of the mag-

net along this x axis, where the magnet is fixed relative

to the y-axis . Here, three domains exist, wherein Reed

Switch closure can take place . Keep in mind the center

domain is much stronger and the graph gives a relative

idea of the closure points on a distance basis along the

y-axis . The hold areas shown, demonstrates the hyster-

esis of the Reed Switch and will vary considerably for

different Reed Switches. In fluid level controls, having

a wider hold area can be beneficial, particularly if there

is constant disruption to the fluid level as in a moving

vehicle. Using the configuration shown in Figure #24,

the maximum distance away from the Reed Switch for

closure is possible . This approach has the best magnetic

efficiency.

Fig. #24 The opening, closing and holding points are shown

for a magnet passing in parallel to Reed Switch and being

affected by the center magnetic lobe.

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Reed Switch Characteristics

Fig. #25 The opening and closing points are shown for a mag-

net making a close approach in parallel to a Reed Switch. Here

the Reed Switch will close and open three times.

Also, for parallel motion, if the magnet and switch are

close enough, parallel motion can create three closures

and openings as demonstrated in Figure #25 .

Fig. #26 The closing and opening is portrayed for a magnet ap-

proaching a Reed Switch in parallel from an end point.

Passing the magnet by the Reed Switch farther away,

one closure and opening will occur . Another approach

for magnets used in a parallel application with parallel

motion is shown in Figure #26, where the closure point

uses the smaller outer magnetic domain .

Another approach for magnets used in a parallel appli-

cation, but with vertical motion, is shown in Figure #27

where the closure point uses the inner larger magnetic

domain . In Figure #28 the vertical motion uses the outer

magnetic domain .

Fig. #27 The closing, holding, and opening are portrayed for a

magnet parallel to the Reed Switch, but moving perpendicular

to the plane of the Reed Switch and being influenced by the

center magnet lobe.

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Reed Switch Characteristics

Fig. #28 The closing, holding, and opening are presented for a

magnet parallel to the Reed Switch, but moving perpendicular

to the plane of the Reed Switch and being influenced by the

outer magnetic lobe.

Another approach for magnets used in a parallel appli-

cation, but with vertical motion, is shown in Figure #29 .

Please note this view is showing the y-z-axis . The clo-

sure and opening states are clearly shown for several

positions of the magnet .

Fig. #29 Motion of the magnet is depicted in the y-z axis where

the magnet is parallel to the Reed Switch, but moving perpen-

dicular to its plane. The closure, holding, and opening points

are shown.

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Reed Switch Characteristics

In Figure #30, the magnet is perpendicular to the Reed

Switch . Here the x-y axis is shown with the relative

closure, holding and opening points . Parallel magnet

movement is along the x-axis, but displaced at a distance

y from the x-axis . Here two closures and openings can

take place .

Fig. #30 The opening and closing points are shown for a ver-

tically mounted magnet making an approach parallel to the

Reed Switch. Here the Reed Switch will close and open two

times.

In Figure #31, the magnet is again perpendicular to the

Reed Switch . Magnet movement is still parallel but on and

along the x-axis . No Reed Switch closure takes place .

Fig. #31 The opening and closing points are shown for a verti-

cally mounted magnet making an approach parallel to the axis

of the Reed Switch. Here the Reed Switch will close and open
two times.

In Figure #32, the magnet is perpendicular to the Reed

Switch . Here the x-y axis is shown with the relative

closure, holding and opening points . Magnet movement

is along the y-axis, but displaced a distance x from the

y-axis . Here two closures and openings can take place

as shown .

Figure #32 The opening and closing points are shown for a

vertically mounted magnet making an approach perpendicular

to the axis of the Reed Switch through its end point. Here the

Reed Switch will close and open two times.

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Reed Switch Characteristics

In Figure #33, the magnet is perpendicular to the Reed

Switch . Here the x-y axis is shown with the relative mag-

net movement along the actual y-axis and the magnet

movement is fixed relative to the x-axis. Here no closures

take place .

Fig. #33 No closure points are shown for a verti-cally mount-

ed magnet making an approach perpendicular to the axis of

the Reed Switch and through its center point. Here the Reed

Switch will not close at all.

With the above closure and opening boundaries relative

to magnet placement, an assortment of closure and open

configurations can be set up when moving the magnet

in more than one axis of motion, i .e . rotary motion, etc .

Also, in the above cases we held the movement of the

Reed Switch fixed in position. By holding the magnet fixed

and moving the Reed Switch, if the application calls for

it, the same expected closures and opening distances

would be expected . There can be multiple poles existing

in one magnet, and under these conditions the closure

and opening points will change . Experimentation may be

required to determine the closure and opening points .

Biasing a Reed Switch with another magnet will allow

normally closed operation . Bringing another magnet, of

opposite polarity, in close proximity to the magnet/Reed

Switch assembly will open the contacts . See Figure #34 .

Fig. #34 A Reed Switch can be biased closed with a magnet.

When a second magnet with an opposing magnetic field is

brought close, the Reed Switch will open giving rise to a nor-

mally closed sensor.

Also, using a biasing magnet will allow Reed Switch

operation in the hold area or hysteresis area, thereby

creating a latching sensor . (see Figure #35) In this situ-

ation, real care needs to be taken in exact placement of

the biasing magnet and the operating magnet needs to

be restricted to certain areas . To switch from bi-stable

state to bi-stable state the operating magnet’s polarity

or direction needs to be reversed .

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Reed Switch Characteristics

Fig. #35 A Reed Switch can be biased with a magnet in such

a way to establish a latching sensor. When a second magnet

with a given polarity is brought close, the contacts will close.

Withdrawing the magnet the contacts stay closed. Bringing a

magnet with opposite polarity close to the Reed Switch, the con-

tacts will open and remain open when the magnet is withdrawn.

Standex Electronic has developed a bridging sensor

which can operate in either a normally open or normally

closed state . When a sheet of ferro-magnetic material

(metal door, etc .) is brought up to the sensor the Reed

Switch will close; when it is withdrawn, the contacts will

open (Figure #36) No external magnets are required to

operate the bridge sensor (see our MK02 Series) .

Fig. #36 Standex Electronics has designed a patented bridge

sensor requiring no external magnets. When the sensor is

brought close to a ferromagnetic sheet or plate the sensor

contacts will close. When the sheet is withdrawn, the contacts

will open.

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Reed Switch Characteristics

Since their introduction several years ago, the Hall effect

sensor has captured the imagination of design engineers .

Generally, it was thought that if it’s in solid state that it’s a

more reliable approach, particularly when comparing it to

electromechanical devices . However, several remarkably

interesting advantages are observed when comparing

the reed sensor technology to the Hall effect technology .

But first, let’s take a closer look at the reed sensor tech-

nology . The key component in the reed sensor is the reed

switch, invented by Western Electric back in the 1930’s .

The other major component is the magnet or electromag-

net used to open or close the reed switch . Over the last

seventy years the reed switch has undergone several

improvements, making it more reliable, improving it’s

quality and reducing it’s cost . Because of these dramatic

improvements of reed switches, they have become the

designin choice in several critical applications where

quality, reliability and safety are paramount .

Perhaps the most dramatic application and testimony of

the reed’s quality and reliability is its use in Automatic

Test Equipment (ATE) . Here this technology is used

exclusively . The reed switches are used in reed relays,

switching in the various test configurations for integrated

circuits, ASICs, wafer testing and functional printed circuit

board testing . For these applications up to 20,000 reed

relays may be used in one system . Here one relay fail-

ure constitutes a 50-ppm failure rate . Therefore to meet

this requirement, the reed relays need to have quality

levels much better than 50-ppm . Heretofore, it was un-

heard of to have an electromechanical device with this

quality level . Similarly the same holds true for several

semi conductor devices as well . Once beyond the initial

operational quality testing, the reed relays then need to

perform well over life . Here they have been proven to out

perform all other switching devices . Because, in many

cases the automatic test equipment is operated 24 hours

a day and 7 days a week to fully utilize it’s high capital

expense; and therefore, billions of operations may be

required during the reed relay’s lifetime .

Another example of its favored use is in air bag sensors,

where they have passed the test of time in a crucial

safety application . Reed sensors are currently used in

many critical automotive safety equipment (brake fluid

level sensing, etc .), along with many medical applications

including defibrillators, cauterizing equipment, pacemak-

ers and medical electronics where they isolate small

leakage currents .

In both technologies, the sizes are shrinking as is evi-

denced in the enclosed picture . However, when compar-

ing the reed sensor over a Hall effect sensor we see

several advantages:

Cost-Effective

Generally the cost of the Hall effect device is low, but it

requires power and circuitry to operate . Also, its signal

output is so low it often times requires amplification cir-

cuitry as well . The net result, the Hall effect sensor can

be considerably more expensive than the reed sensor .

High Isolation

The reed switch has superior isolation from in-put to

output and across the switch up to 1015 Ohms . This

reduces leakage currents to femto amps (1015 amps)

levels . On the other hand, Hall effect devices have sub-

micro amp leakage levels . In medical electronic devices

inserted into the human body as probes (invasive use) or

pacemakers it’s very important not to have any leakage

current near the heart, where micro amp and sub-micro

amp currents can alter the heart’s key electrical activity .

Hermetically Sealed

The reed is hermetically sealed and can therefore operate

in almost any environment .

Low Contact Resistance

The reed has very low on resistance typically as low

as 50 milliohms, whereas the Hall effect can be in the

hundreds of ohms .

Reed Sensors vs. Hall Effect Sensors

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Reed Switch Characteristics

Switching Power

The reed can directly switch a host of load ranging from

nano volts to kilovolts, femto amps to Amps, and DC to

6 GHz . The Hall effect devices have very limited ranges

of outputs

High Magnetic Sensitivity

The reed sensor has a large range of magnetic sensitivi-

ties to offer .

Easy Mounting

Reed sensors are not susceptible to E .D .I ., where elec-

trostatic discharge may often times severely damage the

Hall effect device .

High Voltage

Reed sensors are capable of withstanding much higher

voltages (miniature sizes are rated up to 1000 Volts) .

Hall effect devises need external circuitry for ratings as

high as 100 Volts .

High Carry Current

The reeds are capable of switching a variety of loads,

where the Hall effect sensor delivers only smaller volt-

ages and currents .

High Shock Resistance

The reed sensor is typically tested to withstand a three-

foot drop test, which is comparable to the Hall effect

sensor .

Long Life Expectancy

Because the reed sensor has no wearing parts, low

level loads (<5V @ 10 mA and below), will operate sat-

isfactorily well into the billions of operations . This rivals

semiconductor MTBF figures.

Wide Temperature Range

The reed sensor is unaffected by the thermal environ-

ment, and is typically operated from -50° C to +150° C

with no special additions, modifications or costs. The Hall

effect sensors have a limited operational range .

No external Power

Ideal for portable and battery-powered devices .

There are many very good applications of reed products .

Selection of the proper reed in the proper application,

often time is critical . Some reed/relay companies are

excellent at designing in reeds in critical applications

where quality, reliability and safety are paramount .

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Reed Switch Characteristics

Comparative Table: Reed Sensors vs. Hall Effect Sensors

Specifications

Reed Sensor

Hall Effect Sensor

Input requirements

External magnet field >5 Gauss time

External magnetic field >15 gauss time

Sensing distance

Up to 40mm effectively

Up to 20mm effectively

Output requirements

None

Continuous current >10mA, depending

on sensitivity

Power required all the time

Yes

Requirements beyond sensing device

None

Voltage regular, constant current source,

hall voltage generator, small-signal

amplifier, chopper stabilization, Schmitt

trigger, short-circuit protection, external

filter, external switch

Hysteresis

Ability to adjust to meet design requirement

Fixed usually around 75%

Detection circuit required

None

Yes, and generally needs amplification

Ability to switch loads directly

Yes, up to 2A and 1,000V, depending on

the reed selection

No, requires external switching

Output switching power

Up to 1,000W, depending on switch

selection

Low millitwatts

Voltage switching range

0 to 200V (1,000V available)

Requires external switch

Current switching range

0 to 3A

Requires external switch

Output sensitivity to polarity

Yes, critical for proper operation

Output offset voltage sensitivity

None

Yes, exacerbated by sensitivity to over-

coming, temperature dependencies, and

thermal stress

Chopper circuit requirements

None

Yes, helps reduce output offset voltage;

requires additional external output

capacitance

Frequency range

DC to 6 GHz

Switching frequency 10,000 Hz

Closed output on resistance

0 .050 Ohm

>200 Ohm

Expected life switching >5A @ 10mA

> 1 billion operations

Unlimited

Capacitance across output

0 .2 pF typ

100 pF typ

Input / Output isolation

Ohm min .

Ohm min

Isolation across output

Ohm min .

Ohm min

Output dielectric strength

Up to 10kV available

<10 V typical

EDI (ESD) susceptibility

No, requires no external protection

Yes, requires external protection

Hermeticitiy

Yes

Shock

> 150g

>150 g

Vibration

> 10g

>50 g

Operating temperature

-55°C to 200°C

0 °C to 70°C, typ

Storage temperature

-55°C to 200°C

-55°C to 125°C

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Reed Switch Characteristics

Specifications

Reed Switch

Mechanical Microswitch

Sensing Distance - Touch

Up to 40mm effectively

Touch (zero distance)

Power Required all the Time

Input Requirements

External magnetic field >5 gauss min.

Mechanical Force

Hysteresis

Ability to adjust, to meet design request

Differential Travel (D .T .)

Life Expectancy: Low Level

cycles

Switching Voltage

Up to 200V (10,000V available)

250VAC

Switching Current / Carry Current

Up to 3A / Up to 5A

Up to 25A

Switching Load Minimum

No load required (μV / pA)

50mW

Switching Load Maximum

Up to 100 Watts

Up to 5,000 Watts

Insulation Resistance

Ohm

Contact Resistance

50 milliohm

100 milliohm

Noise

Almost no switching noise

Switching Noise

Overload

Very sensitive

Insensitive

Hermeticity

Yes

General

Galvanic isolation (air gap)

Galvanic Isolation (air gap)

Assembly

20,000 pcs

5,000 pcs

Reed Switches in Comparison

with Mechanical Switches

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About Magnets

Magnets are available in multiple specifications on the

market . Almost all dimensions and geometries can be

realized . To activate the reed switch a magnet (magnet

field) is needed. The different magnet materials have

either more positive or negative specifications, depend-

ing on the dimension and geometries as well as on the

environment . Most preferred and used forms are cylin-

ders, rectangles, and rings . Depending on the different

requirements, magnets can be magnetized in many

different ways (figure #1).

Furthermore each magnet material has a different mag-

net force as well as a different flux density. Additionally

to dimension and material, other factors exist that define

the energy of a magnet . These are mounting position,

environment and other magnetic field witch influence the

interaction between reed sensor/switch and magnet . In

applications were a magnet is used to activate a reed

sensor/switch, the environmental temperature needs to

be considered (in the application as well as in storage) .

High temperatures can cause irreversible damage (so-

called Curie temperature) and will have heavy impact

on the magnetic force and the long term stability . AlNiCo

magnets are best suitable for applications up to 450°C .

Magnets and their Specifications

Fig. #1 An assortment of magnets are shown. Magnets can be formed and made into almost any shape.

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About Magnets

Costs

Ferrite

AlNiCo

NdFeB

SmCo

Energy (WxHmax .)

Ferrite

AlNiCo

SmCo

NdFeB

Working Temperature

NdFeB

Ferrite

SmCo

AlNiCo

Corrosion - Resistant

NdFeB

SmCo

AlNiCo

Ferrite

Opposing Field - Resistant

AlNiCo

Ferrite

NdFeB

SmCo

Mechanical Strength

Ferrite

SmCo

NdFeB

AlNiCo

Temperature Coefficient

AlNiCo

SmCo

NdFeB

Ferrite

LOW

HIGH

AINICo Features

Standard Geometric and Magnetization

Rectangle

Cylinder

•

Working Temperature

from -250 to 450

•

Low Temperature

Coefficient

AINiCo

Magnetic Values according to

DIN 17410

Min .

Typ .

Max .

Units

Energy Product

(B x

max .

kJ/

Remanence

600

1300

Coercivity

kA/m

Coercivity

kA/m

Density

7 .3

g/cm

Max . Operating Temperature

450

Curie Temperature

850

All details correspond to manufacturers information & magnet material

General Information to Magnet Material

Magnets have reversible and irreversible demagneti-

zation specifications. Be specially careful with shock,

vibration, strong and close external magnetic fields as

well as high temperatures. All these factors influence

the magnetic force and the long term stability in dif-

ferent intensities . Preferably the magnet is mounted

on the moving part of the application . Professional

tuning of magnet and reed switch can improve the

functionality of the whole sensor-magnet system .

AlNiCo – Magnets

Raw materials for AlNiCo magnets are aluminium nickel,

cobalt, iron and titanium . AlNiCos are produced in a

sintering - casting procedure . The hard material needs

to be processed by grinding to be cost effective . Due to

its specifications, the best dimension is a remarkably

longer length than its diameter . In combination with reed

sensors / switches we recommend a length / diameter

ratio of more than 4 . AlNiCo magnets have an excellent

temperature stability . Cylindrical AlNiCo magnets can be

used with all Standex Electronics reed sensors / switches

without any problems .

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About Magnets

SmCo Features

Standard Geometric and Magnetization

Disc

Rectangle

Cylinder

•

High energy density

•

Small size

•

Working temperature

up to 250

•

Best opposing field-

resistance

•

Available plastic

bounded

NdFeB Features

Standard Geometric and Magnetization

Disc

Flat Rectangle

Ring

•

High energy density

•

Small size

•

Working temperature

up to 180

•

Lower prices com-

pared to SmCo

•

Available plastic

bounded

Rare earth magnets like SmCo and NdFeB have the

highest energy density per volume and weight and also

the best demagnetization resistance . Following below,

we compare other magnets with the same energy:

• Hartferrit = Volumes 6 cm

• AlNiCo

= Volumes 4 cm

• SmCo

= Volumes 1 cm

• NdFeB

= Volumes 0 .5 cm

Rare – Earth Magnets (NdFeB & SmCo)

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About Magnets

SmCo

Magnetic Values according to DIN 17410

Min .

Typ .

Max .

Units

Energy Product

(B x H)

max .

150

220

kJ/ m

Remanence

900

1050

Coercivity

700

kA/m

Coercivity

1500

kA/m

Density

8 .3

g/cm

Max . Operating Temperature

250

Curie Temperature

750

All details correspond to manufacturers information & magnet material

NdFeB

Magnetic Values according to DIN 17410

Min .

Typ .

Max .

Units

Energy Product

(B x H)

max .

200

400

kJ/ m

Remanence

1020

1400

Coercivity

800

kA/m

Coercivity

955

2000

kA/m

Density

7 .6

g/cm

Max . Operating Temperature

160

Curie Temperature

330

All details correspond to manufacturers information & magnet material

Both magnets are produced by sintering and can only be

processed by grinding, due to the strength and brittle of

the material . The temperature range goes up to + 250 °C .

Very small magnets can be produced . Disadvantages are

the high raw material prices and the limited availability

of special alloys .

The supply of different geometry, size and magnetization

allow many creative combination of reed sensor / switch

and magnet and help to find the best functionality of the

sensor – magnet system for each application .

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About Magnets

Ferrite Features

Standard Geometric and Magnetization

Disc

Rectangle

Cylinder

Ring

•

Cheapest magnet

material

•

Working temperature

up to 300

•

Many options in form

and magnetiuation

•

Available plastic

bounded

Ferrite

Magnetic Values according to DIN 17410

Min .

Typ .

Max .

Units

Energy Product

(B x H)

max .

kJ/ m

Remanence

200

410

Coercivity

200

kA/m

Coercivity

240

kA/m

Density

4 .8

g/cm

Max . Operating Temperature

250

Curie Temperature

450

All details correspond to manufacturers information & magnet material

Hard ferrite magnets are produced with iron oxide and

barium or strontium oxide . The raw materials are mixed

together and normally pre sintered, to generate the

magnetic phase . The pre sintered mixture then gets

crushed . The resulting powder gets pressed together

(wet or dry) either in a magnetic field (an - isotropic) or

without a magnetic field (isotropic) and in the end sin-

tered . Proceedings are only possible by grinding . Due

to the low cost of the raw material, hard ferrite magnets

are the cheapest magnet type out of the actual supply of

magnets . Ferrites have a very good electrical isolation

effect and are hard to demagnetize even in strong exter-

nal magnetic fields. Corrosion tendency is low. Preferred

shapes are long and thin but also round forms are easy

to produce . Disadvantages are the high breakability and

the low tensile strength . The strength and brittleness of

hard ferrites are similar to ceramics . Furthermore the

temperature resistance is limited and they have only a

low energy to volume ratio .

Hard Ferrite – Magnets

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About Magnets

The strong magnetic forces of attraction can cause skin bruises. Sufficient

security distances need to be kept between each magnet and all other

ferromagnetic elements!

A crash of magnets with high energy can produce splinters . Therefore al-

ways wear protection gloves and glasses!

Grinding dust of Rear – Earth Magnets is spontaneously inflammable. Al-

ways process with water!

Crashes of magnets can cause sparks . Handling and processing in EX –

environment is therefore strictly prohibited!

Strong magnetic fields can influence electronic and electrical devices as

well as data mediums . Don’t bring magnets close to peace makers, navi-

gation instruments, diskettes, plug-in boards etc .

For air cargo a special declaration maybe possible .

Radioactivity as well as joining together equal poles can reduce the ma-

gnetic force .

The highest defined working temperature must not be passed.

For all questions concerning magnets and, of course, reed products, please consult us!

Handling Information for Magnets

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About Magnets

Magnetization

Applications

Arrangement

Magnetized in height

(preferred orientation)

Motors, magnetic couplings,

ABS-systems, locking sys-

tems, cutter, press cylinder

Isotropic

Anisotropic

Axial magnetization

Loud-speakers, pot-magnet

systems, holding systems,

magnet switch, protection gas

control

Isotropic

Anisotropic

Axial, sector-shaped

magnetized, e .g . 6-pole

Synchronized motors, ma-

gnetic couplings, brakes, hall

sensors, hard-disc-drive

Isotropic

Anisotropic

Radial magnetization

Lifting magnets, holding sys-

tems, magnet bearings

Isotropic

Anisotropic 1)

Diametric magnetization

Synchronous motors, pumps

Isotropic

Anisotropic 1)

Magnetized on sec-

torshaped surface, e .g .

6-pole magnet

Magnetic separation, brakes,

holding systems, hall sensors,

hard-disc-drive

Isotropic

Anisotropic

Pole-oriented

Multi-pole in circumfe-

rence magnetized e .g .

4-pole

Dynamos, engines, magnetic

couplings, brakes, hall sen-

sors, tachometer

Isotropic

Pole-oriented

Two- or multi pole

magnetized at inside -ø,

e .g . 4-polig

Magnetic couplings, brakes,

motors, hall sensors, tacho-

meter

Isotropic

Anisotropic

Magnetized on lamellar-

shaped surface P = pole

pitch

Holding systems, protection

gas control . hall sensors,

brakes

isotropic

anisotropic

pole-oriented

Radial magnetization

Motors, magnetic couplings

Isotropic

Anisotropic

Diametric magnetization

Motors, magnetic couplings

Isotropic

Anisotropic

s s

Magnetization

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Notes

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Precautions

Many users of Reed Switches for sensor and reed relay

applications try to make the sensors and or relays them-

selves internally . Often however, they do not observe

some basic precautions and preventive measures to

insure reliable operation of the switch . Below we try to

cover the key areas that users and manufacturers must

observe .

Reed Switch modifications can be very dangerous to

the Reed Switch if not done properly . Primarily, this is

because the reed lead is large by comparison to the glass

seal . Here a balance is achieved in Reed Switch sen-

sitivity and mechanical strength . If the lead of the Reed

Switch was much smaller than the glass, seal stress

and glass breakage would not be an issue . However, to

achieve the sensitivity and power requirements in the

Reed Switch, a larger lead blade is necessary . With that

in mind, it cannot be emphasized enough, any forming

or cutting of the Reed Switch leads must be done with

extreme caution . Any cracking or chipping of the glass

are signs that damage has occurred . Internal damage can

occur with no visible signs on the seal . In these instances,

seal stress has occurred, leaving a torsional, lateral, or

translational stress in the seal . This produces a net force

on the contact area that can affect the operate charac-

teristics (Pull-In and Drop-Out), contact resistance, and

life characteristics .

Most Reed Switch suppliers can perform value added

cutting and shaping of the leads in a stress free environ-

ment using proper tooling and fixtures. Often times this

is the most economical approach for users, although it

may not seem so at the time .

Many times the user will often choose to make their own

modifications, and only after manufacturing and quality

problems with the product, do they go back and choose

the approach of letting the Reed Switch manufacturer

perform the value added requirements. Below, in figure

#1 and #2, is the proper approach for cutting and/or

bending the Reed Switch . The effect on the Pull-In and

Drop-Out characteristics of cutting and bending the Reed

Switch will be explained later in more detail .

Fig. #1 Presentation of the proper and improper way of bend-

ing a Reed Switch. Supporting the switch lead while bending

is a must.

Handling and Load Precautions when using Reed

Switches in various Sensor and Relay Applications

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Precautions

Fig. #2 Properly supported the switch lead while cutting is re-

quired, otherwise damage can occur to the Reed Switch.

Soldering and Welding

Many times soldering or welding of the Reed Switch is

required . Reed Switches are usually plated with a suit-

able solderable plating . Welding is also easily carried

out on the nickel/iron leads of the Reed Switch as well .

However, in both processes, if not done properly, stress,

cracking, chipping or breaking of the Reed Switch can

occur . When soldering or welding, the farther one is away

from the glass seal the better . Many times, this may not

be possible . Welding can be the most dangerous if one

is welding very close to the seal . Here a heat front of up

to 1,000 °C can conduct its way to the seal .

Since it arrives on one end of the seal first, the other

end of the seal may be at 20 °C . This causes a dramatic

thermal gradient to exist across the seal which can disrupt

the seal in many ways, all of which, will give rise to faulty

Reed Switch operation. See figure #3.

Fig. #3 Soldering and welding can generate a heat front to

the glass to metal seal of the Reed Switch causing potential

damage.

Soldering, in a similar manner, close to the seal can have

the same effect to a lesser extent because of the lower

solder temperatures involved (200°C to 300°C) .

Two ways to improve the likelihood of success are by

heat sinking the lead of the Reed Switch (figure #4) or by

preheating the Reed Switch and/or assembly .

Fig. #4 Use of Heat Sinking or preheating Reed Switches for

soldering or welding can prevent heat stress damage.

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Precautions

Most commercial wave soldering machines have a pre-

heating section before the PCB or assembly is immersed

into the solder wave . Here the thermal shock is reduced

by the existing higher ambient temperature preexisting

before the solder wave, thereby reducing the thermal

gra-dient to the reed switch seal .

Printed Circuit Board (PCB)

Mounting Reed products mounted on PCBs can some-

times be a problem. If the PCBs have a flex to them

after wave soldering, removing this flex may be required

when mounting the board to a fixed position. When the

flex is removed, the hole distance, where a Reed Switch

for instance may be mounted, can change by a small

amount . If there is no provision in the mounting to take

this small movement into consideration, the Reed switch

seal will end up absorbing the movement, which leads to

seal stress, glass chipping or cracking . Care should be

taken in this area, particularly when very thin PCBs are

used and flexing or board distortion is common.

Using Ultrasonics

Another approach to making a connection to a Reed

Switch is ultrasonic welding . Reed Switch Sensors and

Reed Relays may also be sealed in plastic housings

where the sealing process uses ultrasonic welding . In

addition, cleaning stations use ultrasonic welding . In all

these areas the Reed Switch can be damaged by the

ultrasonic frequency . Ultrasonic frequencies range from

10kHz to 250kHz, and in some cases even higher . One

does not only have to be concerned with the resonant

frequency of the Reed Switch and its harmonics, but also

of the resonant frequency of the assembly in which the

Reed Switch resides . Given the right frequency and the

exact conditions severe damage can occur to the con-

tacts . If using ultrasonics in any of the above conditions,

be very cautious and perform exhaustive testing to insure

there is no interaction or reaction with the Reed Switch .

Dropping Reed Switch Products

Dropping the Reed Switch, a Reed Sensor, or a Reed

Relay on a hard object, typically on the floor of a manu-

facturing facility, can induce a damaging shock to the

Reed Switch . Shocks above 200 Gs should be avoided

at all costs . (See Figure #5) . Dropping any of the above

on a hard floor from 30 cm or more (greater than one foot)

can and will often destroy a Reed Switch where G forces

greater than 100 G are not uncommon . Not only can

the glass seal crack under these circumstances, but the

reed blades may be dramatically altered . Here the gaps

may have been drastically increased or the gaps may be

closed, due to these high G forces . Simple precautions of

placing rubber mats at assembly stations can eliminate

these problems . Also, instructing operators that if a reed

product is dropped it can not be used until it is re-tested .

Fig. #5 Dropping the Reed Switch on a hard surface can in-

duce several 100 G to the contacts many times altering the

switch characteristics.

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Precautions

Encapsulating Reed Switch Products

Further damage can occur to a Reed Switch when one

attempts to package the Reed Switch by sealing, pot-

ting, or encapsulating . Whether this is done by a one or

two part epoxy, thermoplastic encapsulation, thermoset

encapsulation, or other approaches, damage to the glass

seal can occur . Without any buffer, the encapsulants

crack, chip or stress the glass seal . Using a buffer com-

pound between the Reed Switch and the encapsulant

that absorbs any induced stress is a good approach to

eliminate this problem . Another approach would be to

match the linear coefficient of thermal expansion with

that of the Reed Switch, thereby reducing stress as the

temperature fluctuates. However, keep in mind, this ap-

proach does not take into consideration the shrinkage

that occurs in most epoxies and encapsulants during

the curing stage .

Sometimes a combination of both approaches may be

the best way to seal a product with a Reed Switch .

Temperature Effects and Mechanical

Shock

Temperature cycling and temperature shock if naturally

occurring in a Reed Switch application must be taken

into consideration . Again, temperature changes creat-

ing movement with various materials due to their linear

coefficients of thermal expansion will induce stress to

the Reed Switch if not properly dealt with . All our Reed

Sensors and Reed Relays have been designed to handle

temperature changes and mechanical shock . Through

rigorous qualification testing by exposure to tempera-

ture cycling, temperature shock and mechanical shock,

potential design defects have been eliminated from our

products .

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Precautions

The Reed Switch contact rating is dependent on the

switch size, gap size or ampere turn rating, contact mate-

rial and atmosphere within the glass capsule . To receive

the maximum life for a given load some precautions may

be necessary .

Because a Reed Switch is a mechanical device and has

moving parts, there are circumstances where life will be

shortened due primarily to contact wear . Switching no

load or loads where the voltage is less that 5 Volts @ 10

mA or less, the contacts undergo little or no wear . Here

life times in excess of billions of operations are expected

and realized . In the 10 Volt range, higher contact wear

will take place . The amount of wear is dependent upon

the current switched . Generally speaking, switching 10

Volts @ 10 mA, life times of 50 million to 200 million op-

erations can be expected . If one is looking for more life

under these circumstances and you can not eliminate the

actual switching of the load, mercury wetted contacts may

be the correct solution . Here the contacts actually have

a small amount of mercury on them so that no net metal

is ever transferred from contact to contact . Life for most

‘hot’ switching loads using mercury wetted contacts will

also be in the billions of operations even when switching

100’s of Volts at 10’s of mA .

Switching pure DC loads is always advised . All the data

shown in our life test section, has been taken under this

condition . Avoid loads with a leading or trailing power

factors .

The quick disconnection creates a high induction volt-

age, which will result in arcing . This creates burns on

the contact surface .

When the contacts see a net overall capacitive load, an

inrush of current will occur when closing the contacts .

Contact damage and even sticking will occur depend-

ing up the total capacitance, voltage present and series

resistance .

Tungsten filament lamps, a very popular switching load

for Reed Switches particularly in automotive, have inrush

currents due to their cold filaments. Once the light is on

the resistance in the filament rises rapidly reducing the

current flow. Typically current surges in the order of 10

to 20 times the stead state current can be expected .

Knowing the cold filament resistance is important to

determine the size of the inrush current . Adding some

series resistance to the same circuit can have a dramatic

improvement on the life of the switch .

Capacitive and Inductive Loads

Stray capacitance may be present, to some degree,

when switching any voltage and current . When closing

and switching a given voltage and current, the first 50

nanoSeconds are the most important (figure #6). This is

where the exact amount of arcing will occur . If there is a

significant amount (depends on the amount of voltage

switched), of stray capacitance in the switching circuit,

a much greater arc may occur and thereby reduce life .

When switching any sizable voltage, it is always a smart

idea to place a fast current probe in the circuit to see ex-

actly what one is switching in the first 50 nanoSeconds.

Generally speaking, when switching voltages over 50

Volts, 50 picoFarads or more can be very significant to

the expected life times . If the Reed Switch is operated

remotely with a long cable connection, that cable can

act like a long distributed capacitance . Shields and other

potentially capacitive components can also lend their

capacitance to high inrush currents .

Load Switching and Contact Protection

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Precautions

Fig. #6 Surprisingly large inrush currents can be generated

across the contact when stray capacitance is charged to com-

pliance voltages. Contact life may be dramatically shortened.

When line voltages are present in or near sensitive cir-

cuits, be cautious . Those voltages can be coupled into

the circuit creating havoc with your life requirements .

Typically, a faulty Reed Switch is blamed for this reduced

life, when in actuality, it is a product of unforeseen condi-

tions in the circuit .

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Precautions

Under above conditions, protective circuitry can be added

which will minimize the metal transfer at the time of the

transitions, but not eliminate it. Circuits shown in figure

#7 are very typical . The capacitance can be only a few

pF attributed to stray capacitances or actual capacitive

com-ponents in the mf range . Capacitors in an elec-

tronic circuit store charge . By their nature they like to

give up their entire charge as quickly as possible . With

no resistance or impedance to the flow of the current,

that is exactly what will occur .

Fig. #7 Switching capacitance directly will damage the con-

tacts rapidly with high inrush currents. Adding a resistor or an

inductor will reduce the inrush current and reduce the contact

wear.

Inrush currents are to be avoided or minimized when

closing the contacts of a Reed Switch . If your circuit al-

lows series resistance to be added directly in line with

the Reed Switch, that is generally the best choice . The

higher the resistance the better as shown in Figure #7 .

Using an inductor or adding inductance in the circuit

can be effective as well . Inductors initially impede the

flow of current, thereby reducing inrush currents. Here a

careful balance must be calculated such that too much

inductance is not added, thwarting its effect and creating

another problem when the contacts open .

Switching inductive loads such as relays, sole-noids,

coil driven counters, small motors or inductive circuits

will all require protective circuitry to lengthen the life of

the reed contacts (see figure 8).

Protection Circuitry

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Precautions

Fig. #8 Abruptly opening a circuit with an inductor can pro-

duce a very large back voltage. Adding a diode in parallel with

the coil will dramatically reduce this voltage. An RC network
across the contact will also help.

Inrush Current Loads

Lamp loads can also produce high inrush currents when

they are initially switched on . Here typically tungsten

filaments are used in small bulbs which will have inrush

currents as high as 10 times their normal operating cur-

rent when initially switched on. See figure #9. Adding re-

sistance in series with the lamp can dramatically reduce

the inrush current and play a major role in extending the

life of the Reed Switch .

Another approach is to add a parallel resistor across

the contacts as shown in Figure #9 . In this case, a small

current always flows through the filament keeping it hot

and its resistance high. This current flow is balanced

such that the filament is not ‘glowing’. Now when the

Reed Switch is activated, the current switched is close

to its steady state current .

Fig. #9 Lamps when first turned on have a high inrush current because of their cold filament. Adding series resistance will reduce

the inrush. Having a resistor in parallel with the contacts will allow a trickle current to flow, heating the lamp filament below it. Then

when the contacts close the filament is hot and does not draw an inrush current.

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Ampere-Turns (AT) versus Millitesla (mT)

With the advent of the Reed Switch, developed by Bell

Labs in the 1930s, it was convenient to measure its

operate characteristics using the units of ampere turns .

Since the Reed Switch is cylindrical it is easy to make

the measurement of its closure, release and contact

resistance using a coil with a given geometry, wire size

and number of turns . It is easy to conventionalize this

approach as long as other users, internal or external,

find no problem using ampere-turns (AT) as their unit

of measure .

However, a real problem arises when one finds that

no convention has ever been adopted in the Reed

Switches’ long history; in fact, most manufacturers of

Reed Switches have their own standard . Therefore,

companies who purchase their Reed Switches for making

Reed Relays, Reed Sensors or other reed products find

they have to deal with an assortment of AT standards .

No true standard is offered to customers who use Reed

Relays, Reed Sensors, etc .

Users find themselves selecting reed products with no

idea how to categorize or select them for their own ap-

plications . This results in much time lost and frustration

in trying to select the proper product . Often times, many

thousands of dollars may be lost through high production

failures or production line shut down, before determining

the correct Reed Switch sensitivity selection .

What we plan to present here is a standard that manufac-

turers of Reed Switches, manufacturers of reed products,

and users of reed products can all use . We will present

a simple way to bridge the approach of measuring the

magnetic field strength of a Reed Switch from the Reed

Switch manufacturer/reed product manufacturer to the

reed user’s application .

Before we present this approach, we need to review

a few very important points that generally affect Reed

Switch applications:

1 .

When a Reed Switch is initially measured, it is

made with its given overall length . This length is

established by the manufacturer to offer the users

the most flexibility for short and long length design

requirements . As one cuts the Reed Switch to a

given size for a given application the AT for that

switch will change . If now measured in the same

coil to a given cut length, the AT will be different .

If significant lead length is cut off the AT change

can be dramatic . This occurs because the reed

blades are ferromagnetic and the more magnetic

material present the more efficient the magnetic

field strength. Cutting away the magnetic material

will reduce the magnetic field strength; thereby, re-

ducing the magnetic sensitivity of the Reed Switch .

Some companies for a given special requirement

will supply the AT difference in their specification

for a given cut length . However, if the user cannot

measure his application in the standard test coil

used by the Reed Switch supplier because his ap-

plication does not ‘fit’ into it, which is most times the

case, it becomes impossible to directly correlate

between the two companies when using AT only .

2 .

Reed Switches that are not cut, but bent into a new

configuration, will often undergo an AT change as

well . Here, whenever the magnetic path is altered,

the magnetic field strength may change depending

upon the new given configuration.

3 .

When a Reed Switch is bent into a new configura-

tion with or without cutting the lead length, the AT

may be additionally altered by improperly bending

the Reed Switch . All Reed Switches have some

susceptibility to any stress placed on either end of

its glass to metal seal . Some switches are more

susceptible than others . In any case, a stress to

the seal can alter the mechanical operation and

thereby alter its AT . The Reed Switch gap gener-

ally averages less than 25 microns (0 .001”) . Any

small mechanical change produced by either a tor-

sional, rotational or linear force can give rise to an

A Comparison of the measured Magnetic Field

Strength using Ampere-Turns (AT) and Millitesla (mT)

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Ampere-Turns (AT) versus Millitesla (mT)

AT or contact resistance change . The contact gap,

contact design, blade overlap, lead material, lead

material hardness, lead material length and thick-

ness, seal strength, seal length, glass length and

measurement approach, will all influence the AT of

a Reed Switch .

Since the user in most cases can not measure his mag-

netic field requirements in AT, the easi-est way and more

accepted way is to measure the requirement in Gauss or

Millitesla (mT) . Here 10 Gauss is equal to 1 mT making

the in-terchange between the two units an easy task .

More generally accepted outside the Reed Switch and

reed product manufacturing arena is the use of Gauss

and Tesla or Millitesla (mT) .

Bridging Ampere-Turns (AT) to Milli-

tesla (mT)

The rest of this discussion will be to bridge the gap be-

tween ampere-turns (AT) to Millitesla (mT) . The lower the

AT or mT rating of a Reed Switch the lower the magnetic

field strength required to close the Reed Switch. To ac-

complish this bridge, we have chosen to use its internal

KMS standard coils as our AT standard; and bridging to

mT by using a standard AlNiCo 5 magnet with a given

length and mT rating . We found the easiest way to make

this bridging of units was to do the following:

1 .

First measure a group of Reed Switches in our

standard KMS coil and record the operate AT .

2 .

Using a linear micrometer table, with a 1240 mT

AlNiCo 5 magnet measuring 4 mm by 19 mm in

length, mounted at its axis origin, the magnetic field

strength was measured (in mT) at regular mm inter-

vals along the linear axis . See Figure 1 . Here it is very

important not to have any ferromagnetic material as

part of the test setup or anywhere near the testing . .

3 .

Using the same setup as in step two, we now

measure the operate point in mm of the previously

measured Reed Switches used in step one .

4 .

The mm distance of the closure points is now

mapped with the mT field strength taken in step two

The graphs that follow were produced in exactly this

above described manner . Keep in mind this data is taken

for the full length, uncut Reed Switch . However, this data

can be used for various cut lengths by using another

graph, which presents the percent of change for a given

cut length . This percentage change graph is shown for

various AT switches and the percentage changes not

covered can be extrapolated using the graph data .

Using the graphs in figure 5ff, we can directly convert

to mT .

An example of using this approach with the included

graphs is the following:

1 .

Your application requires you to use our KSK-1A85

Reed Switch, and you need to use only its cut length

of 30 mm .

2 .

You plan to have the Reed Switch close 15 mm

away from the magnet you have chosen .

3 .

You are capable of measuring your magnetic field

strength at this distance with a standard gaussme-

ter, and find you have a 2.2 mT field 15 mm from

your

magnet .

4 .

You next look at figure 7. where the AT and mT

graphs presents the comparison you need for the

KSK-1A85 . But since you are cutting the Reed

Switch to 30 mm you need to determine the per-

cent increase expected . For a 20 AT Reed Switch

being cut to 30 mm the percent increase is approxi-

mately 30% or 6 AT change (see Figure 3) . This

brings the AT of the switch up to 26 AT . Now, look-

ing at Figure 3 you see 26 AT corresponds to about

1 .7

mT .

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Ampere-Turns (AT) versus Millitesla (mT)

5 .

Here the original 20 AT switch will close well under

the field of 2.2 mT giving you plenty of margin. In

this way, depending upon your tolerances, you can

directly select the AT range you require .

Please be aware, that a Hall probe only measures the

field strength at a certain point. Whereas a Reed Switch

absorbs the magnetic field lines of its entire length.

Therefore this approach can only be used for a rough

approximation but, will enable your engineers to make a

preselection of the Reed Switch easily, quickly and cost

effectively for your application . Following this, we would

be able to help you with the precision adjustment .

Fig. #1 Presentation of the equipment and test layout in which the magnetic data was taken using a linear micrometer.

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Ampere-Turns (AT) versus Millitesla (mT)

Pull-In AT vs Reed Switch Cut Length

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56

Reed Switch Cut Length (mm)

12 AT
14 AT
17 AT
20 AT

Pull-In

Pull-In AT vs Reed Switch Cut Length

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

Reed Switch Cut Length in (mm)

Pull-In AT

5 AT
8 AT
12 AT
14 AT
17 AT
20 AT

Pull-In AT vs Reed Switch Cut Length

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

Reed Switch Cut Length (mm)

5 AT

8 AT

12 AT

14 AT

17 AT

20 AT

Pull-In

Pull-In AT vs Reed Switch Cut Length

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

Reed Switch Cut Length (mm)

5 AT

8 AT

12 AT

14 AT

17 AT

20 AT

Pull-In

The following graphs show the AT change for various cut

lengths of Reed Switches .

Fig. #2 Presentation of the operate AT change for various cut
lengths for a given operate AT.

Fig. #3 Presentation of the operate AT change for various cut
lengths for a given operate AT.

Fig. #4 Presentation of the operate AT change for various cut

lengths for a given operate AT.

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Ampere-Turns (AT) versus Millitesla (mT)

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

10,00

14,00

18,00

22,00

26,00

Pull-In AT vs Pull-In Distance in mm

Pull-In Distance mm

Pull-In AT

Pull-In AT vs Pull-In mT

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

0,00

0,50

1,00

1,50

2,00

2,50

Pull-In mT

Pull-In AT

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

45,00

50,00

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

Pull-In AT vs Pull-In Distance in mT

Pull-In mT

Pull-In AT

0,00

5,00

10,00

15,00

20,00

25,00

30,00

35,00

40,00

45,00

50,00

8,00

12,00

16,00

20,00

24,00

Pull-In AT vs Pull-In Distance in mm

Pull-In Distance in mm

Pull-In AT

We have also supplied graphs showing the AT operate

point versus mm distance so that a gaussmeter is not

necessary . Just using these enclosed graphs will allow

you to make the correct selection assuming you are us-

ing a similar magnet as was used in our data selection .

Fig. #5 The Pull-In AT is presented with its corre-sponding mT

Pull-In level.

Fig. #6 The Pull-In AT is presented with its corresponding Pull-

In distance from the magnet, and is measured in mm.

Fig. #7 The Pull-In AT is presented with its corresponding mT

Pull-In level.

Fig. #8 The Pull-In AT is presented with its corresponding Pull-

In distance from the magnet, and is measured in mm.

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Ampere-Turns (AT) versus Millitesla (mT)

0,00

10,00

20,00

30,00

40,00

50,00

60,00

70,00

8,00

12,00

16,00

20,00

24,00

Pull-In AT vs Pull-In Distance in mm

Pull-In mT

Pull-In AT

0,00

10,00

20,00

30,00

40,00

50,00

60,00

0,00

0,50

1,00

1,50

2,00

2,50

3,00

3,50

4,00

4,50

Pull-In AT vs Pull-In mT

Pull-In mT

Pull-In AT

Fig. #9 The Pull-In AT is presented with its corresponding mT

Pull-In level .

Fig #10 The Pull-In AT is presented with its corresponding Pull-

In distance from the magnet, and is measured in mm.

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Application Examples

Introduction

Reading the previous sections on the Reed Switch

basics, key parameters, and operational characteristics

before delving into this section will give one a better

background and more insight into developing require-

ments for your own applications .

Without question, the Reed Switches’ hermeticity lends

itself to more switching applications than any other

switching device . Its ability to be used as a complete

sensing component by itself or the ease of packaging

it into special sensing requirements is done without any

complicated process or high tooling costs .

There are so many existing and potential applications

for Reed Sensors that it would be impossible to discuss

them all here . We will however, cover some of the basic

applications which we hope, will give insight and help

to your application . Once you review this section and

some of the sensor specifications offered in this book,

please feel free to call our applications department, where

qualified engineers will be able to answer questions

pertaining to your application . Free samples are always

available as well .

Obviously if your application requires one of our stan-

dard sensors from our catalog, that is clearly the best

approach and the quickest solution to satisfying your

design requirement . However, more than half of our

shipped sensors are from special requirements . Since

many sensing requirements are unique, working with

customers on their special applications is expected .

Using the Reed Switch by itself can seem like the sim-

plest approach . However, without proper consideration

and precaution it could become disastrous . If you decide

to go this route, be sure to read our precautions section .

Most important to keep in mind, the Reed Switch is a

glass capsule and is susceptible to breakage . Observing

this, and properly mounting the switch in a stress free en-

vironment, will prove to be a winning combination . If you

do have failures or erratic operation, please discuss your

problem with our applications engineering . Many times

we have taken over the application and manufactured

the entire sensor thereby producing a fault free sensor .

In the end, it would have been less expensive having

us design and manufacture the entire sensor from the

beginning . Keeping this in mind, we really are open to

working with you on your application in either manner .

Reed Switch Selection

Initially the most important step is the proper selection

of the correct Reed Switch for a given application . If

the sensor is simply switching the gate of a transistor

or digital gate any Reed Switch will handle that require-

ment . The question then becomes one of size and cost .

Looking through our Reed Switch selection chart will

help you arrive at the best choice . If you are switching a

load, ‘hot’ switching a voltage at some current level, care

must be taken to select the proper wattage Reed Switch

with the corresponding required voltage and current

level . Sensors requiring long life times (10’s of millions

of operations) will need special attention to the load you

are switching . If you are switching 5 Volts @10 mA or

less you will not have a life problem; above this level care

must be taken . Talking with our applications engineers

and reviewing our life testing section will be helpful .

Reed Sensor Packaging

Usually packaging is the safest approach when devel-

oping a Reed Sensor . Carefully protecting the glass to

metal seal from potential damage or stress will result in a

fault free application . When packaging the Reed Switch,

even when it ‘looks’ fine, stress may have been induced

through bending, cutting, soldering, welding, potting, or

encapsulating the Reed Switch, with erratic behavior

resulting . Packaging the Reed Switch without inducing

any stress is critical to proper operation and long life,

whether it is packaged by the user or by the Reed Switch

Sensor manufacturer . Collaboration on the application,

between the user and the Sensor manufacturer must be

carried out in a detailed fashion .

Reed Switch and Reed Sensor Applications

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Application Examples

Using our Reed Switch Sensor selector guide will give

the user some ideas as to packaging styles and sizes .

Special packages with specific connectors or connections

are very much a norm . So do not hesitate, to offer your

special packaging requirement . Our special packages are

far too numerous to show in our Data Book . When deter-

mining the closure and opening distance care must be

taken to include the distance within the package as part

of the sensing distance . Standard packages offered by

Standex Electronics will take this distance into consider-

ation in the design . However, on special packages, keep

this distance in mind because it does affect sensitivity .

Plastic packages are easiest to tool and are the least

expensive . However, if a rugged enclosure is required,

use of a non-ferromagnetic material may be the best

approach . Be careful not to include any nickel, iron, or

cobalt in the package. They will shunt the magnetic field.

Lead lengths and connectors are wide open with hun-

dreds of possibilities for all potential requirements .

Reed Sensor Mounting

Mounting a Reed Sensor is generally quite open with a

multitude of options . However, care must be taken not

to mount the sensor on any ferro-magnetic material or

be within its influence. Keep in mind, magnetic flux lines

prefer to travel in ferromagnetic material, which in effect,

will have a shunting effect on the magnetic field.

We have shown cases where this effect can be used for

positive results in some applications in our operational

section, but one must give consideration to magnetic

materials in the vicinity of the application . Also, any

magnetic components that are also in the vicinity of an

application, such as inductors, transformers, toriods,

etc. must be given consideration to their influence in the

magnetic sensing circuit . Our Reed Sensors come with

an assortment of ways in which to be mounted . Many

have simple slots for screw hole mounting; some have

doubleback sticky tape; some simply screw into panels;

others have pins for PCB through hole mounting; others

have surface mount ‘J’ or ‘gull’ leads for mounting on SMT

boards . Variations of the above are available as well, to

meet all your application mounting possibilities .

Reed Switch Electrical Connections

All our Reed Sensors are manufactured with an assort-

ment of ways in which to be electrically connected . Most

of the popular ways are PCB mount, leads of varying

length for soldering, leads with connectors and surface

mount soldering . Some lead wires will have an array of

terminals available as options for making the electrical

connection . Most of our series offer terminals on the leads

for quick solderless connections . Surface mount solder-

ing is becoming increasingly popular . Our MK1, MK15,

MK16, and MK17 were all designed with that in mind .

Reed Switch Sensing Applications As stated, the list for

different sensing applications is endless . We will make

an attempt at presenting some of the more common

sensing applications, which we hope will nurture ideas

that may offer solutions to your sensing application . Keep

in mind, no external power is required in a Reed Sensor

application . The Reed Switch in most cases, once closed

will switch the load you require .

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Application Examples

Trailer Hitch Detection and Lights

Headlights and Taillights

Brake Fluid Sensor

Tire Pressure Monitoring

Anti-Theft Alarms

Ignition Immobilizer

Anti Lock Brakes

Interior and Dashboard Lighting / LED

Washer Level Sensor

Windshield Wiper Operation

Remote Door Locks and Keyless Entry

Coolant Level Sensor

Crash Safety Sensing

Sunroofs

Gas Cap Detection

Fuel Level Sensors

Antennas

Engine Transformers,

Coils and

Ignition Assemblies

Door Latch Sensors

Gear Shift

Music Video Systems

Seat Belt Detection

Brake Pedal

Battery Deactivation

Exhaust Fumes Detection

HVAC Controls

Mirror Controls

Hood Latch Verification

Standex Electronics dynamic capabilities and solutions

provide reed switches, relays, and sensors, magnetics,

and fluid level sensing products throughout the trans-

portation industry . Think of anything that turns on/off,

opens/closes, requires power transfer, lighting, starting,

measuring, or detecting – and we can play a role within

that application .

From read outs on the dashboard to measurement of

coolant, brake, windshield, water in fuel, tire pressure,

and emissions – our components perform within vital

processes within automobiles, heavy-duty trucks, recre-

ational vehicles, airplanes, trains, motorcycles, E-Cars,

E-Bikes, boats, and more .

Automotive & Transportation Market Applications

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Application Examples

Liquid Level Detection

More and more level sensing of brake fluid, window

washer fluid, and water cooling fluids are controlled

by Reed Sensors. A float, with a magnet mounted in it,

is generally placed in the container . The Reed Switch

is placed either inside or under the container for float

detection .

In the past, automotive manufacturers used the Reed

Switch in the brake fluid application in the following man-

ner: when the container is full the float opens the Reed

Switch. When the liquid level drops, the float goes down

and activates the Reed Switch . A lamp is then activated

on the dashboard . Nowadays, automotive manufactur-

ers use the Reed Switch in reverse order . When the

container is full, the float with the magnet, actuates and

closes the Reed Switch. When the level of the float drops,

the Reed Switch opens . The change in monitoring the

opening instead of the closure has the advantage that a

malfunction of the switch can be detected much easier .

If the on-board computer on the automobile can electri-

cally detect a level sensor, then an advanced level sensor

can be used . This sensor has more electronic compo-

nents than a Reed Switch . It is made with a PC board

on which a resistor is mounted in series that protects

the Reed Switch, and a second resistor is mounted in

parallel so that the computer detects that the sensor is

connected and in place .

Liquid level sensor applications range from one switch to detect

a high or low, to arrays of many to accurately monitor fluid levels.

Brake Fluid Detection

Convertible Roof Position Sensor

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Application Examples

Battery Deactivation Controlled by a

Reed Sensor

Brake Pedal Position

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Application Examples

Reverse osmosis

Anchor position

Hatch position

Battery detection

Winch level

Warning signals

Wind speed and direction sensor

Shore power detection

Bilge pump smart sensor

Fluid level sensors: fuel, oil, water, etc.

Clean water supply, toilet control

Kill switch

Sensor systems in

joysticks and control panel

Gearshift position,

Speed indicator

GPS positioning system

Radar antenna sensor

Detect our drive position,

Rudder position,

Trim tap position

Similar to automotive applications, Reed Sensors are

used in marine and boat applications for level sensing

and position detection .

Smart Bilge Pump Sensor

Marine and Boat Applications

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Application Examples

Security

Door Sensors

Window Sensors

Control Panels

Smoke Alarms

HVAC and Plumbing

Furnaces

Air Conditioning Compressors

Air Conditioning Condensers

Dehumidifiers
Humidifiers

Solar Panels

Gas Smart Meters

Electric Smart Meters

Instant Water Heaters

Standard Water Heaters

Water Meters

Shower Pumps

Pool and Spa Pumps

Sprinkler System Controllers

Transportation

Washer Level Sensor

Coolant Level Sensor

Keyless Entry

Ignition Immobilizer

Anti Lock Brakes

Dashboard Lighting

Marine Coils

Ignition Assemblies

Hood Latch Verification

Dashboard Lighting

Appliances

Dishwasher

Range

Oven

Microwave

Coffeemaker

Refridgerater

Ice Maker

Washers & Dryers

Other

Designer Lighting

Automatic Shades

Tablet Keyboards

Sound Sensors for Toys

Guitar Amplifiers

Microphones

Organs

Fitness Equipment

Garage Door Openers

Speakers

Household appliances and electronics feature much

higher efficiency and are now being designed in conjunc-

tion with smart metering devices . Detecting door posi-

tion, and monitoring fluid levels are just a few examples

of how reed switch sensors are making their way into

household appliances .

Dishwasher Spray Arm Detection

Smart Home Applications

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Application Examples

Security Control for Appliance Door

Detection

The white industry of refrigerators, freezers, microwave

ovens, stoves, etc . requires safety elements that detect

the status (open/closed) of the appliance doors . These

door sensors are designed in many sizes and shapes de-

pending upon the specific application. Many are specifi-

cally designed with special tooling . Generally, both a

Reed Switch and a magnet are used, and in many cases,

added circuitry is built into a PCB for smart sensing .

If the sensor does not activate after a specified period

of time, an alarm will sound, alerting one that the door

is ajar . In the case of a freezer, several hundred dollars

in frozen meats and other foods can be saved from

spoilage if an open door is detected . The Reed Sensor

is usually mounted in the chassis of the appliance and

the permanent magnet is placed in the doorframe . Thus,

when the door is closed, the magnet’s position is above

or parallel to the sensor . When someone opens the door,

the circuit is broken .

Water Flow Sensor

In this application, the sensor recognizes the movement

of water . The Reed Switch, in going from an open to a

closed state, produces a fast response to the initiation

of water flow; in turn, an action sequence is initiated.

Applications such as electric water heaters, air condi-

tioners, etc. represent some examples. A baffle plate

with a magnet mounted to it is used in the water flow

line. When water begins to flow, the baffle plate moves

parallel to the water flow. A Reed Switch is strategically

positioned to pick up, or sense, this movement, and once

the magnetic field is sensed, the Reed Switch closes. In

the case of a water heater, it instantly detects the water

flow, and in turn triggers the heating element to be turned

on . The alternative method of detecting the temperature

change when cold water is added into the tank can take

a much longer time for detection, resulting in the loss

of valuable heating time, particularly when high water

usage is involved

Measuring the Quantity of a Liquid or Gas

Water or fluid flow can be easily measured by mounting

a propeller just outside the water pipe and connecting

it underneath the plastic casing of the meter . The water

flows through the pipe and spins the propeller. A mag-

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Application Examples

net is mounted on the propeller and a Reed Sensor is

mounted outside of the plastic casing . In this case, each

propeller rotation is counted as the magnet rotates past

the Reed Sensor . The rotations are tallied and electronic

circuitry converts the rotations to volumes of water (or

other liquid) flowing through the pipes. In a similar man-

ner, the flow of gas and electricity can also be measured.

Our MK3 Sensor is often used for such applications, but

we have many other sizes and shapes also worthy of

your consideration .

Consumer Electronics

Sensors and switches make their way into many types

of consumer electronics . Just about any application in-

volving movement or the the need to switch something

on and off . A proximity sensor used in a cell phone or

digital camera has a switch housed in the device and a

permanent magnet positioned within the moving screen .

Once the screen is rotated or slid to one side the magnet

lines up with the switch contacts causing the screen to

activate the phone or camera .

Another use for a reed sensor in a cell phone is when the

phone is used with a docking station . When the phone is

place into the docking station, the magnet activates the

switch causing the phone to go into hands-free mode or

switches into car mode for the use of a global position-

ing system GPS .

Applications

•

Barcode Scanner

•

Camera screen activation

•

Cell phone screen activation

•

Chair lift

•

Copier position sensors

•

Electric toothbrush

•

Hotel security card reader

•

Hot Tubs & Spas

• Interlocking

• Keyboards

•

Laptop closure sensing

•

Massage chair

•

Printer sensors

•

Water flow sensor

•

Utility meter sensors

Hobby & Toy

Today’s toys are being designed with more and more

moving parts requiring simple, reliable and inexpensive

sensing solutions . Our magnetic reed sensors are a

perfect fit in countless toy sensor applications.

For example: a baby doll that drinks a bottle may have a

reed sensor positioned beneath the mouth and a perma-

nent magnet molded into the bottle and when the bottle

is held up to the it’s mouth, the baby makes a drinking

sound or stops crying .

Applications

•

Car race track

•

Baby doll position sensor

•

Electronic board game position sensor

•

Mechanical movement sensing

•

Model train

•

Video game peripherals

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Application Examples

Fire and Safety doors in public buildings, hospitals, gov-

ernment buildings, hotels and other buildings regularly

frequented by people, require the doors to be shut at all

times except in an emergency . By law, the doors must

be electronically controlled; if they are opened, proper

warnings must be given .

The topic of security gets more and more important –

and Standex Electronics has the solutions for a lot of

applications .

Safety Applications

•

Passive infrared detectors

•

Smoke and fire alarms

•

Dial-up modems

•

Ultrasonic detectors

•

Cargo & freight theft prevention

•

Door sensor

•

Emergency door sensor

•

Explosive Proof

•

Fire extinguisher

•

Hotel security

•

Position sensor

•

Vehicle restraint

•

Window sensor

Door Sensor for Fire, Safety & Emer-

gency Exit

Hotel Security

Safety and Security

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Application Examples

In portable and implantable devices it is equally important

to utilize a switch that is ultra miniature and one that

consumes the least amount of power . Reed switches

and sensors consume no power in their normally open

state . Reed Relays are used in many types of medical

equipment that require high current and/or high voltage .

Equipment such an electrosurgical generator requires a

high voltage relay to aid in regulating the right amount of

current used to cauterize vessels during surgery . Similar

equipment may use RF energy combined with saline to

seal off vessels therefore high frequency relays would

provide a maker solution .

Medical Applications

•

Camera pill

•

Handheld surgical tools

•

Glucose monitor

•

Hearing aid

•

Implantable cardioinverter defibrillator ICD

•

Orthopedic micro power instruments

• Pacemaker

•

Portable defibrillators

•

Surgical Instruments

•

Spine stimulator implant

•

Video camera pill

•

Hospital bed

•

Lift chair position

•

Mobility scooter

•

Patient lift

•

Power wheelchair

•

Stair lift position

•

Wheelchair ramp position

•

Cleaning equipment

•

Drug dispensing systems

•

Electrosurgical generators

•

EKG equipment

•

Insulin pumps

•

Intravenous pumps

Portable medical equipment - Defibrillator

Medical

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Application Examples

With the ever increasing requirements for electronics

and electronic systems, the need exists to be able make

voltage and current measurements covering several or-

ders of magnitude . From nano-volts to kilovolts and from

fempto-amps to amps . To do this with one instrument is

almost impossible; however, multimeter designers have

been able to expand the order of magnitude of these

measurements in recent years . To be able to do this,

the reed relay has become an essential component . Our

specialized reed relays have helped designers meet this

challenge .

Test and Measurement Applications

•

Automated Test Equipment

•

Battery powered

•

Cable testers

•

Chip testers

•

Data Acquisition/Scanning Systems

• Electrometer

•

Functional PCB testers

•

High voltage

• Industrial

•

Integrated circuit testers

•

Linear distance

•

Medical equipment testers

•

Modular Instrumentation

• Multimeters

•

Network Analyzers

• Oscilloscopes

•

RF Attenuators

•

TVS Tester

•

Wafer testers

•

Weather meters

High End Multimeters Use Reed Relays to Measure Low & High Voltages

Test and Measurement

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Application Examples

The hermetically sealed Reed Switches can switch low

signals, which are required for the various applications

within the telecommunication sector .

Telecommunication Applications

•

Device disabling

• Interlocking

•

Mobile phone position sensing

•

Off hook sensing

•

Switching a cellular phone on/off in a flip phone

•

Telephone line switching

•

Cellular phone antenna switching

•

Line sensing

•

Line switching

•

Modem switching

•

Pager T/R switching

•

Portable radios

•

RF Receivers

•

Test equipment

Reed relays for portable radios and communication systems

Telecommunication

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Application Examples

Many more Applications possible…

•

Motor rotor sensing

• Thermostats

•

Test and measurement equipment

•

Rain gauge sensor

•

Wind speed and direction sensing

•

Barometric sensing

•

Inside / outside temperature sensing

•

Position sensor for exact window sun-shade

control

•

Solar panels

•

E-Bikes brake detection

•

Sensor solutions for agriculture, forestry and cons-

truction machinery

•

Many more

E-Cars connector detection

Visit our website at www .standexelectronics .com to explore our diverse range of animated applications, including

and beyond what is offered in this book .

Further Applications

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Reed Relays

In a Reed Relay, the Reed Switch uses an electromag-

netic coil for activation and is shown in its simplest form

in Figure # 1 . Reed Relays require relatively little power

to operate and are generally gated using transistors,

TTL directly or cmos drivers . Reed Relay contacts,

when switched dry, (currentless closure or less than 5

Volts @ 10 mA), will literally operate well into the billions

of operations . In areas like automatic test equipment,

where Reed Relays may be called upon to switch tens

of millions of operations per year, the Reed Relay rises

to the challenge .

Fig. #1 A Reed Relay consists of a copper insulated wire wound

coil with a Reed Switch traditionally mounted on its center axis.

Using the proper design, materials, placing an electro-

static shield around the Reed Switch internal to the coil

and driving the shield, will allow coupling or passage of

very small signals (nanoVolt signals or femptoAmpere

currents) through the relay with little or no interference .

See figure #2. This is virtually impossible with other

technologies except at very high cost .

Fig. #2 Depiction of a Reed Relay showing the coil, Reed Switch,
and shield (coaxial) placement.

Using a coaxial shield internal to the coil, the Reed Relay

looks like a transmission line to high frequency signals .

With Reed Switches becoming smaller and smaller,

overall Reed Relay packages have shrunk to less than

8 mm long, reducing the distributed capacitance (switch

to shield) to less than 0.8 pF. See figure # 3 This has al-

lowed Reed Relays to carry frequencies up to 6 GigaHz

without serious loss of signal strength (3 dB down) .

Typically, insertion losses as low as 0 .2 dB and VSWR

of 1 .1 out to 2 GHz are now realizable . Reed Relays’ RF

characteris-tics rival the gallium arsenide mosfets and at

1 GHz and above are very cost competitive . Reed Relays

are now commonly used in semiconductor test equipment

and cellular telecommunication equipment because of

their superior better RF characteristics .

Fig. #3 A Reed Switch mounted internal to a coaxial shield

provides and excellent RF path for Giga Hertz frequencies.

Numerous applications for Reed Relays exist today and

are increasing every day . Please see our applications

section for more detailed Reed Relay usage .

The Reed Switch used as a Reed Relay

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Reed Relays

Introduction

Reed Relay applications continue grow every year

despite severe competition from other small switching

devices such as semiconductors and electromechanical

armature style relays .

Because the contacts in a Reed Relay are hermetically

sealed, the contacts can switch low level signals as low

as femtoamps and nanovolts . Electromechanical relays

cannot do this because they are not hermetically sealed

and have polymer films build up on their contacts that

require a voltage arc to break through this layer before

conduction can take place . Similarly, semiconductors

have capacitance, leakage currents and semiconductor

offsets to deal with that clearly limit the switching and

detection of low voltages and currents .

Also, electromechanical relays, at best, can switch up to

low millions of operations . Because its armature moves

about a pivot point, wearing occurs, reducing life . The

Reed Switch has no wearing parts and therefore, under

signal conditions will switch into the billions of operation

with fault free operation .

Reed Relays are ideally used for switching applications

requiring low and stable contact resistance, low capaci-

tance, high insulation resistance, long life and small size .

For specialty requirements such as high RF switching,

very high voltage switching, extremely low voltage or

low current switching, again Reed Relays are also ideal .

Reed Relay Features

•

Long life (10

operations)

•

Multi-pole configurations up to 8 poles

•

Form A (normally open switching)

•

Form B (normally closed switching)

•

Form C (single pole double throw - normally

closed contacts break before the normally open

makes)

•

Form D (single pole double throw - normally open

contacts make before the normally closed breaks)

•

Form E (latching – bi-stable state switching )

•

Low contact resistance (less than 50 milliohms)

•

High insulation resistance (greater than 10

ohms)

•

Ability to switch up to 10,000 volts

•

High current carrying ability

•

Ability to switch and carry signals as low as 10

nanovolts

•

Ability to switch and carry signals in the femtoamp

range

•

Capable of switching and carrying signals up to 10

Gigahertz

•

Operate times in the 100μs to 300 μs range

•

Operating temperature from –55 to 100°C

•

Capable of operating in all types of environments

including air, water, vacuum, oil, fuels, and dust

ladened atmospheres

•

Ability to withstand shocks up to 200 Gs

•

Ability to withstand vibration environments of 50 Hz

to 2,000 Hz at up to 30 Gs

•

Very small sizes now available

• Auto-insertable

•

Standard pin-outs

•

Large assortment of package styles available

•

Large assortment of Reed Switch options available

•

Large assortment of coil resistances

•

Relays can be driven in a current or voltage mode

•

UL, CSA, EN60950, VDE, BABT 223ZV5 approved

on many of our relays

•

Magnetic shielding available on many of our

relays

Reed Relay Applications

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Reed Relays

Reed Relays are susceptible to magnetic effects which

may degrade performance under certain conditions . This

report presents a practical approach to reducing magnetic

effects among and between Reed Relays . The guidelines

can be applied to many cases .

With the trend toward reducing the size of electronic

equipment, Reed Relays are typically placed in proximity

to one another . Magnetic coupling between relays can

affect parameters such as pull-in and dropout voltage . In

some circumstances, adjacent relays will be adversely

affected by their neighbors .

Experimental data is provided for some basic Reed

Relay arrays under worst-case conditions . An analysis

of the data is presented with equations . The data was

gathered on single in-line package (SIL) Reed Relays,

but applies to most Reed Relay packages because the

basic physical principles are the same .

A checklist for designing a relay array or matrix covers

the factors necessary to minimize the electromagnetic

effects most likely to be encountered . Systematically

progressing through the checklist will aid in reducing or

eliminating many troublesome variables

Factors Affecting Reed Relay Magnetic

Interaction

A host of factors, internal and external, determine how a

Reed Relay will perform when installed in a matrix assem-

bly and subjected to electromagnetic interference (EMI) .

Internal Factors:

Early in the design phase, the user

and the manufacturer must discuss the application and

consider all the internal factors:

•

Coil wire gauge

•

Coil resistance

•

Coil ampere-turns (AT)

•

Coil winding direction

•

Coil winding terminations

•

Type of Reed Switch assembly

•

Number of Reed Switches in the relay

•

Internal magnetic shielding

External Factors:

Controlling external factors generally

is accomplished by giving proper attention to the operat-

ing environment of the Reed Relay . How much effort is

expended on these factors will depend on how strongly

they adversely affect design performance . Consideration

should be given to these factors:

•

Nearby magnetic fields

•

Relay spacing in the relay matrix

•

Magnetic polarity arrangement

•

External magnetic shielding

Magnetic Coupling between Reed Relays

To better understand the magnetic coupling between

adjacent Reed Relays, consider this example . Figure #4

shows a portion of a relay matrix with two adjacent Reed

Relays mounted on a PC board . The relays, K1 and K2,

are identical in construction and the direction of current

flow is the same in each.

Magnetic field lines are shown when both relays are

energized . When K1 and K2 are energized, their op-

posing magnetic fields will adversely affect each other.

This is shown where the field of K2 is extended into the

body of K1 .

When K2 is energized and K1 is not operating, the pul-

lin and dropout voltage of K2 is within the range of the

manufacturer’s specifications. Attempting to energize

K2 when K1 is operating results in an increase of the

pull-in and dropout voltage for K2, perhaps beyond the

manufacturer’s limits .

Reducing Magnetic Interaction in

Reed Relay Applications

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Reed Relays

(a)

(b)

(c)

(d)

(e)

When K1 is energized with a current flowing opposite

to that in K2, an attempt to energize K2 results in lower

pull-in and dropout voltages .

Fig. #4 Magnetic Interaction Effects in Reed Relays

Experimental Data on Topical Relay

Matrices

Relay matrices can be configured in a number of ways.

In this analysis, data is presented on five typical configu-

rations . Polarity considerations also have been limited .

The configurations and magnetic polarities presented

demonstrate some worst-case effects of relay-magnetic

interaction .

Experimental Setup

Experimental data was gathered on 0 .20“ wide molded

SIP relays. The test matrix configurations are shown in

figure #5.

Data was taken while all relays surrounding the rela-

yunder-test (RUT) were energized with the same mag-

netic polarity . Once all relays were energized, the RUT

(with the same magnetic polarity as its neighbors) was

incrementally energized to the pull-in point . The dropout

voltage data was taken in a similar manner .

All data was gathered using a 5V coil drive . With higher

voltage coils that generate equivalent ampere turns, the

results are similar . Higher ampere-turn coils produce

slightly higher interaction effects . For data presented on

magnetically shielded relays, the magnetic shielding is

internal and an integral part of the relay (fig. #6).

Fig. #5 Relay Test Configurations: (a) Two In-Line Matrix; (b) Three In-Line Matrix; (c) Five In-Line Matrix; (d) Stacked Matrix of

10 and (e) Stacked Matrix of 15.

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Reed Relays

(a)

(b)

Data Analysis

In general, near worst-case magnetic interaction condi-

tions for pull-in voltage in a matrix exist when all relay

fields have the same polarity and all the fields are from

adjacent relays (fig. #5). The interaction is somewhat

reduced when the matrices have relays mounted end-

to-end (figure 5d and 5e). This effect can be seen in

figure 6b.

Under the expected worst-case conditions presented,

dropout voltage is not really a concern since it increases

and tracks the pull-in voltage, maintaining approximately

the same voltage change . The dropout voltage may

become a major concern if the magnetic polarity of

adjacent relays is opposite to that of the RUT (aiding) .

This situation can be avoided by assigning appropriate

voltage polarities to the relays and using relays of con-

sistent manufacture .

The change in pull-in voltage (ÄPI) is defined as the

pull-in voltage with interaction effects minus the pull-in

voltage without interaction effects . Percent increases for

the pull-in voltages presented were calculated at the 5-V

nominal coil voltage . Stated mathematically .

% ΔPI = ΔPI(100)/5 volts

Equation #1

For a given matrix, the change in pull-in voltage essen-

tially remains the same for all relays having pull-in volt-

ages of various levels . If one relay without interaction, for

example, has a pull-in voltage of 2 .3 V, it will shift to 2 .7

V with interaction (ΔPI of 0.4 V). Now consider a second

relay in the same matrix under the same conditions that

has an initial pull-in of 2 .6 V . With interaction, the pull-in

voltage will rise to 3 .0 V

(again the ΔPI is 0.4 V).

Calculating the Effects of Magnetic

Interaction

To further examine the magnetic interaction effects on

Reed Relays, consider an example using the three-relay

matrix of 5-V SIL relays in figure #5b on 0.20“ centers

(no magnetic shielding) . All testing will be performed on

the center relay which has an actual pull-in voltage of

2 .6 V by itself . The outer two relays are activated with 5 V

applied to the coil .

Fig. #6 (a) Percent Pull-In Voltage Increase vs. On-Center Distance Between SIL Relays. Data was taken using the three-relay

test matrix (see Figure 5). (b) Percent Pull-In Voltage Increase vs. the Number of Relays in the test configuration, using matrix

for up to 15 relays.

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Reed Relays

The center relay will be energized and the expected

pull-in voltage change can be calculated .

First calculate the pull-in voltage change . For the ex-

ample, these equations will be used:

ΔPI =(% ΔPI x Vnom)/100

Equation #2

Where ΔPI = the expected pull-in voltage change.

% ΔPI =the percent interaction calculated at the nominal

voltage and shown in the graphs of experimental data .

Vnom = the nominal coil voltage as specified by the

manufacturer .

PIwc = Plact + ΔPI

Equation #3

PIwc = the worst-case increased pull-in voltage under

interactive conditions .

Plact = the actual pull-in voltage without external mag-

netic interference .

Referring to figure #6a, at a nominal coil voltage of 5 V,

the magnetic interaction is 14 .2% . Using equation #2 to

calculate ΔPI:

ΔPI = (14.2 x 5)100 = 0.71 volts

The relay has an actual pull-in voltage of 2 .6 V . Therefore,

the near worst-case pull-in voltage can be calculated

using Equation #3:

PIwc = 2 .6 + 0 .71 = 3 .31 volts

Equation #4

The value calculated for Plwc is perhaps the worst case

for the given matrix under all possible polarity (magnetic

and electrical) conditions. The value calculated for, ΔPI

is a close approximation over the entire pull-in voltage

range .

Furthermore, ΔPI ~ ΔDO; that is, the dropout voltage

change in the matrix closely follows the change in pul-

lin voltage . For example, in the calculation for PIwc, if

the dropout voltage was measured to be 1 .4 V without

magnetic influence, its value will change to 2.11 V for

the conditions described . Except for rare cases where

special dropout conditions are required, dropout voltage

changes as described do not present a problem .

Ways to reduce Magnetic Effects

•

Specify Reed Relays with internal shielding

•

Use external magnetic shielding on the matrix

•

Provide for larger spacing between relays

•

Avoid simultaneous operation of adjacent relays

•

Design a special matrix configuration

Special Conditions

For conditions presented in figure #6, the data was taken

on single unenergized relays surrounded by energized

relays . In many actual applications, the relays are ener-

gized under a host of different circumstances . Typically,

banks of relays are energized together .

For example, the data gathered in Figure 5a will be

reduced by approximately a factor of two by energizing

the relays in the same matrix in this manner: energize

all relays simultaneously with a ramping voltage while

monitoring the center relay .

Here the interaction effects will be reduced by a factor

of two . This same effect will be observed with faster and

faster ramp speeds (approximately a step function) if the

relays are still energized simultaneously .

This reduction in interaction occurs because of the re-

duced surrounding magnetic fields present at the time

of contact closure, where the actual pull-in voltages are

typically half the nominal voltage .

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Reed Relays

Fig. #7 Alternative Pairs Relay Test Configurations.

Special Matrix Applications

Under certain conditions, the consistent direction in

which the coils are wound and terminated, particularly

when mounted close together, can reduce the magnetic

influence.

The matrix shown in figure #7 uses the opposing mag-

netic polarities and consistent coil manufacture to reduce

interaction without the added cost of magnetic shielding .

This effect (fig. #8) is achieved by wiring the matrix as

shown in figure #7.

The data presented in figure #5 can be compared to the

data presented in figure #6 for a similar nonmagnetically

shielded SIL matrix of 15 where the polarities are in the

same direction . The improvement or reduced interaction

is 2.5 % in figure # 8 compared to 6% in figure #6.

Checklist for Designing a Relay Matrix

1 .

Applied Voltage

2 .

Temperature Effects

3 .

Available PC board space

4 .

Distance between adjacent relays

5 .

Energizing the matrix

6 .

Magnetic shielding

7 .

Life Characteristics

8 .

Design Analysis

Fig. #8 Alternate Pairs Matrix.

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Reed Relays

Applied Voltage:

The power supply under maxi-

mum load and at 50 °C can be as low as 4 .9 V min-

imum . Under some circumstances, the load volt-

age may be in series with transistor/diode drops of

0 .7 V maximum over the operating temperature

range . The working voltage of the power supply is

reduced to 4 .3 V, the actual voltage applied to the

relay coil .

Temperature Effects:

If the maximum system

operating temperature is 50 °C and the specified

relay pull-in voltage is 3 .6 V maximum at 25 °C for

a 5-V nominal coil voltage, a rise in voltage from

3 .6 V to 3 .96 V maximum at 50 °C can be expected .

Available PC Board Space:

A 5 x 10 relay ma-

trix (50 relays) is required. To fit the relays on the

board, a crowded arrangement must be employed

(only 7 .75 in .2 of board space is available) .

Distance between Adjacent Relays:

The relays

must be placed on 0.20“ centers, five rows of 10

relays

each .

Energizing the Matrix:

In this application, a maxi-

mum of three relays is energized simultaneously .

Figure #6a presents the interaction data required

for this application . Here the worst case occurs for

the non-magnetically shielded 0 .20“ separation

and is 7 .5 % . By using equation 2, the interaction

effects are calculated as a worst-case pull-in volt-

age increase of 0 .38 V .

Magnetic Shielding:

It is decided not to use mag-

netic

shielding .

Life Characteristics.

In general, when switching

intermediate to high-level loads, the coil voltage

overdrive should be about or equal to 100% (about

or equal to two times the actual pull-in voltage) for

best-life characteristics . Here the relay coil over-

drive is small; however, only low-level switching is

expected . Therefore, the life characteristics should

not be affected .

Design Analysis:

If the results found in item 5

were added to the results in item 2, the maximum

pull-in voltage will rise to 4 .34 V under interactive

conditions . This exceeds the minimum voltage of

4 .3 V . Probably the two simplest approaches at this

point are increasing the power supply voltage or

lowering the initial maximum pull-in voltage rating

from 3 .6 V to at least 3 .2 V maximum . This would

leave sufficient added overdrive under worst-case

conditions .

Summary

Magnetic interaction effects on Reed Relays can rep-

resent a significant problem if ignored. Many solutions

are possible .

The foundation for determining worst-case scenarios

on the basic matrix types is presented in this article . A

systematic approach to designing a relay matrix can be

achieved by referring to the checklist provided .

It is strongly suggested that the user consult with the

relay manufacturer early in the design process . Follow-

ing this methodology will greatly diminish the potential

for unpredictable relay matrix performance .

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Reed Relays

Specifications

Reed Relay

Mechanical Relay

Solid State Relay

Switching Time

100 μs - 1 ms

> 5 ms

< 100 μs

Life Expectance: Low Level

cycles

Nearly unlimited

Power Consumption

3mW possible

50 mW

3mV possible

Switching Voltage

10 kVDC

1 .5 kVDC

Switching Current /

Carry Current

Max . 3A / Max . 5A

Up to 40A

Load Minimum

No load requirement (μV/pA)

50mW

Insulation Resistance

Ohm

Noise

No switching noise

Partly high switching noises

No switching noise

Insertion Loss

Low (0 .5dB)

High (2dB)

Overload

Very sensitive

Insensitive

General

Linear graph from DC to

GHz range

Linear graph from DC to

GHz range

Distortion of the signal

General

Galvanic isolation (air gap)

No galvanic isolation

(low/high)

Reed Relays in Comparison with Solid State

and Mechanical Relays

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Reed Relays

CRR- and CRF-Reed Relays for usage

in Automated Test Equipment

CRR for functional test systems . Functional test systems

continue to grow in size, pin count and complexity . Each

pin usually requires 3 to 5 test connections . Each test

connection needs to be isolated from all the others . In-

troducing any leakage paths thwarts the signals under

test potentially shunting them to the point where they

lose their functionality .

Because of the high pin counts, the number of test

connections grows dramatically . Here the need to sat-

isfy these test connections with an ultra small surface

mounted relay (CRR series) becomes ideal with the

following specifications:

1 .

Extremely small size (8 .6 x 4 .4 x 3 .55 mm)

2 .

Ability to mount the Reed Relays on both sides of

the board

3 .

Standard internal magnetic shielding eliminating

any magnetic interaction even in the tightest ma-

trices

4 .

Insulation resistance to all points typically 1014

ohms

5 .

Over 200 volts isolation across the contacts

6 .

A minimum of 1500 volts isolation between switch

and coil

7 .

Thermal offset voltage across the contacts in the

one microvolt or less range

8 .

Contact capacitance is less than 0 .2 pf

CRF-Relays

for wafer, memory, and integrated circuit

test systems . Integrated circuit and wafer testers have

continued to take on an ever more complex format with

the need for faster and faster clock rates . With clock rates

in the 2 GHz range, components must be able to pass

continuous wave signals with frequency responses in

the 8 to 10 GHz range . These fast switching high speed

digital signals require these new frequency responses

so that signals are not slewed or reflected going through

the switching components in these systems . .

The CRF Reed Relay represents an ideal switch in these

component testers for the following reasons:

1 .

The frequency response of 7 GHz is a current criti-

cal need

2 .

Rise time change through the relay of 40 pico-

seconds typical

3 .

High return loss

4 .

Insertion loss less than 1 dB at 6 GHz

5 .

Extremely small size

6 .

Ability to mount the Reed Relays on both sides of

the board (with internal magnetic shielding elimi-

nating any magnetic interaction)

7 .

Insulation resistance to all points typically 1014

ohms

8 .

Over 200 volts isolation across the contacts

9 .

A minimum of 1500 volts between switch and coil

10 .

Thermal offset voltage across the contacts in the

one microvolt or less range

11 .

Contact capacitance less than 0 .2 pf

12 .

Open contacts to shield capacitance 0 .6 pf

7 GHz RF-Reed Relays – Applications

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Reed Relays

Height: max .

*All dimensions in mm (inches)

DIMENSIONS

PIN OUT

(Top View)

POST REFLOW

PAD LAYOUT

Instrumentation (CRR and CRF)

1 .

On measuring input of multimeters where voltage

isolation is required, low voltage offsets (on the

order of 1 microvolt or less) and very low sub-pi-

coamp leakages are needed .

2 .

Feedback loops where high frequency, low leak-

age, and voltage isolation are required

3 .

In Attenuators where a high frequency response is

required, low leakage paths are essential, long life

(in excess of 100 million operations), and elimina-

tion of any inter-modular distortion is a clear need .

Multi-pole Configurations

When circuits require common points tied together, ca-

pacitance becomes a real problem . Trying to reduce this

capacitance can be a real effort with no clear solution .

Using our new relay approach multipole relays with com-

mon tie points are no problem configuring with resulting

reduced capacitance . Relay drivers, connectors, etc .

can be easily added forming RF switching modules, RF

attenuators, T/R switches, ‘T’ switches, etc .

Fig. #9 Mechanical layouts with Ball Grid Array (BGA).

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Reed Relays

Most important in the testing of any component for

frequency response over 100 MHz is a good Network

Analyzer and carefully designed test fixtures for calibra-

tion as well as for the actual testing . The same is true

when testing in the time domain . When measuring rise

time characteristics, one must be aware of overshoot

and undershoot of the rise time pulse that may affect the

signal quality adversely .

Fixture design starts with suitable SMA connectors on

high frequency board material . There are several materi-

als suitable for this including FR-4, G-Tech materials, and

several Rogers PCB materials . Many feel FR-4 material

is suitable since the fixture zeroing process will eliminate

its high frequency loss characteristics . As a general rule,

below 6 GHz is okay; above 6 GHz use of Rogers high

frequency circuit materials such as, RO3203 or RO4350

will improve the test performance . Rogers has several

other materials available depending upon the TCE match-

ing of the component/s or performance requirements .

Most of these materials are ceramic filled.

Figures 15, 16, 17 and 18 below show calibration board

layouts for a shorted to ground, and open circuit, through

line transmission, a 50 ohm impedance termination,

and the layout used to test the device . As many ground

points as possible were used along with avoiding and

sharp corners . All signal path transitions were made as

gradual as possible .

Once the calibration testing was completed, our test pro-

cess was as follows using an Agilent Network Analyzer

Model number 8720ES (See test layout in figure #14).

All calibration boards were entered into the network ana-

lyzer and stored . The relay under test was then measured

and stored . The calibration data was then entered and

the losses due to the board under the various configu-

rations was extracted yielding the results shown below .

This was compared with data extracted from a MIMICAD

pro-gram using the equivalent circuit presented and the S

parameters; and it was found both tracked very closely .

See the results from the data shown below taken from

network analyzer . Included are the isolation, insertion

loss, and VSWR . Also, included below is a Smith chart

indicating the impedance for a given frequency over the

entire frequency range .

Applications Notes for RF-Relay Measurement in

both the Frequency and Time Domain

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Reed Relays

Dielectric

Conductor

Ground

Fixture Design

Definition of the exact geometry your test fixture will take

is the first key step. Listed below are four geometries

and their corresponding equations for calculating the

characteristic impedance .

Fig. #10 Coaxial cable geometry

= 60/(

√

(

)) ln ((2h)/d)

Equation #5 (for a coaxial cable)

Where h and d are defined above and εr is the dielectric

constant for the material between conductors .

Fig. #11 Round wire over a ground geometry.

= 60/(

√

(

)) ln ((4hk

)/d)

Equation #6 (for a round wire over ground)

for a round wire over ground

Here k

is the proximity factor for round wire over ground,

which is near unity when the ratio h/d is large; but for

close spacing is approximately

= ½ + (

√

(4h2 – d2))/4h

Equation #7

is reduced to ½ when the round wire touches the

ground at d = 2h . The proximity effect results from the

same mechanism as skin effect . Mutual repulsion drives

like currents to the extreme outer edges of individual

conductors carrying current in the opposite direction . This

crowds the current in round wires toward the side nearest

a ground . As is the case while signal is going through

the relay, the proximity effect and skin effect are indistin-

guishable for a coaxial line because the entire surface of

the round center conductor is at the same distance from

the shield . Proximity effect is not normally considered for

thin rectangular conductors, but skin effect does drive the

currents toward the edges of the conductors .

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Reed Relays

TransmissionTest Port

(RF IN)

Reflection Test Port

(RF OUT)

8720ES Network Anaylzer

300KHz - 20 GHz

TEST

BOARD

Condcutor

Fig. #12 Buried microstrip geometry.

= 60/(

√

(

)) ln ((5.98h)/(0.8w + t))

Equation #8 (buried microstrip over ground)

Fig. #13 Stripline geometry.

= 60/(

√

(

)) ln (3.8(h +0.5t)/(0.8w + t))

Equation #9 (Stripline between ground planes)

Test Setup and Test Fixtures

Key to the proper testing of a component in an RF circuit

is the proper use of test fixtures.

Fig. #14 Test Setup.

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Reed Relays

Calibration Approach Critical

•

The fixtures were constructed to serve as calibra-

tion boards to allow for better characterization of

the relays. All the fixture boards used to test the

relays under test (RUT) used SMA connectors for

connection to and from the test equipment and for

terminations . The following are the makeup of the

boards under test:

•

RUT calibrated with a 50 ohm line and open termi -

nation

•

RUT calibrated with a 50 ohm line and shorted

termination

•

RUT calibrated with a 50 ohm line and 50 ohm

termination

•

RUT calibrated with a 50 ohm through line

Fig. #15 50 Ohm termination board.

Fig. #16 Open/short termination board.

Fig. #17 Through line transmission termination board.

Fig. #18 Relay under test termination board.

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Insertion Loss

Copper Wire Insertion Loss

Fig. #19 Insertion loss tested to 7 GHz for the Reed Relay

shown in figure #9.

Horizontal full scale: 7.0 GHz.

Vertical scale:10 dB/div referenced from the 0 mark.

Fig. #20 Insertion loss tested to 7 GHz for the Reed Relay

shown in Figure # 9 but with the internal Reed Switch replaced

with a bare copper wire.

Horizontal full scale: 7.0 GHz.

Vertical scale: 10 dB/div referenced from the 0 mark.

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VSWR

Isolation

Fig. #21 Voltage Standing Wave Ratio (VSWR) tested to 6.5

GHz for the Reed Relay shown in figure #9.

Horizontal full scale: 6.5 GHz.

Vertical scale: 1.0/div referenced from the bottom line 1.0

mark.

Fig. #22 Isolation tested to 7 GHz for the Reed Relay shown

in figure #9.

Horizontal full scale: 7.0 GHz.

Vertical scale: 10 dB/div referenced from the 0 mark.

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Return Loss

Fig. #23 Return loss tested to 6.5 GHz for the Reed Relay

shown in figure #9.

Horizontal full scale: 6.5 GHz.

Vertical scale: 10 dB/div referenced from the 0 mark.

Fig. #24 Represents the characteristic impedance going

through the Reed Relay shown in figure #9. Waves 1 through

5 depict calibration points.

Horizontal full scale: 750 ps.

Vertical scale: 150 milliUnit/div referenced from the 0 unit

mark. The vertical scale measures the reflection coefficient.

1 - Short Before Relay

2 - Open Contacts

3 - Close Contacts

4 - Closed Contacts - Shorted

5 - Closed Contacts - 50 Ohm

Smith Chart

Fig. #25 Shows a Smith Chart plotted for frequencies to 4 GHz.

The second dotted circle starting from the right is the 50 Ohm

impedance point.

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Reed Relays

Test Results

Figures #19 through 25 represent the results of our test-

ing using the procedures previously described and the

fixtures presented. The fixtures used were made from

FR-4 printed circuit board material . Improvements to the

fixturing, using some of the Rogers PCB material may

improve the results .

Insertion Loss

As explained earlier, insertion loss is the loss of power

going through the relay . Insertion loss is one of the most

important measurements in RF because it is simply

a measure of the loss of the signal going through the

component (Reed Relay) . Minimizing this loss is a key

interest .

First, it can be clearly seen that the insertion loss looks

excellent up to 7 GHz, as shown in Figure # 19 . As shown,

the insertion loss curve is very flat and begins to tail off

at 7 GHz . Clearly, this indicates signals, whether digital

or analog, will fare very well when switched and pass-

ing through this CRF ceramic Reed Relay . When using

semiconductors as a switching element, intermodulation

distortion may sometimes occur, giving rise to distortions

in the frequency response . With a passive device such

as the Reed Relay, no intermodulation distortion exists,

resulting in a very flat insertion loss up to 7 GHz. Hav-

ing this very flat insertion loss allows the user the ability

to switch, carry or deal with a multitude of different fre-

quencies or different width digital pulses, without having

to worry about having different switches to handle the

different frequencies .

At higher and higher frequencies, it has been proposed

that a Reed Relay, because it uses nickel/iron as its

center conductor, will not have very good performance

characteristics . Skin effect is often the proposed culprit,

because nickel and iron, being ferromagnetic, have a high

magnetic permeability μ. However, this is not the case as

shown in figure #20, where the Reed Switch in figure #9

was replaced with a pure copper wire. Comparing figures

#19 and #20, one sees little or no difference . Under high

power transmission conditions, a difference would prob-

ably be seen . But as is the case in many applications,

the power being switched is very low; and therefore, we

only see a negligible effect up to seven GHz .

VSWR

VSWR represents the effects of the transmission of

power due to standing waves . When standing waves

are present on a line, some power is being reflected

back on the line and re-reflecting again from the source.

This back and forth reflection produces standing waves.

These standing waves interfere with the transmission of

the original signals from the source because they are

continuously present and continually absorb power . Fig-

ure #21 presents the VSWR for the Reed Relay shown

in Figure # 9 . While still an important RF characteristic

for analog continuous wave analysis, insertion loss is

looked on more for RF characteristics .

Isolation

Isolation is the ability of a component to isolate the RF

signal from propagating further in a circuit . For a Reed

Relay, the isolation is a measure of the ability to halt fur-

ther progress of the signal when it is in the open state . We

all think of a switch in the open state as meaning no signal

passes beyond those open contacts . However, with RF

we know an open circuit is not totally open because the

capacitance across the contacts represents a leakage

path; and indeed with high enough frequencies, that’s

exactly what occurs . Presented in Figure #22 one can

see isolation of –50 dB or greater at low RF frequencies

which drops to –15 dB at 3 GHz and continues to a level

of –10 dB at 7 GHz . Contributing to this drop off in isola-

tion is the contact gap . Increasing the gap on the Reed

Switch is very difficult to do because it would require a

larger capsule, which would increase the package size .

Also, a larger gap will make the switch less sensitive for

closure, requiring more coil power . If the isolation is a

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Reed Relays

critical parameter in an application, stringing more than

one Reed Relay together will help . Also using a ‘T’ switch

or half ‘T’ switch will yield much higher isolations .

Return Loss

Return loss is also an RF parameter that is not used as

much as the insertion loss or isolation . As stated, it is a

measure of the power of the RF signal being reflected

back to the source . As can be seen in Figure #23, the

return loss has only 35 db of reflected signal at the lower

frequencies and about 10 db reflected back at 6.5 GHz.

Here the larger the dB level the lower the percentage of

the signal being reflected.

Characteristic Impedance

To gain most information from a characteristic im-

pedance measurement of the relay it is fruitful to make

measurements of the signal up to certain points while

going through the relay . Since this measurement is a

spatial measurement, the actual impedance at each point

of the relay can be measured . The following points of

reference were made as shown in Figure #24:

1 .

A short before the relay defining when the signal

enters the relay

2 .

Open contacts define the signal up to the middle

of the relay

3 .

Closed contacts define the signal path up to the

end of the relay

4 .

Closed contacts with the relay shorted

5 .

Closed contacts with the relay terminate in 50 ohms

Superimposing the 5 traces on the actual trace through

the relay, a full picture of the characteristic impedance

can be seen at each point though the relay . This is very

valuable particularly if the relay or component is slightly

off the 50 ohm impedance . As shown in the trace in

Figure #24, the relay is slightly above 50 ohms . With

the trace being high, this indicates a slightly inductive

entrance into and out of the relay . Compensating with a

little capacitance on each end of the relay will tune the

impedance to the desired level . This will in turn improve

the performance of the relay in a given circuit and in-

crease its performance at higher RF frequencies as well .

Smith Chart

If one is looking at different RF frequencies in a given

application or at a specific frequency, a Smith Chart can

help by presenting the characteristic impedance over a

given frequency range . The Smith Chart presents a plot

of the response of fre-quencies every 50 KHz up to 4

GHz . Shown in Figure # 25, the plot of points is centered

around the 50 ohm real point . To better understand this

Smith Chart, the second dotted circle starting from the

right center point of the large circle is the 50 ohm imped-

ance circle . The center line of the circle running horizon-

tally, is the real axis . Plots above this line are inductive

and plots below this line are capacitive . As shown, the

plot of the CRF relay is in a tight circle around the real

axis, and centered around the 50 ohm circular axis .

Summary

As can be seen the CRF Reed Relay is an excellent Reed

Relay for switching and carrying RF signals at least up

to 7 GHz and beyond . Our current efforts are to improve

its characteristics up to 10 GHz and beyond . This is a

reachable goal as we try to continually develop new RF

relays, pushing the current bandwidth and current ‘state

of the art’ . As higher and higher frequencies are used and

components are needed to develop these circuits, the

need for Reed Relays like the CRF series and subse-

quent improvements on performance over existing data

will be needed . Our engineers are up for this challenge .

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Life Test Data

The life of a Reed Switch can vary widely depending on

the exact switching circumstances . Over the years, many

improvements have been made to the Reed Switch,

which have played a major role in improving its reliability .

Reed Switches, because of their hermetically sealed

properties and no wearing parts, will switch no load or

signal loads into the billions of operations, in most cases

with minimal contact resistance changes . In fact, over

long life at no load, the contact resistance will often times

drop approximately 5 mOhms to 10 milliohm .

Standex Electronics offer several different types of

switches ranging from 4 mm long to 50 mm long, ca-

pable of switching nanoVolts up to 10,000 Volts; capable

of switching femtoAmps up to 5 Amps, and capable of

switching DC on up to 6 GigaHz . Generally speaking,

we offer Reed Switches in Sensor or Relay applications

having tungsten, rhodium, ruthenium, palladium or iridium

contacts .

When trying to optimize your life requirement be sure

to consult our precautions selection . Several areas of

concern are discussed both mechanically and electroni-

cally . The load section in particular will give you important

insight when switching any loads with inductance, capaci-

tive, or inrush current loads .

It is always best to test the particular switch under actual

switching loads for the life you require . A life test offers

a high level of safety .

Life Requirements

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Activate Distance

Resulting from position and movement of the actuator magnet .

Activate Distance

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Activate Distance

308

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MEDER electronic

ACTIVATE DISTANCE

Typ

Artikel-Nr.

Magnetische

Empfindlichkeit

Position und Bewegung

max. Anzugsdistanz in mm

Position und Bewegung

min. Abfalldistanz in mm

MK03

-1A66

-500W

2232711054

> 1,70

15,0

6,5

9,3

8,5

17,5

8,0

11,4

10,1

MK03

-1A66

-500W

2233711054

> 2,30

13,0

4,4

7,4

7,2

16,5

6,5

9,9

9,5

MK03

-1A66

-500W

2234711054

> 2,70

11,0

4,0

5,7

6,5

14,5

5,5

8,5

9,0

MK03

-1A66

-500W

2235711054

> 3,10

10,0

3,5

4,5

5,7

13,5

5,0

8,0

8,5

K04

-1A66

-500W

2242661054

> 1,70

15,0

6,5

9,3

8,5

17,5

8,0

11,4

10,1

MK04

-1A66

-500W

2243711054

> 2,30

13,0

4,4

7,4

7,2

16,5

6,5

9,9

9,5

MK04

-1A66

-500W

2244711054

> 2,70

11,0

4,0

5,7

6,5

14,5

5,5

8,5

9,0

MK04

-1A66

-500W

2245661054

> 3,10

10,0

3,5

4,5

5,7

13,5

5,0

8,0

8,5

MK05

-1A66

-500W

2252711054

> 1,70

15,0

6,5

9,3

8,5

17,5

8,0

11,4

10,1

MK05

-1A66

-500W

2253711054

> 2,30

13,0

4,4

7,4

7,2

16,5

6,5

9,9

9,5

MK05

-1A66

-500W

2254661054

> 2,70

11,0

4,0

5,7

6,5

14,5

5,5

8,5

9,0

MK05

-1A66

-500W

2255661054

> 3,10

10,0

3,5

4,5

5,7

13,5

5,0

8,0

8,5

MK06-4-A

2206040000

< 1,70

8.5

13.5

9.5

13.5

MK06-4-B

2206040001

> 1,70

7.5

12.5

10.5

13.5

11.5

MK06-4-C

2206040002

> 2,30

10.5

9.5

7.5

MK06-4-D

2206040003

> 2,70

6.5

11.5

10.5

MK06-4-E

2206040004

> 3,10

5.5

8.5

9.5

MK12

-1A66

-500W

9122711054

> 1,70

11.5

20.5

14.5

MK12

-1A66

-500W

9123711054

> 2,30

11.5

9.5

8.5

12.5

11.5

MK12

-1A66

-500W

9124711054

> 2,70

7.5

5.5

6.5

11.5

9.5

MK12

-1A66

-500W

9125711054

> 3,10

5.5

3.5

8.5

MK11/M8

-1A66

-500W

9118266054

> 1,70

15,0

6,5

9,3

8,5

17,5

8,0

11,4

10,1

MK11/M8

-1A66

-500W

9118366054

> 2,30

13,0

4,4

7,4

7,2

16,5

6,5

9,9

9,5

MK11/M8

-1A66

-500W

9118066054

> 2,70

11,0

4,0

5,7

6,5

14,5

5,5

8,5

9,0

MK11/M8

-1A66

-500W

9118566054

> 3,10

10,0

3,5

4,5

5,7

13,5

5,0

8,0

8,5

MK13

-1A66

-500W

9132661054

> 1,70

15,0

6,5

9,3

8,5

17,5

8,0

11,4

10,1

MK13

-1A66

-500W

9133711054

> 2,30

13,0

4,4

7,4

7,2

16,5

6,5

9,9

9,5

MK13

-1A66

-500W

9134711054

> 2,70

11,0

4,0

5,7

6,5

14,5

5,5

8,5

9,0

MK13

-1A66

-500W

9135661054

> 3,10

10,0

3,5

4,5

5,7

13,5

5,0

8,0

8,5

MK14

-1A66

-100W

9142711054

> 1,70

MK14

-1A66

-100W

9143711054

> 2,30

6.5

7.5

MK14

-1A66D-100W

9144711054

> 2,70

4.5

6.5

MK14

-1A66

-100W

9145711054

> 3,10

2.5

4.5

MK15

-2

9151710022

> 1,70

6.5

9.5

MK15

-2

9151710023

> 2,30

6.5

8.5

6.5

7.5

8.5

7.5

MK15

-3

9151710024

> 2,70

5.5

7.5

5.5

8.5

MK15

-3

9151710025

> 3,10

4.5

3.5

Note: The mentioned distances in millimeters in the table are typical values, they can differ in applications.

Type

Part-No.

Magnetic

Sensitivity

Position and Movement

max. Pull-in Distance in mm

Position and Movement

min. Drop-out Distance in mm

Note: The mentioned distances in millimeters in the table are typical values, they can differ in applications .

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Activate Distance

309

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MEDER electronic

ACTIVATE DISTANCE

Typ

Artikel-Nr.

Magnetische

Empfindlichkeit

Position und Bewegung

max. Anzugsdistanz in mm

Position und Bewegung

min. Abfalldistanz in mm

MK16

-2

9161870022

> 1,70

9.5

MK16

-2

9161870023

> 2,30

14.5

9.5

MK16

-2

9161870024

> 2,70

5.5

7.5

6.5

9.5

MK16

-2

9161870025

> 3,10

5.5

13.5

9.5

8.5

MK17

-2

9171009022

> 1,70

7.5

12.5

13.5

MK17

-2

9171009023

> 2,30

14.5

9.5

15.5

7.5

11.5

10.5

MK17

-2

9171009024

> 2,70

12.5

9.5

MK17

-2

9171009025

> 3,10

5.5

8.5

7.5

13.5

6.5

10.5

8.5

MK18

-300W

9182100034

> 1,70

16.5

14.5

18.5

9.5

16.5

10.5

MK18

-300W

9183100034

> 2,30

9.5

15.5

12.5

MK18

-300W

9184100034

> 2,70

5.5

7.5

9.5

MK18

-300W

9185100034

> 3,10

13.5

10.5

9.5

MK20/1

-100W

9202100014

> 1,70

5.5

6.5

7.5

11.5

MK20/1

-100W

9203100014

> 2,30

10.5

5.5

6.5

7.5

MK20/1

-100W

9204100014

> 2,70

4.5

5.5

6.5

10.5

MK20/1

-100W

9205100014

> 3,10

9.5

4.5

5.5

6.5

MK21M

-1A66

-500W

9212100054

> 1,70

5.5

4.5

6.5

5.5

MK21M

-1A66

-500W

9213100054

> 2,30

2.5

6.5

1.5

4.5

8.5

3.5

MK21M

-1A66

-500W

9214100054

> 2,70

9.5

3.5

11.5

2.5

MK21M

-1A66

-500W

9215660054

> 3,10

2.5

3.5

Alle angegebenen Distanzen sind gültig mit folgenden Magneten:

2500000002 / M2, Schraubmagnet

2500000004 / M4, Schraubmagnet

2500000013 / M13, Schraubmagnet

2500000021 / M21, Schraubmagnet

Diese Tabelle enthält nur einen Teil unseres Sensorsortiments. Schaltdistanzen für alle anderen Serien, Schaltertypen, Sonderausführungen und mit anderen

Magneten auf Anfrage.

Note: The mentioned distances in millimeters in the table are typical values, they can differ in applications.

4003004003 / offener Rundmagnet, ø 4x19 mm

2500000005 / M5, Schraubmagnet

Type

Part-No.

Magnetic

Sensitivity

Position and Movement

max. Pull-in Distance in mm

Position and Movement

min. Drop-out Distance in mm

Note: The mentioned distances in millimeters in the table are typical values, they can differ in applications .

All distance data above are valid for the magnets below:

4003004003 / open cylindrical magnet, ø 4x19 mm

2500000005 / M5, screw magnet

2500000002 / M2, screw magnet

2500000013 / M13, screw magnet

2500000004 / M4, screw magnet

2500000021 / M21, screw magnet

The table on this page contains only part of our sensor product range . Switching distances for other series, switch types, special sen-
sors and with other magnets can be obtained upon request .

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Glossary

Notes

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Glossary

The following definitions refer to the generally used terms

relating to Reed Switches, Reed Sensors, Reed Relays

and Electromechanical Relays. Some of the definitions

have multiple names. The most popular name was chosen

for this listing and is listed under that name. However, we

have tried to list those other common names under the most

popular name.

Actuation Time

is the time from initial energization to the

first closing of open contact or opening of a closed contact,

not including any bounce .

Ampere Turns

(AT or NI) is the product of the number of

turns in an electromagnetic coil winding times the current

in amperes passing through the winding. AT usually defines

the opening and closing points of contact operate conditions .

Armature

is the moving magnetic member of an electro-

magnetic relay structure .

Bias Magnet

is a steady magnetic field (permanent magnet)

applied to the magnetic circuit of a relay or sensor to aid or

impede operation of the contacts .

Bias

, Magnetic is a steady magnetic field applied to the

magnetic circuit of a switch .

Blade

is used to define the cantilever portion of the reed

switch contained within the glass envelope .

Bobbin

is a spool, coil form or structure upon which a coil

is wound .

Bobbinless Coil

(self supporting coil) is a coil formed with-

out the use of a bobbin .

Bounce

, Contact is the intermittent opening of closed con-

tacts occurring after initial contact actuation or closure of the

contacts due to mechanical rebound, or mechanical shock

or vibration transmitted through the mounting .

Break

defines the opening of closed contacts.

Breakdown Voltage

is that voltage at which an arc or break

over occurs between the contacts .

Breakdown Voltage

, Pre-ionized is the voltage level at

which the voltage breaks down across the contacts, after

which, the voltage had been recently broken down across

the contacts, creating an ionized state in the glass capsule .

Usually the break-down voltage in the pre-ionized state is

a lower value and more repeatable . It is a truer measure of

the breakdown voltage level .

Bridging

is the undesired closing of open contacts caused

by a metallic bridge or protrusion developed by arcing caus-

ing the melting and resolidifying of the contact metal .

Changeover Contact

(also referred to as a Form C or single

pole double throw (SPDT)) has three contact members, one

of them being common to the two contacts . When one of

these contacts is open, the other is closed and vice versa .

Coaxial Shield

is an electrostatic shield grounded at both

the input and output .

Coil

is an electromagnetic assembly consisting of one or

more windings of copper insulated wire usually wound on a

bobbin or spool . When current is applied to the coil, a mag-

netic field is generated, operating the contacts of a Reed

Relay or Electromechanical Relay .

Common Mode Voltage

usually refers to a voltage level

as measured between one or more lines and ground (com-

mon) or a current flowing between one or more lines and

ground (ground) .

Contact

refers to the contact blades making up a Reed

Switch or Electromechanical Relay .

Contact, Bifurcation

is a forked, or branching of contacting

member so formed or arranged, as to provide some degree

of independent dual contacting .

Contact, Break-before-make

(Form C) defines the se-

quence in which one contact opens its connection to another

contact and then closes its connection to a third contact .

Contact Force

is the force which two contact points exert

against each other in the closed position under specified

conditions .

Contact Form

describes the type of contacts used for a

given design or applications (ex . 1 Form A, 1 Form B, etc .)

Contact, Form A

is a single pole single throw (SPST) nor-

Glossary of Commonly used Terms Relating to

Reed Switch Products

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Glossary

mally open (N .O .) switch .

Contact, Form B

is a single pole single throw (SPST) nor-

mally closed (N .C .) switch .

Contact, Form C

is a single pole double throw (SPDT)

where a normally closed contact opens before a normally

open contact closes .

Contact, Form D

is a single pole double throw where the

normally open contact closes before normally closed contact

opens (continuity transfer) .

Contact, Form E

is a bistable contact that can exist in either

the normally open or normally closed state . Reversing the

magnetic field causes the contacts to change their state.

Contact, Current Rating

is the current which the contacts

are designed to handle for their rated life .

Contact, Gap

is the gap between the contact points when

the contacts are in the open state .

Contact, Make-before-break

(Form D) defines the se-

quence in which one contact remains connected to a second

contact while closing on a third contact and then the second

contact opens its connection .

Contact, Rating

refers to the electrical load-handling ca-

pability of relay contacts under specified conditions for a

prescribed number of operations .

Contact, Reed

defines a Reed Switch whereby a glass

enclosed, magnetically operated contact using thin, flex-

ible, magnetic conducting leads or blades as the contacting

members .

Contact Resistance

is the electrical resistance of closed

contacts; measured at their associated contact terminals

after stable contact closure .

Contact Seal

refers to a contact assembly sealed in a

compartment separate from the rest of the relay .

Contact Separation

is the distance between mating con-

tacts when the contacts are open .

Contact, Snap Action

describes the crisp closure and

opening of contacts at or around the operate points where

the contact resistance remains constant and stable .

Contact, Stationary

is a member of a contact combination

that is not moved directly by the actuating system .

Contact Tip

is the point at the end of a contact where the

contacts come together when closed .

Contact Transfer Time

(in a Form C switch) is the time dur-

ing which the moving contact first opens from a closed posi-

tion and first makes with the opposite throw of the contact.

Contact Weld

is a fusing of contacting surfaces to the extent

that the contacts fail to separate when intended .

Contact Wipe

occurs when a contact is making the rela-

tive rubbing movement of contact points after they have

just touched .

Contacts, Mercury Wetted

are contacts that make closure

via a thin film of mercury maintained on one or both contact

surfaces by capillary action .

Control Voltage

is another name for the voltage applied

across the coil of a relay and refers to that point where the

relay will operate .

Crosstalk

is the electrical coupling between a closed contact

circuit and other open or closed contacts on the same relay

or switch, expressed in decibels down from the signal level .

Current

is the rate of flow of electrons in a circuit measured

in amperes (unit A) .

Current, AC

is alternating current flow from positive to

negative .

Current, DC

is current flow in one direction.

Current, Carry

is the amount of current that can safely be

passed through closed switch contacts .

Current, Inrush

is the surge of current a load may draw

at initial turn on and may be many times greater than the

steady current draw .

Current Leakage

is that parameter measuring the unwanted

leakage of current across open contacts and/or leakage

current between the coil and contacts .

Current Rated Contact

is the current which the contacts

are designed to handle for their rated life .

Currentless Closure

refers to contacts closing with no

voltage existing or current flowing at the time of closure.

Cycling

refers to the minimum number of hours during which

a relay may be switched between the off state and the on

state at a fixed, specific cycle rate, load current, and case

temperature without failure .

De-energize

is the act of removing power from a relay coil .

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Glossary

Dielectric Strength

or Breakdown Voltage is the maximum

allowable voltage, usually measured in DC Volts or Peak

AC, which may be applied between two specified test points

such as input-output, input-case, output-case and between

current-carrying and non-current-carrying metal members .

Dropout

refers to maximum value of coil current or voltage at

which a Reed Switch or Relay resumes its natural condition .

Dropout Value

is the measured current, voltage or distance

when the contacts open .

Duty Cycle

is the percentage of time on versus time off or

duty cycle = Ton/Toff .

Dynamic Contact Resistance

is the repetitive measure-

ment of contact resistance measured 1ms to 3 ms after

contact closure .

Electrostatic Shield

is a copper alloy material terminated

to one or more pins and located between two or more

mutually insulated elements within a relay which minimizes

electrostatic coupling between the coil and Reed Switch in

a Reed Relay .

Energization

is the application of power to a coil winding

of a relay .

Frequency, Operating

represents the rate or frequency at

which contacts be switched on and off .

Frequency Response

is the frequency at which the output

signal decreases by 3 dB from the input signal .

Gap, Magnetic

describes the nonmagnetic portion of a

magnetic circuit .

Hermetic Seal

is an encapsulation process where the

contacts are sealed in a glass to metal seal in the case of

a Reed Switch . In the case of a relay, the contacts and coil

are sealed .

Holding Current

is the minimum current required to main-

tain closed contacts .

Holding Voltage

is the minimum voltage required to main-

tain closed contacts .

Hysteresis

1 . The lag between the value of magnetism in a magnetic

material, and the changing magnetic force producing it;

magnetism does not build up at the same rate as the force,

and some magnetism remains when the force is reduced to

zero . Also, the difference in response of a device or system

to an increasing and a decreasing signal .

2 . Hysteresis is also referred to the difference between

the operate voltage and the release voltage and can be

expressed as a percentage of release/operate .

I/0 Capacitance

is the capacitance between the input and

output terminals or between the coil and contacts .

I/0 Isolation Voltage

refers to the voltage value before volt-

age breakdown occurs . It is the same as breakdown voltage .

Impedance

refers to the resistance in ohms composed of DC

resistance, inductive reactance, and capacitive reactance

added vectorally in an RF circuit .

Insulation Resistance

is the DC resistance in ohms

measured from input to output or across the contacts .

Measurement is usually done by applying 100 Volts to one

of the points to be measured and the other is connected to

a picoameter .

Latching Relay

is a relay that maintains its contacts in the

last assumed position without needing to maintain coil ener-

gization . To change the state of the contacts, the magnetic

field must be reversed.

Leakage Current

is the current flow from input to output or

across the contacts when the contacts are in the open state .

Load, Contact

is the electrical power encountered by a

contact set in any particular application .

Load Power Factor

is the phase angle (cos) between load

voltage and load current in an electrical circuit caused by

the reactive component of the load .

Load Voltage

refers to the supply voltage range at the output

used to normally operate the load .

Low Thermal Relay

is a Reed Relay designed specifically

to switch very low microvolt or nanovolts signals without

distorting their signal level .

Magnetic Flux

is the total magnetic induction, or lines of

force, through a given cross section of a magnetic field.

Magnetic Interaction

is the undesired effect when relays

are mounted in close proximity, the flux produced when the

coils are energized affects the pickup and dropout values of

the adjoining relays . This either increases or decreases both

pickup and dropout values . The direction of the parameter

shift is determined by whether the stray flux aids or bucks

the flux produced by the coil of the relay under consideration.

Problems may result from bucking flux raising the pickup

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Glossary

V
R

V
W

V
R

A R

V A

√

WATTS

VOLTS

OHMS

AMPS

voltage close to the coil drive voltage or by aiding the flux of

sufficient magnitude that the relay will not drop out when its

drive is removed . To calculate the change in pull-in voltage

and dropout voltage, multiply the percent change shown

by the relay’s nominal voltage . For example, if the percent

change in pull-in voltage is 14% for a 5V nominal relay, the

pull-in voltage will increase by 0 .7 volts .

Magnetic Pole

is the end of a magnet, where the lines of

the flux coverage, and the magnetic force is strongest (north

or south pole) .

Magnetic Shield

is a thin piece of ferromagnetic metal

surrounding a relay to enhance its magnetic field internally

while reducing the stray magnetic field external to the relay.

Magnetostrictive Force

usually refers to the force produced

on the contacts with current flowing and the coil energized.

Here the magnetic field of the coil and the magnetic field

produced by the current flowing through the contacts interact

with each other producing a torsional force .

Make

refers to the closure of open contacts .

Mechanical Shock, Non-operating

is the mechanical

shock level (amplitude, duration and wave shape) to which

the relay or sensor may be subjected without permanent

electrical or mechanical damage (usually during storage or

transportation) .

Mechanical Shock, Operating

is the mechanical shock

level (amplitude, duration and wave shape) to which the relay

or sensor may be subjected without permanent electrical or

mechanical damage during its operating mode .

Miss, Contact

is the failure of a contact mating pair to close

in a specified time or with a contact resistance in excess of

a specified maximum value.

MOV (Metal Oxide Varistor)

is a voltage-sensitive, non-

linear resistive element . MOV’s are clamp-type devices

that exhibit a decrease in resistance as the applied voltage

increases . They are usually characterized in terms of the

voltage drop across the device while it is conducting one

milliamp of current . This voltage level is the conduction

threshold . The voltage drop across an MOV increases

significantly with device current. This factor must be taken

into consideration when determining the actual protection

level of the device in response to a transient .

Normally Closed (N.C.)

, Contacts (Form B) represents a

state of contacts before any magnetic field is applied to them

in which they exist in the closed state .

Normally Open (N.O.)

, Contacts (Form A) represents a state

of contacts before any magnetic field is applied to them in

which they exist in the open state .

OHM’s Law

the following is a table of common electrical

conversions

Operate Time

or (contact operate time or Pull-in time) is

the total elapsed time from the instant power is applied to

the energizing coil until the contacts have operated and all

contact bounce has ended .

Operating Temperature Range

is the normal temperature

range in which a Reed Switch, Sensor, or Relay will suc-

cessfully operate .

Output

is the portion of a relay which performs the switching

function required .

Output Capacitance

is capacitance across the contacts .

Output

Offset Voltage or thermal offset usually measured

in microvolts is voltage existing across closed contacts in

the absence of any signals . The voltage which appears at

the output of the isolation amplifier with the input grounded.

Overdrive

is the amount of voltage or ampere turns applied

after the exact point of closure of contacts is reached . Contact

resistance is usually measure with 40 % overdrive .

Permeability

is a characteristic of a magnetic material which

describes the ease of which it can conduct magnetic flux.

Pickup Value

refers to the measure of current or voltage

applied to a relay when the contacts just close .

Pickup Pulse

is a short, high-level pulse applied to a relay;

usually employed to obtain faster operate time .

Pole, Double

is a term applied to a contact arrangement

to denote two separate contact combinations, that is, two

single-pole contact assemblies

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Glossary

Pole, Single

is a term applied to a contact arrangement to

denote that all contacts in the arrangement connect in one

position or another to a common state . Pressure, Contact

refers to the force per unit area on the contacts .

Rating, Contact

is the maximum rating of the allowable volt-

age and current that a particular contact is rated to switch .

Reed Relay

is a relay that uses a glass-enclosed hermeti-

cally sealed magnetic reed as the contact members .

Reed Switch or Reed Sensor

is a switch or relay using

glass-enclosed magnetic reeds as the contact members

which includes mercury-wetted as well as dry contact types .

Relay, Antenna switching

is a special RF relay used to

switch antenna circuits .

Relay, Close Differential

is a relay having its drop-out value

specified close to its pickup value.

Relay, Crystal Can

defines a relay housed in a hermeti-

cally sealed enclosure that was originally used to enclose

a frequency control type of quartz crystal .

Relay, Current Sensing

is a relay that functions at a pre-

determined value of current typically used in teleco Reset

refers to the return of the contacts to their normal state

(initial position) .

Resonant Frequency

is the tendency of the contacts to

resonate at certain frequencies determined by their size

and makeup .

Retentivity

is the capacity for retaining magnetism after the

magnetizing force is removed .

Saturation

exists when an increase of magnetization applied

to a magnetic material does not increase the magnetic flux

through that material .

Sensitivity

refers to the pull-in of a Reed Switch usually

expressed in ampere-turns .

Shield, Electrostatic

is the grounded conducting member

located between two or more mutually insulated elements

to minimize electrostatic coupling .

Slew Rate

is the rate of change in output voltage with a large

amplitude step function applied to the input .

Small Signal Bandwidth

is the frequency range from DC

to a frequency where the signal strength is down 3 dB from

its original signal strength .

Thermal Offset

usually measured in microvolts is the volt-

age existing across closed contacts in the absence of any

signals .

Thermal Shock Non-operating

is the temperature shock

induced into a group of Relays, Switches or Sensors to

determine their robustness .

Turn Off or Dropout Time

refers to the time from initial

de-energization to the first opening of a closed contact time.

Turn On

or (contact operate time or Pull-in time) is the total

elapsed time from the instant power is applied to the ener-

gizing coil until the contacts have operated and all contact

bounce has ended .

Varistor

see

MOV

Vibration, Non-operating

is the vibration level and fre-

quency span to which the relay may be subjected without

permanent electrical or mechanical damage .

Voltage, Nominal

is the typical voltage intended to be ap-

plied to the coil or input .

Voltage, Peak AC

is the maximum positive or negative volt-

age swing of an alternating current signal or power supply .

Voltage, Peak to Peak AC

is the maximum positive threw

negative voltage swing of an alternating current signal or

power supply . Vp-p =2Vp when no DC offset is present .

Voltage, RMS

is the Root Mean Square of the positive and

negative voltage swing of an alternating current signal or

power supply .

Winding

refers to the electrically continuous length of insu-

lated wire wound on a bobbin, spool or form .

Winding, Bifilar

represents two windings with the wire of

each winding alongside the other, matching turn for turn .

Wipe, Contact

refers to the sliding or tangential motion

between two mating contact surfaces as they open or close .

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Notes

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Notes

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Contact Information:

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World Headquarter

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Fairfield, OH 45014 USA

Standex Americas (OH)

(+1 .866 .782 .6339)

+1 .866 .STANDEX

info@standexelectronics .com

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+86 .21 .37606000

salesasia@standexelectronics .com

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+49 .7733 .9253 .200

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