selecting_the_appropriate_circuit_protection

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Introduction



Selecting the appropriate circuit protection
component is critical to a safe and robust design



Sometimes, selecting the incorrect component
can lead to catastrophic failures



This presentation will show examples of proper
and improper device selection and the
consequences

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Glowing Reviews of the (Wrong) GDT



Background:

– Selecting the appropriate GDT for power line applications



Surge protection on AC or DC power lines is typically done by using MOVs



GDTs are typically used in signal applications or one N

– PE leg due to minimal

available currents



Problem

– Upon seeing overvoltage surge, GDTs will break-over (crow-bar) by

creating a sustained arc across the electrodes; surge current then shunted
to ground, usually.

– When surge event subsides, the GDT arc will be extinguished and system

will return to normal

– If power is applied to the line, the “follow current” will sustain the arc and

the GDT may not be able to turn off.

– GDT will then thermally fail due to sustained currents (glow red hot)



Solution:

– Littelfuse has AC power optimized GDTs (AC120/240 Series)

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Glowing Reviews of the (Wrong) GDT



Test Set-up:

– 120V tube, AC coupled, 6KV/3KA, limited to 10A

follow thru current

– Littelfuse AC120 GDT (designed for power lines)
– Littelfuse SL series GDT (designed for signal apps)



Images:

– 1. Littelfuse AC120 (GOOD) –

see next slide

– 2. Littelfuse SL series (BAD) –

see next slide

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Images Before, During and After



Good

– Before

– During

– After



Bad

– Before

– During

(longer glow,
more heat)

– After

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Glowing Reviews of the (Wrong) GDT



Additional Information:



MOVs can be placed in series with GDTs



MOV will help cut off the follow current and allow GDT to turn
off



During surge event, MOV will clamp and conduct first into a
low impedance state ; then GDT will break-over and create the
arc.



When surge subsides, the MOV will go back to high
impedance state and will quench the follow current and allow
GDT arc to be extinguished

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MOV End of Life Failures are Really HOT!



Background:

– MOV (Metal Oxide Varistors) can degrade over lifetime due to surge events
– MOV material can weaken due to multiple surges and develop “memory” path
– MOV at end-of-life will start to leak current with nominal system voltage applied



Problem:

– Leakage will heat up the MOV and impedance will continue to drop leading to

thermal run-away failure

– MOV protection solutions needing to meet UL1449 3

Ed which includes

Abnormal Overvoltage testing which simulates this fault condition



Solution:

– Select Littelfuse TMOV series products to control MOV end-of-life (EOL)

conditions.

– TMOV™ MOVs have integrated thermal protector built inside the disc which

will open upon thermal heating of MOV.

– Use of TMOV will prevent catastrophic failure of MOV disc during EOL

condition

– TMOVs will help equipment makers pass UL1449 Abnormal Overvoltage

Limited Current test requirements without the need for external fuse

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MOV End of Life Failures are Really HOT!



Test Set-up:

– 150V MOV with 240V/10A fault, AC coupled – simulating EOL

condition

– Side-by-side testing 150V TMOV (thermally protected MOV)



Images:

– Competitor MOV (Left) ; Littelfuse MOV (Middle) ; Littelfuse

TMOV (Right)

See next slide for before,
during & after pictures

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Images Before, During and After

– Before

– During

– After

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Don’t Let Your Diode Die an Untimely Death



Background:

– TVS diodes can be used for AC or DC input power protection
– Caution to stay under the surge rating of the TVS diode
– While TVS diodes offer fast and efficient clamping capability,

they have limited surge robustness

– IEC61000-4-5 and C.62.41-2002 are popular surge immunity

standards

– Maximum indoor surge condition typically is 6kV/3kA, 8/20us

surge combo wave



Problem:

– TVS diodes can undergo catastrophic failure if over stressed

beyond surge ratings

– Traces need to be sized according or will open up as well!



Solution:

– Select the correct TVS diode surge rating for your application

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Don’t Let Your Diode Die an Untimely Death



Test Set-up:

– SMCJ TVS diode, 1500W diode, bidirectional –

6kV/3kAa surge applied



Images:

See next slide for before,
during & after pictures

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Images Before, During and After

– Before

– During

– After

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

Additional Information:

– Diodes should be selected for a given application by their:



Power Rating,



Maximum surge current,



Standoff Voltage, and



Breakdown Voltage

– Though sometimes not as robust as a MOV, a TVS Diode will have the

lowest dynamic resistance (the resistance between the I/O and ground);
therefore, a TVS Diode will clamp

better and reduce the overall amount

of energy seen by the sensitive electronics

downstream.

– The area between the curves represents

the amount of energy that DOES NOT

get to the chip when an MLV was

replaced by an equivalent TVS Diode

Don’t Let Your Diode Die an Untimely Death

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Ethernet Vs. Power Cross



Background:

– Ethernet ports needing to meet GR-1089 Inter-Building Power

Cross requirements need appropriate overcurrent protection

– Typically, protection is a surge tolerant fuse that will open fast

enough during Power cross testing



Problem:

– Prevent SEP SIDACtor (overvoltage protector) from getting

damaged during power cross testing

– Proper fusing required to comply with GR-1089 Power cross and

prevent equipment damage/safety hazard



Solution:

– Use Littelfuse 461 Series Telelink fuse (typically 1.25A rating) at

port input on cable side

– Use low capacitance, C or D Rated, SIDACtor overvoltage

protector (Littelfuse SEP series)

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Ethernet Vs. Power Cross



Test Set-up:

– Littelfuse Ethernet demo board - Power cross 425V/40A GR-1089

fault

– with and without fuse protection

– SEP Series Ethernet surge protector on cable side
– Fuse – Littelfuse 461 series, 1.25A Telelink fuse



Images:

– With fuse:
– Without fuse:

See next slide for before,
during & after pictures

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Images Before, During and After



With Fuse

– Before

– During

– After



Without Fuse

– Before

– During

– After

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“Fuse Interrupted” (the exploding DVD version)



Background:

– Fuse max voltage and max interrupt rating are safety

critical specifications

– When fuse opens during fault, the higher voltage applied

will cause arc to form longer duration

– Higher voltage and higher current faults will cause plasma

formation and molten metal

– Fuse body, fillers, and fuse element designed to quench

arc and safely open fuse



Problem:

– Deviating from fuse max specs and over-stressing the

device will cause catastrophic failures



Solution:

– Stay under the fuse voltage and interrupt ratings

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“Fuse Interrupted” (the exploding DVD version)



Test Set-up:

– Littelfuse 215 series, 5x20mm ceramic fuse ; 3.15A rating ;

250VAC/1500A Interrupt rating

– We applied 250VAC/1500A short circuit fault
– We applied 400VDC, 200A short circuit fault (above fuse voltage

rating)



Images:



Within fuse voltage rating:



Above fuse voltage rating:

See next slide for before,
during & after pictures

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Images Before, During and After



Within Fuse Voltage
Rating

– Before

– During

– After



Above Fuse Voltage
Rating

– Before

– During

– After

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The Non-Resettable Resettable Fuse



Background:

– Just like fuses, PTC Resettable fuses can experience

overvoltage stress and fail

– PTC’s most dangerous failure mode is overvoltage stress
– The higher voltage causes damage to the polymer material and

will damage the conductive carbon particles



Problem:

– Choosing wrong voltage rating can lead to catastrophic failure

mode



Solution:

– Stay under the max voltage rating of your PTC

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See next slide for before,
during & after pictures

The Non-Resettable Resettable Fuse



Test Set-up:

– Littelfuse 16R series PTC Resettable fuse being used

in 60VDC short circuit fault

– 16R series has max voltage rating of 16VDC
– Littelfuse 60R or 72R series is recommended for this

application.



Images:

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Images Before, During and After

– Before

– During

– After

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Fusible Resistors are Irresistible (buyer beware!)



Background:

– Fusible resistors are poor alternatives to using a properly specified

fuse.

– These fusible resistors are frequently used in LED bulb or charger

applications due to their low cost.

– FusR will tend to get very hot during overload and burn open

causing potential safety hazard.

– Smoke will be generated from burning fusible resistor which is a

customer satisfaction issue.



Problem:

– Unlike a fuse which is designed to open safely during overload

condition, a fusible resistor (FusR) will not have a controlled and
consistent opening mode.



Solution:

– Select a Littelfuse fuse designed to meet the specified

requirements.

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Fusible Resistors are Irresistible (buyer beware!)



Test Set-up:

– Fusible resistor vs. fuse during overload condition
– 392 series TE fuse vs. 10ohm FusR
– 240vac, 200% Overload over the fuse rating



Images:

– 392 series fuse – GOOD
– 10 Ohm Fusible resistor – BAD

See next slide for before,
during & after pictures

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Images Before, During and After



Fuse

– Before

– During

– After



Fusible Resistor

– Before

– During

– After

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SMOV

– The Superhero of MOVs



Background:

– UL1449 3

Ed, Abnormal Overvoltage Intermediate current

testing requires up to 150A fault current when testing MOVs

– Intermediate current testing required for Type 3 SPDs and

above.



Problem:

– Passing the UL1449 Intermediate current test standards typically

requires an external fuse

– Fuse will open before MOVs fail but difficult to select due to

6kv/3ka high surge withstand requirements

– Integrated thermal protection inside Littelfuse TMOV is limited to

max 10A fault current



Solution:

– Select Littelfuse SMOV Series instead of TMOV to pass UL1449

Intermediate current requirements

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See next slide for before,
during & after pictures



Test Set-up:

– 150V TMOV and SMOV tested at 240VAC/150A Intermediate

current per UL1449



Images:

– TMOV failing at 150A – BAD
– SMOV opening safely at 150A – GOOD

SMOV

– The Superhero of MOVs

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Images Before, During and After

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TMOV

– Before

– During

– After



SMOV

– Before

– During

– After

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Background selection information:

Maximum operating current

– the maximum current that the fuse will experience during normal

operation of the application

Ambient temperature

– the temperature in the area surrounding the fuse

Normal operating voltage

– the voltage level of the line that the fuse is protecting; this is also the

voltage that the fuse will have to safely support after it has opened

Current pulses

– these are short duration pulses for which the fuse should not open

• In-rush and start-up currents are examples
• The shape, magnitude and quantity of the pulses is needed to ensure no nuisance tripping
of the fuse

Maximum fault current

– this determines the Interrupt Rating (Breaking Capacity) that the fuse

must meet

Mounting requirements of fuse (surface mount, through hole) is considered secondary selection
criteria (to meet mechanical needs)

Fuse Selection Process

Basics

– definitions for selecting fuses

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Step 1) Collect information to calculate minimum fuse rating

• Maximum operating current

• Normal operating voltage

• Ambient temperature

Minimum fuse rating =

Maximum operating current

fuse re-rating factor x thermal de-rating factor

Use the following equation to calculate the minimum fuse rating:

Fuse Selection Process

Process for calculating minimum fuse current rating (Amps)

(This is explained in the Littelfuse Catalog starting on page 9)

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Fuse re-rating factor:

• Use 0.75 if the fuse is UL or CSA Listed or Recognized
• Use 1.00 if the fuse is IEC Designed

Thermal de-rating factor (TDR):

Determine the thermal de-rating factor by using the appropriate curve for the ambient
temperature that the fuse will experience (found on page 9 of Fuse Catalog)

Curve A = Thin Film Fuses
Curve B = Wire-in-air Fuses

•

(Cartridge, Nano

)

Curve C = Resettable PTCs

For example:

Thin Film Fuse

Use Curve A

If temp is 40ºC

Use TDR of

95%

Chip fuses

Wire-in-air fuses

Fuse Selection Process

Process for calculating minimum fuse rating (amperage)

Resettable PTCs

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maximum operating current

fuse re-rating factor x thermal de-rating factor

For this example, it is given that a

surface mount thin film fuse

is desired, and that the

maximum operating current is 0.50A and ambient temperature is 40

Maximum operating current:

0.50A

Fuse re-rating factor:

0.75

Thermal de-rating factor:

0.95

Then, minimum fuse rating =

0.700 A

Since this value is the minimum requirement, find the closest fuse rating that is higher.

So, the minimum fuse rating that can be used is

0.750 A.

0.50 A

0.75 x 0.95

Step 2) Calculate the minimum fuse rating

Minimum fuse rating =

Fuse Selection Process

Process for calculating minimum fuse rating (amperage)

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Step 3) Calculate minimum nominal melting I

t rating of fuse

Determine

Pulse I

of the application (in-rush current, inductive load switching, etc.)

Calculate nominal melting I

t of the fuse

Pulse I

use the waveshape chart to determine appropriate formula

For example:

• Assume that current measurements show

Type A waveshape

• Peak current was measured to be

1.5A

• Duration of pulses are

1 millisecond

Then,

Pulse I

= Ip

x t = (1.5)

x 0.001s =

0.00225 A

Fuse Selection Process

Process for calculating minimum melting i

t of fuse

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Step 4) Calculate minimum nominal melting I

t rating of fuse

Determine

Pulse I

of the application

Determine Rating Factor for the application

For example:

•

Pulse

Pulse I

was calculated to be

0.00225 A

•

Assume that the fuse needs to survive

100,000 pulses

Use Chart II to determine the Rating Factor

•

For 100,000 pulses,

Rating Factor is 22%

Then,

Minimum

nominal melting

rating = Pulse I

t / rating factor

= 0.00225 A

s / 0.22

0.0102 A

Fuse Selection Process

Process for calculating minimum melting i

t of fuse

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Step 5) Compare the calculated nominal melting I

t to actual fuses:

Surface mount thin film fuses were specified earlier

So, compare Nominal melting

value of 0.750A-rated thin film fuses to target value (0.0102 A

s):

0433.750

1206, very fast acting

0.0170 A

63VDC

0434.750

0603, very fast acting

0.0171 A

32VDC

0435.750

0402, very fast acting

0.0120 A

24VDC

0435.750 .75 24 0.105 0.0120

Since the nominal melting

value for all of these fuses is

greater than

the required value of the

application (0.0102 A

s), they are all valid for usage. The specific part can be chosen according the

amount of board space available, the rated voltage, etc.

The information can be found on
the product data sheet. The
chart to the right is for the
SlimLine 0402, 0435 series fuse.

Fuse Selection Process

Process for calculating minimum melting i

t of fuse

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Understand the application
and circuit parameters

Compare calculated and actual nominal
melting

values to ensure fuse will not suffer

nuisance opening. If there are multiple fuses
qualified for the application, use secondary
characteristics (size, voltage rating, etc.) to
determine best solution

Determine minimum current
rating of fuse (fuse re-rating,
thermal de-rating)

0.750 A

Determine minimum

Nominal melting

value of the Fuse

0.0102 A

Determine

Pulse I

value

of the application

0.00225 A

IMPORTANT!! Even though care may
be used during the fuse selection
process, it is recommended that
application-level testing be performed
to verify coordination of fuses to the
circuit conditions

Fuse Selection Process

Summary of steps to select fuse

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Details of in-rush current

•  System voltage = 12VDC
•  Peak current = 35A
•  t = 40



• Number of pulses required = 70,000

Calculations:
I

t = (1/2)Ip

t = (1/2) x (35A)

x (.00004)

Pulse I

t = 0.0245 A

Nominal melting I

t = (0.0245 / 0.23) =

0.1065 A

The 0467003.NR fuse had been selected

•

0.2403 A

is the listed value

• This value is greater than the
calculated value, so the fuse should
withstand 70,000 pulses
•

Testing at Littelfuse confirmed that

the fuse could indeed survive 70,000
of these pulses

Fuse Selection Example

Verification of calculated melting i

Screen shot is actual in-rush current from HDD hot-plug

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Pulse Energy vs. Fuse Melting Energy

Calculated Pulse I

t = 0.0245 A

(previous page)

0467003 Fuse I2t = 0.2403 A

Ratio of Calculated Pulse I

t / Fuse Melting I

= 0.0245 A

s / 0.2403 A

s = ~10.2%

>100,000
pulses at 10.2%
melting I²t

Fuse Selection Example (continued)

Using Ratio of Calculated Pulse I

t to Melting I

t of selected fuse to

determine Pulse Cycle Withstand Capability

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Metal Oxide Varistor (MOV)

•

Shunts high pulse-current and high-energy

transients to ground; thereby protecting the
application

•

Industry standard form factors

•

Thermally-protected version is available (TMOV)

•

Key feature is the durability to repeatedly

handle high peak pulse current, high-energy
surge transients

Surge Protection Component

Overview of MOV product

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-8

-6

-2

Current (A)

Leakage

Region

Normal Varistor

Operation

Upturn

Region

oltage

(

SLOPE =

I = kV

100

200

500

1000

(TYPICAL V130LA2OA)

-4

Surge protection component

Functional regions of MOV (based on V-I curve)

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Example of MOV selection

Circuit conditions and requirements:

-120VAC circuit
- Current waveform for surge is 8x20



s; voltage is 1.2x50µs

- Peak current during the surge is 3,000A
- Requirement is to survive 40 surges
- Other components (transformer, capacitors, etc.) are rated to
withstand 1,000V maximum.

Approach to finding a solution:

- To find the voltage rating of the MOV, allow for 20% head room
to take into account voltage swells.

• 120VAC x 1.2 = 144VAC

• So look at 150VAC rated MOVs

• Determine which MOV disc size to use – identify those
that minimally meet the 3,000A surge requirement

-Use

Pulse Rating Curves

to determine pulse capabilities of

each series per the 40 pulses @ 3,000A requirement
- Use

V-I Curve

of selected MOV to verify that the peak voltage

will be below the 1,000V ceiling.

MOV Selection Process

Example of selecting a MOV for lightning protection

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MOV Selection Process

Determine which disc size is needed

(see page 112 of MOV Catalog)

Data sheet review

– Peak Current rating

•  From the problem statement, need > 3,000A capability for 150VAC disc
•  Per the table, the 14mm disc can pass at least one 3,000A surge pulse
•  Since the LA series is the least robust, we’ll start the evaluation there

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LA Series

Pulse Rating Curves for 14mm LA series

•  Locate pulse width (20µs) on the x-axis
•  Find where vertical line intercepts 3,000A point
•  In this case, we find that the LA MOV can survive 1 to 2 pulses

MOV Selection Process

Determine if 14mm LA Series is suitable

(see page 117, Fig 11 of the MOV Catalog)

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UltraMOV Series

Pulse Rating Curves for 14mm UltraMOV series

•  Locate pulse width (20µs) on the x-axis
•  Find where vertical line intercepts 3,000A point
•  In this case, we find that the UltraMOV can survive 2 to 10 pulses

MOV Selection Process

Determine if 14mm UltraMOV Series is suitable

(see page 88, Fig 9 of the MOV Catalog)

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Pulse Rating Curves for 14mm C-III series

•  Locate pulse width (20µs) on the x-axis
•  Find where vertical line intercepts 3,000A point
•  In this case, we find that the C-III can survive 10 to 100 pulses

C-III Series

MOV Selection Process

Determine if 14mm C-III Series is suitable

(see page 105, Fig 6 of the MOV Catalog)

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MOV Selection Process

So, how many pulses can 14mm C-III varistor take?

(see page 103 of the MOV Catalog)

Pulse Rating Curves for 14mm C-III series

•  Consult the data sheet for verification of surge pulse capabilities
•  From the table, the 14mm disc can survive 40 pulses
•  So, the V150LA10C(P) is the best part for the requirements

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MOV Selection Process

Determine the peak voltage that the 3,000A surge will create

(see page 105 of the MOV Catalog)

V-I Curves for 14mm C-III series

•  Consult the data sheet for verification of surge pulse capabilities
•  From the table, locate the peak current on the x-axis (3,000A)
•  Find where it intercepts the curve for V150LA10C(P) product
•  In this case, the maximum voltage is found to be 850V

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Example of MOV selection

Circuit conditions and requirements:

-120VAC circuit
- Current waveform for surge is 8x20



s; voltage is 1.2x50µs

- Peak current during the surge is 3,000A
- Requirement is to survive 40 surges
- Other components (transformer, capacitors, etc.) are rated to
withstand 1,000V maximum.

Approach to finding a solution:

- To find the voltage rating of the MOV, allow for 20% head room
to take into account voltage swells.

• 120VAC x 1.2 = 144VAC
• So look at 150VAC rated MOVs

• Determine which MOV disc size to use – identify those
that minimally meet the 3,000A surge requirement

-Use

Pulse Rating Curves

to determine pulse capabilities of

each series per the 40 pulses @ 3,000A requirement
- Use

V-I Curve

of selected MOV to verify that the peak voltage

will be below the 1,000V ceiling.

MOV Selection Process

Compare V150LA10C(P) to requirements

Compare requirements to
V150LA10C(P)

Voltage rating of 150VAC

Disc size of 14mm

Can meet 40 surge pulses

Peak voltage of 850V

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iDesign

Online Fuse Design and Selection Tool

Registration page

Free to register now !!!

https://littelfuse.transim.com/login.aspx

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Additional Literature

Design and Selection Guides



Electronic Products Selection Guide



Available on the Littelfuse website



Includes all Littelfuse technologies



Quick reference for all product specifications and applications



System Level Design Guide



Available on the Littelfuse website



Discusses multiple applications such as:



USB1.1/2.0/3.0



HDMI/DVI



10/100/1000 Ethernet



eSATA



Audio (Speaker/Microphone)



Keypad/Push button



And many more…



Ethernet Design Guide



Includes both TVS Diode Arrays, SIDACtor Devices, and TVS Diodes (for PoE)

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Additional Literature

Sample Kits



TVS Diode Arrays



Contains over 55 products and includes all 2012 new product releases



TVS Diodes



Axial Lead 400-1500W



SA5.0A, SA12CA, SAC5.0, P6KE27CA, P6KE200A,

1.5KE91A, 1.5KE440A, LEC28A



Surface Mount 400-1500W



SMAJ5.0A, SMAJ58A, P4SMA20CA, P4SMA200CA,

SMBJ15A, SMBJ33CA, P6SMB36A, P6SMB200CA,

1KSMB47CA, 1KSMB160A, SMCJ24CA, SMCJ64A,

1.5SMC6.8A, 1.5SMC550CA

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Additional Literature

Miscellaneous



TVS Diode Array App



Only for the iPhone/iPad



Help in finding the right product for your application



Product Catalogs



Found on Littelfuse.com



Catalogs are available under the

respective product category

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Who is Littelfuse?



Founded 1927 in Chicago, Ill., USA



Traded on the U.S. NASDAQ; Symbol: LFUS



6,300 employees



35 facilities worldwide:

– Americas
– Europe
– Asia

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Littelfuse Products

Global Presence

– Local Resources

Electronics
600,000 sq. ft.

Automotive
and Electrical
375,000 sq. ft.

Electrical
70,000 sq. ft.

Automotive
70,000 sq. ft.



Founded in 1927



World Headquarters in Chicago, IL



More than 5,000 employees



Publicly held company since 1992

– NASDAQ

LFUS



“

world class

”

manufacturing sites

*Yahoo Finance

PROTECT

CONTROL

SENSE

The #1 Brand in Circuit Protection

— Emerging

Player in Power Control and Sensing

Electrical
(16%)



Power Fuse



Relay/Custom

Littelfuse has the broadest and deepest portfolio of circuit protection products serving three major
market segments.

Automotive
(35%)



Auto Fuse



Commercial Vehicle



Sensors

Electronics
(49%)



Passives



Semis



Sensors

PROTECT

CONTROL

SENSE

Littelfuse Protects Against Common Threats
to Electrical Circuits and Components

Overcurrent

Protection

Overvoltage

Protection

Power Monitoring and

Protective Switching

Power Cross

Overloads &

Short Circuits

ESD Protection

Lightning

Protection

Equipment

Protection

Ground-Fault Protection

Power Distribution

and Control

Mining Control

Consoles

Power Distribution Centers

Every product that uses electrical energy needs circuit protection
to ensure safety, reliability and performance.