Littelfuse_Fuseology_Selection

Selection Guide

Fuse Characteristics, Terms

and Consideration Factors

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FUSEOLOGY

Selection Guide

Fuse Characteristics,

Terms and Consideration

Factors

Table of Contents

Page

Fuse Characteristics, Terms and Consideration Factors

2–4

Fuse Selection Checklist

5–7

PTC Characteristics and Terms

8–9

PTC Product Applications

Typical PTC Circuit Protection Designs

Standards

12–14

PTC Selection Worksheet

Fuse and PTC Products Selection Guide

16–18

Packaging and Part Numbering

Legal Disclaimers

About this guide

Fuses are current-sensitive devices that provide reliable protection for discrete components
or circuits by melting under current overload conditions. Choosing the right fuse for your
application can be an overwhelming, time-consuming process, even for a seasoned
electronics design engineer. This user-friendly Fuseology Selection Guide makes the fuse
selection process quick and easy-helping you optimize the reliability and performance of
the application.

Specifications descriptions and illustrative material in this literature are as accurate as known at the time of publication,

but are subject to changes without notice. Visit

littelfuse.com

for more information.

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Fuseology Selection Guide

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www.littelfuse.com

The purpose of this introductory section is to promote
a better understanding of both fuses and common
application details within circuit design.

The fuses to be considered are current sensitive devices
designed to serve as the intentional weak link in the
electrical circuit. Their function is to provide protection of
discrete components, or of complete circuits, by reliably
melting under current overload conditions. This section
will cover some important facts about fuses, selection
considerations and standards.

The application guidelines and product data in this guide
are intended to provide technical information that will
help with application design.

The fuse parameters and

application concepts presented should be well understood

in order to properly select a fuse for a given application.

Since these are only a few of the contributing parameters,
application testing is strongly recommended and should be
used to verify performance in the circuit / application.

Littelfuse reserves the right to make changes in product
design, processes, manufacturing location and information
without notice. For current Littelfuse product infomation,
please visit our web site at

www.littelfuse.com

AMBIENT TEMPERATURE:

Refers to the temperature

of the air immediately surrounding the fuse and is not to
be confused with “room temperature.” The fuse ambient
temperature is appreciably higher in many cases, because
it is enclosed (as in a panel mount fuseholder) or mounted
near other heat producing components, such as resistors,
transformers, etc.

BREAKING CAPACITY:

Also known as interrupting rating

or short circuit rating, this is the maximum approved
current which the fuse can safely break at rated voltage.
Please refer to the INTERRUPTING RATING definition of
this section for additional information.

CURRENT RATING:

The nominal amperage value of the

fuse. It is established by the manufacturer as a value of
current which the fuse can carry, based on a controlled set
of test conditions (See RERATING section).

Catalog Fuse part numbers include series identification
and amperage ratings. Refer to the FUSE SELECTION
CHECKLIST section for guidance on making the proper
choice.

RERATING:

For 25ºC ambient temperatures, it is

recommended that fuses be operated at no more than
75% of the nominal current rating established using the
controlled test conditions. These test conditions are part of
UL/CSA/ANCE (Mexico) 248-14 “Fuses for Supplementary
Overcurrent Protection,” whose primary objective is
to specify common test standards necessary for the
continued control of manufactured items intended for
protection against fire, etc. Some common variations of

these standards include: fully enclosed fuseholders, high
contact resistances, air movement, transient spikes, and
changes in connecting cablesize (diameter and length).
Fuses are essentially temperature-sensitive devices. Even
small variations from the controlled test conditions can
greatly affect the predicted life of a fuse when it is loaded
to its nominal value, usually expressed as 100% of rating.

The circuit design engineer should clearly understand
that the purpose of these controlled test conditions is to
enable fuse manufacturers to maintain unified performance
standards for their products, and he must account for
the variable conditions of his application. To compensate
for these variables, the circuit design engineer who is
designing for trouble-free, long-life fuse protection in his
equipment generally loads his fuse not more than 75%
of the nominal rating listed by the manufacturer,keeping
in mind that overload and short circuit protection must be
adequately provided for.

The fuses under discussion are temperature-sensitive
devices whose ratings have been established in a 25ºC
ambient. The fuse temperature generated by the current
passing through the fuse increases or decreases with
ambient temperature change.

The ambient temperature chart in the FUSE SELECTION
CHECKLIST section illustrates the effect that ambient
temperature has on the nominal current rating of a fuse.
Most traditional Slo-Blo

Fuse designs use lower melting

temperature materials and are, therefore, more sensitive to
ambient temperature changes.

DIMENSIONS:

Unless otherwise specified, dimensions

are in inches.

The fuses in this catalog range in size from the approx.
0402 chip size (.041”L×.020”W×.012”H) up to the 5
AG, also commonly known as a”MIDGET” fuse (13/32”
Dia.×11/2” Length). As new products were developed
throughout the years, fuse sizes evolved to fill the various
electrical circuit protection needs.

The first fuses were simple, open-wire devices, followed
in the 1890’s by Edison’s enclosure of thin wire in a lamp
base to make the first plug fuse. By 1904, Underwriters
Laboratories had established size and rating specifications
to meet safety standards. The renewable type fuses and
automotive fuses appeared in 1914, and in 1927 Littelfuse
started making very low amperage fuses for the budding
electronics industry.

The fuse sizes in following chart began with the early
“Automobile Glass” fuses, thus the term “AG”. The
numbers were applied chronologically as different
manufacturers started making a new size: “3AG,” for
example, was the third size placed on the market. Other
non-glass fuse sizes and constructions were determined by
functional requirements, but they still retained the length
or diameter dimensions of the glass fuses. Their

Fuse Characteristics, terms and Consideration Factors

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designation was modified to AB in place of AG, indicating
that the outer tube was constructed from Bakelite, fibre,
ceramic, or a similar material other than glass. The largest
size fuse shown in the chart is the 5AG, or “MIDGET,”
a name adopted from its use by the electrical industry
and the National Electrical Code range which normally
recognizes fuses of 9/16”× 2” as the smallest standard
fuse in use.

FUSE SIZES

SIZE

DIAMETER (Inches)

LENGTH (Inches)

1AG

1/4

.250

5/8

.625

2AG

–

.177

–

.588

3AG

1/4

.250

1¼

1.25

4AG

9/32

.281

1¼

1.25

5AG

13/32

.406

1½

1.50

7AG

1/4

.250

7⁄8

.875

8AG

1/4

.250

TOLERANCES:

The dimensions shown in this catalog

are nominal. Unless otherwise specified, tolerances are
applied as follows. Tolerances do not apply to lead lengths:

± .010” for dimensions to 2 decimal places.

± .005” for dimensions to 3 decimal places.

Contact Littelfuse should you have questions regarding
metric system and fractional tolerances.

FUSE CHARACTERISTICS:

This characteristic of a fuse

design refers to how rapidly it responds to various current
overloads. Fuse characteristics can be classified into three
general categories: very fast-acting, fast-acting, or Slo-Blo

Fuse. The distinguishing feature of Slo-Blo

fuses is that

these fuses have additional thermal inertia designed to
tolerate normal initial or start-up overload pulses.

FUSE CONSTRUCTION:

Internal construction may vary

depending on ampere rating. Fuse photos in this catalog
show typical construction of a particular ampere rating
within the fuse series.

FUSEHOLDERS:

In many applications, fuses are

installed in fuseholders. These fuses and their associated
fuseholders are not intended for operation as a “switch”
for turning power “on” and “off “.

INTERRUPTING RATING:

Also known as breaking

capacity or short circuit rating, the interrupting rating is
the maximum approved current which the fuse can safely
interrupt at rated voltage. During a fault or short circuit
condition, a fuse may receive an instantaneous overload
current many times greater than its normal operating
current. Safe operation requires that the fuse remain intact
(no explosion or body rupture) and clear the circuit.

Interrupting ratings may vary with fuse design and range
from 35 amperes for some 250VAC metric size (5×20mm)

fuses up to 200,000 amperes for the 600VAC KLK series.
Information on other fuse series can be obtained from the
Littelfuse.

Fuses listed in accordance with UL/CSA/ANCE 248 are
required to have an interrupting rating of 10,000 amperes
at 125V, with some exceptions (See STANDARDS section)
which, in many applications, provides a safety factor far in
excess of the short circuit currents available.

NUISANCE OPENING:

Nuisance opening is most often

caused by an incomplete analysis of the circuit under
consideration.

Of all the “Selection Factors” listed in the FUSE
SELECTION CHECKLIST, special attention must be given
to items 1, 3, and 6, namely, normal operating current,
ambient temperature, and pulses.

For example, one prevalent cause of nuisance opening in
conventional power supplies is the failure to adequately
consider the fuse’s nominal melting I

t rating. The fuse

cannot be selected solely on the basis of normal operating
current and ambient temperature. In this application, the
fuse’s nominal melting I

t rating must also meet the inrush

current requirements created by the input capacitor of the
power supply’s smoothing filter.

The procedure for converting various waveforms into
I

t circuit demand is given in the FUSE SELECTION

CHECKLIST. For trouble-free, long-life fuse protection, it is
good design practice to select a fuse such that the I

t of

the waveform is no more than 20% of the nominal melting
I

t rating of the fuse. Refer to the section on PULSES in the

FUSE SELECTION CHECKLIST.

RESISTANCE:

The resistance of a fuse is usually an

insignificant part of the total circuit resistance. Since the
resistance of fractional amperage fuses can be several
ohms, this fact should be considered when using them
in low-voltage circuits. Actual values can be obtained by
contacting Littelfuse.

Most fuses are manufactured from materials which have
positive temperature coefficients, and, therefore, it is
common to refer to cold resistance and hot resistance
(voltage drop at rated current), with actual operation being
somewhere in between.

Cold resistance is the resistance obtained using a
measuring current of no more than 10% of the fuse’s
nominal rated current. Values shown in this publication for
cold resistance are nominal and representative. The factory
should be consulted if this parameter is critical to the
design analysis.
Hot resistance is the resistance calculated from the

Fuse Characteristics, terms and Consideration Factors (continued)

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stabilized voltage drop across the fuse, with current equal
to the nominal rated current flowing through it. Resistance
data on all Littelfuse products are available on request.
Fuses can be supplied to specified controlled resistance
tolerances at additional cost.

SOLDERING RECOMMENDATIONS:

Since most fuse

constructions incorporate soldered connections, caution
should be used when installing those fuses intended to
be soldered in place. The application of excessive heat can
reflow the solder within the fuse and change its rating.
Fuses are heat-sensitive components similar to semi-
conductors, and the use of heat sinks during soldering is
often recommended.

Lead-Free Soldering Parameters (most instances):

Wave Solder — 260ºC, 10 seconds max
Reflow Solder — 260ºC, 30 seconds max

TEST SAMPLING PLAN:

Because compliance with certain

specifications requires destructive testing, these tests are
selected on a statistical basis for each lot manufactured.

TIME-CURRENT CURVE:

The graphical presentation of

the fusing characteristic, time-current curves are generally
average curves which are presented as a design aid but
are not generally considered part of the fuse specification.
Time-current curves are extremely useful in defining
a fuse, since fuses with the same current rating can
be represented by considerably different time-current
curves. The fuse specification typically will include a life
requirement at 100% of rating and maximum opening
times at overload points (usually 135% and 200% of rating
depending on fuse standard characteristics). A time-current
curve represents average data for the design; how ever,
there may be some differences in the values for any one
given production lot. Samples should be tested to verify
performance, once the fuse has been selected.

UNDERWRITERS LABORATORIES:

Reference to “Listed

by Underwriters Laboratories” signifies that the fuses
meet the requirements of UL/CSA/ANCE 248-14 “Fuses
for Supplementary Overcurrent Protection”. Some 32
volt fuses (automotive) in this catalog are listed under
UL Standard 275. Reference to “Recognized under the
Component Program of Underwriters Laboratories”
signifies that the item is recognized under the component
program of Underwriters Laboratories and application
approval is required.

VOLTAGE RATING:

The voltage rating, as marked on a

fuse, indicates that the fuse can be relied upon to safely
interrupt its rated short circuit current in a circuit where the
voltage is equal to, or less than, its rated voltage.

This system of voltage rating is covered by N.E.C.
regulations and is a requirement of Underwriters
Laboratories as a protection against fire risk. The standard
voltage ratings used by fuse manufacturers for most small-
dimension and midget fuses are 32, 63, 125, 250 and 600.

In electronic equipment with relatively low output power
supplies, with circuit impedance limiting short circuit
currents to values of less than ten times the current rating
of the fuse, it is common practice to specify fuses with
125 or 250 volt ratings for secondary circuit protection of
500 volts or higher.

As mentioned previously (See RERATING section), fuses
are sensitive to changes in current, not voltage, maintaining
their “status quo” at any voltage up to the maximum rating
of the fuse. It is not until the fuse element melts and
arcing occurs that the circuit voltage and available power
become an issue. The safe interruption of the circuit, as it
relates to circuit voltage and available power, is discussed
in the section on INTERRUPTING RATING.

To summarize, a fuse may be used at any voltage that is
less than its voltage rating without detriment to its fusing
characteristics. Please contact the factory for applications
at voltages greater than the voltage rating.

DERIVATION OF NOMINAL MELTING I

Laboratory

tests are conducted on each fuse design to determine the
amount of energy required to melt the fusing element. This
energy is described as nominal melting I

t and is expressed

as “Ampere Squared Seconds” (A

sec.).

A pulse of current is applied to the fuse, and a time
measurement is taken for melting to occur. If melting
does not occur within a short duration of about 8
milliseconds (0.008 seconds) or less, the level of pulse
current is increased. This test procedure is repeated until
melting of the fuse element is confined to within about 8
milliseconds.

The purpose of this procedure is to assure that the heat
created has insufficient time to thermally conduct away
from the fuse element. That is, all of the heat energy (I

is used, to cause melting. Once the measurements of
current (I) and time (t) are determined, it is a simple matter
to calculate melting I

t. When the melting phase reaches

completion, an electrical arc occurs immediately prior to
the “opening” of the fuse element.

Clearing I

t = Melting I

t + arcing I

The nominal I

t values given in this publication pertain to

the melting phase portion of the “clearing” or “opening”.
Alternatively the time can be measured at 10 times of the
rated current and the I

t value is calculated like above.

Fuse Characteristics, terms and Consideration Factors (continued)

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Fuse selection Checklist

The application guidelines and product data in this guide
are intended to provide technical information that will help
with application design. Since these are only a few of the
contributing parameters, application testing is strongly
recommended and should be used to verify performance in
the circuit/application.

Many of the factors involved with fuse selection are listed
below. For additional assistance with choosing fuses
appropriate to you requirements, contact your Littelfuse
products reprentative.

Selection Factors

1.  Normal operating current
2.  Application voltage (AC or DC)
3.  Ambient temperature
4.  Overload current and length of time in which the

fuse must open

5. Maximum available fault current
6. Pulses, Surge Currents, Inrush Currents, Start-up

Currents, and Circuit Transients

7. Physical size limitations, such as length, diameter,

or height

8. Agency Approvals required, such as UL, CSA, VDE,

METI, MITI or Military

9. Fuse features (mounting type/form factor, ease of

removal, axial leads, visual indication, etc.)

10. Fuseholder features, if applicable and associated

rerating (clips, mounting block, panel mount, PC
board mount, R.F.I. shielded, etc.)

11. Application testing and verification prior to

production

1. NORMAL OPERATING CURRENT:

The current rating

of a fuse is typically derated 25% for operation at 25ºC to
avoid nuisance blowing. For example, a fuse with a current
rating of 10A is not usually recommended for operation at
more than 7.5A in a 25ºC ambient. For additional details,
see RERATING in the previous section and AMBIENT
TEMPERATURE below.

2. APPLICATION VOLTAGE:

The voltage rating of the

fuse must be equal to, or greater than, the available circuit
voltage. For exceptions, see VOLTAGE RATING.

3. AMBIENT TEMPERATURE:

The current carrying

capacity tests of fuses are performed at 25ºC and will be
affected by changes in ambient temperature. The higher
the ambient temperature, the hotter the fuse will operate,
and the shorter its life. Conversely, operating at a lower
temperature will prolong fuse life. A fuse also runs hotter
as the normal operating current approaches or exceeds the
rating of the selected fuse. Practical experience indicates
fuses at

room temperature

should last indefinitely, if

operated at no more than 75% of catalog fuse rating.

Ambient temperature effects are in addition to the

normal re-rating, see example. Example: Given a normal

operating current of 1.5 amperes in an application using
a traditional Slo-Blo

fuse at room temperature, then:

Normal Operating Current

Catalog Fuse Rating = ——————————————————

0.75

- or -

1.5 Amperes

——————— = 2.0 Amp Fuse (at 25ºC)

0.75

Similarly, if that same fuse were operated at a very high

ambient temperature of 70°C, additional derating would

be necessary. Curve "A" (Traditional Slo-Blo

Fuse) of

that ambient temperature chart shows the maximum

operating "Percent of Rating" at 70°C to be 80%, in

which case;

Normal Operating Current

Catalog Fuse Rating = ——————————————————

0.75 x Percent of Rating

- or -

1.5 Amperes

——————— = 2.5 Amp Fuse (at 70ºC)

0.75 x 0.80

This charts shows typical ambient temperature effects on
current carrying capacity of Littelfuse products. For specific
re-rating information, please consult the product data sheet
at

www.littelfuse.com

or contact a Littelfuse representative.

Curve A: Thin-Film Fuses and 313 Series (.010 to .150A)
Curve B: FLAT-PAK

, TeleLink

, Nano

2®

, PICO

, Blade

Terminal and other leaded and catridge fuses

Curve C: Resettable PTC’s

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4. OVERLOAD CURRENT CONDITION:

The current level

for which protection is required. Fault conditions may be
specified, either in terms of current or, in terms of both
current and maximum time the fault can be tolerated
before damage occurs. Time-current curves should be
consulted to try to match the fuse characteristic to the
circuit needs, while keeping in mind that the curves are
based on average data.

5. MAXIMUM FAULT CURRENT:

The Interrupting Rating

of a fuse must meet or exceed the Maximum Fault Current
of the circuit.

6. PULSES:

The general term “pulses” is used in this

context to describe the broad category of wave shapes
referred to as “surge currents”, “start-up currents”, “inrush
currents”, and “transients”. Electrical pulse conditions can
vary considerably from one application to another. Different
fuse constructions may not react the same to a given
pulse condition. Electrical pulses produce thermal cycling
and possible mechanical fatigue that could affect the life
of the fuse. Initial or start-up pulses are normal for some
applications and require the characteristic of a Slo-Blo

fuse. Slo-Blo

fuses incorporate a thermal delay design

to enable them to survive normal start-up pulses and still
provide protection against prolonged overloads. The start-
up pulse should be defined and then compared to the time-
current curve and I

t rating for the fuse. Application testing

is recommended to establish the ability of the fuse design
to withstand the pulse conditions.

Nominal melting I

t is a measure of the energy required

to melt the fusing element and is expressed as “Ampere
Squared Seconds” (A

Sec.). This nominal melting I

and the energy it represents (within a time duration of
8 milliseconds [0.008 second] or less and 1 millisecond
[0.001 second]or less for thin film fuses), is a value that is
constant for each different fusing element. Because every
fuse type and rating, as well as its corresponding part
number, has a different fusing element, it is necessary to
determine the I

t for each. This I

t value is a parameter of

the fuse itself and is controlled by the element material
and the configuration of the fuse element. In addition
to selecting fuses on the basis of “Normal Operating
Currents”, “Rerating”, and “Ambient Temperature” as
discussed earlier, it is also necessary to apply the I

design approach. This nominal melting I

t is not only a

constant value for each fuse element design, but it is also
independent of temperature and voltage. Most often, the
nominal melting I

t method of fuse selection is applied to

those applications in which the fuse must sustain large
current pulses of a short duration. These high-energy
currents are common in many applications and are critical
to the design analysis.

The following example should assist in providing a better
understanding of the application of I

EXAMPLE:

Select a 125V, very fast-acting PICO

II fuse that

is capable of withstanding 100,000 pulses of current (I) of
the pulse waveform shown in Figure 1.

The normal operating current is 0.75 ampere at an ambient
temperature of 25ºC.

Step 1

— Refer to Chart 1 and select the appropriate

pulsewaveform, which is waveform (E) in this example.
Place the applicable value for peak pulse current (i

) and

time (t) into the corresponding formula for waveshape (E),
and calculate the result, as shown:

t = — (i

)

5
1

= —×8

×.004 = 0.0512 A

Sec.

This value is referred to as the “Pulse I

t”.

Step 2

— Determine the required value of Nominal Melting

t by referring to Chart 2. A figure of 22% is shown in

Chart II for 100,000 occurrences of the Pulse I

t calculated

in Step 1. This Pulse I

t is converted to its required value of

Nominal Melting I

t as follows:

Nom. Melt I

t = Pulse I

t/.22

0.0512/.22 = 0.2327 A

Sec.

Step 3

— Examine the I

t rating data for the PICO

II, 125V,

very fast-acting fuse. The part number 251001, 1 ampere
design is rated at 0.256 A

Sec., which is the minimum

fuse rating that will accommodate the 0.2327 A

Sec.

value calculated in Step 2. This 1 ampere fuse will also
accommodate the specified 0.75 ampere normal operating
current, when a 25% derating factor is applied to the 1
ampere rating, as previously described.

7. PHYSICAL SIZE LIMITATIONS:

Please refer to the

product dimensions presented in current Littelfuse product
data sheets for specific information.

8. AGENCY APPROVALS:

For background information

about common standards, please consult the STANDARDS
section of this guide or visit our Design Support web site
at

www.littelfuse.com/design-support.html

. For specific

agency approval information for each Littelfuse product,
please refer to the data sheets within this catalog and
information presented on

www.littelfuse.com

. As agency

approvals and standards may change, please rely on the
information presented on

www.littelfuse.com

as current

information.

9. FUSE FEATURES:

Please consult the specific product

features presented within this catalog and on our web
site at

www.littelfuse.com

. For additional information and

support contact your Littelfuse product representative.

Fuse Selection Checklist (continued)

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.001

.002

.003

.004 .005

.006

Time (Seconds)

Cur

rent (Amperes)

Figure 1

Normal Operating Current

l t

Pulse

Energy

CHART 1

WAVESHAPES

FORMULAS

i = k
I

t = i

i = i

-kt

t = (1/3)(i

+ i

) t

i = i

sin t

t = (1/2) i

t = (1/3) i

i = kt

OR i = i

(1-kt)

t = (1/5) i

i = i

–kt

)

t (1/2) i

CHART 2

PULSE CYCLE WITHSTAND CAPABILITY

100,000 Pulses

Pulse I

t = 22% of Nominal Melting I

10,000 Pulses

Pulse I

t = 29% of Nominal Melting I

1,000 Pulses

Pulse I

t = 38% of Nominal Melting I

100 Pulses

Pulse I

t = 48% of Nominal Melting I

100000

10000

1000

100

10%

100%

Number of P

ulses

Pulse I

t / Average Melting I

Note: Adequate time (10 seconds) must exist between pulse events

to allow heat from the previous event to dissipate.

10. FUSEHOLDER FEATURES AND RERATING:

For

information about the range of Littelfuse fuseholders and

specific features and characteristics, please consult with

a Littelfuse products representative or visit our web site

(

www.littelfuse.com

For 25ºC ambient temperatures, it is recommended that

fuseholders be operated at no more than 60% of the

nominal current rating established using the controlled test

conditions specified by Underwriters Laboratories. The

primary objective of these UL test conditions is to specify

common test standards necessary for the continued

control of manufactured items intended for protection

against fire, etc. A copper dummy fuse is inserted in

the fuseholder by Underwriters Laboratories, and then

the current is increased until a certain temperature rise

occurs. The majority of the heat is produced by the contact

resistance of the fuseholder clips. This value of current

is considered to be the rated current of the fuseholder,

expressed as 100% of rating. Some of the more common,

everyday applications may differ from these UL test

conditions as follows: fully enclosed fuseholders, high

contact resistance,air movement, transient spikes, and

changes in connecting cable size (diameter and length).

Even small variations from the controlled test conditions

can greatly affect the ratings of the fuse-holder. For

this reason, it is recommended that fuseholders be

derated by 40% (operated at no more than 60% of the

nominal current rating established using the Underwriter

Laboratories test conditions, as previously stated).

11. TESTING:

The factors presented here should be

considered in selecting a fuse for a given application. The

next step is to verify the selection by requesting samples

for testing in the actual circuit. Before evaluating the

samples, make sure the fuse is properly mounted with

good electrical connections, using adequately sized wires

or traces. The testing should include life tests under normal

conditions and overload tests Under fault conditions, to

ensure that the fuse will operate properly in the circuit.

FIGURE 1

Fuse selection Checklist (continued)

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Overcurrent circuit protection can be accomplished
with the use of either a traditional fuse or PTC (positive
temperature coefficient) device.

PTCs are typically used in a wide variety of telecom,
computer, consumer electronics, battery and medical
electronics product applications where overcurrent events
are common and automatic resettability desired.

Littelfuse offers PTCs with the following general forms and
features, and come in a variety of sizes and capacities:

Surface Mount Devices:

• A full range of compact footprints

• Low hold current

• Very fast trip time

• Low resistance

Radial Leaded Series:

• Protection devices up to 600Vdc

• A very high hold current

• Low trip-to-hold current ratio

• Low resistance.

Battery Strap Devices:

• A narrow low profile design

• A weldable band Nickel terminal

• Low resistance–for extended battery run time

If your application requirements fall outside of our product
range, in certain instances we can offer customized
solutions. Please contact Littelfuse for more information.

Traditional Fuses Vs. PTCs

Fuses and PTCs are both overcurrent protection
devices, though each offer their own unique operating
characteristics and benefits. Understanding the differences
between the two technologies should make the choice in
selection easier, depending on the application.

The most obvious difference is that PTCs are automatically
resettable whereas traditional Fuses need to be replaced
after they they are tripped. Whereas a fuse will completely
stop the flow of current (which may be desired in critical
applications) after most similar overcurrent event, PTCs
continue to enable the equiment to function, except in
extreme cases.

Because they reset automatically, many circuit designers
choose PTCs in instances where overcurrent events are
expected to occur often, and where maintaining low
warranty and service costs, constant system uptime, and/
or user transparency are at a premium. They are also often
chosen in circuits that are difficult to access or in remote
locations, were fuse replacement would be difficult.

There are several other operating characteristics to be
considered that distinguish PTCs and fuses, and it is also
best to test and verify device performance before use
within the end application.

Littelfuse PTC Characteristics

Both Polymeric (Positive Temperature Coefficient) PTC
and traditional Fuse devices react to heat generated by
the excessive current flow in a circuit. A fuse melts open,
interrupting the current flow whereas a PTC limits current
flow as it rises in temperature, changing from low to high
resistance state. In both
cases this condition is
called "tripping." The
graph at right shows the
typical response of a
PTC to temperature.

Littelfuse Polymer PTCs
are made chiefly of high
density polyethylene
mixed with graphite.
During an overcurrent
event, a Polymer PTC
will heat and expand, which in turn causes the conducting
particles to break contact and stop the current.

The general procedure for resetting the device after an
overload has occurred is to remove power and allow the
device to cool down.

Log resistance (ohms)

Temperature (C)

Trip Point

PtC Characteristics and terms

To view a short online tutorial on using the Littelfuse
iDesign tool, visit: http://vimeo.com/90118164

Littelfuse iDesign

Online Fuse Design

and Selection Tool

Littelfuse_Fuseology_Selection_Guide-html.html

www.littelfuse.com

Fuseology Selection Guide

Leakage Current:

When a PTC is in a "tripped state" it

protects the circuitry by limiting the current flow to a low
leakage level. Leakage current can range from less than
a hundred milliamps (mA) at rated voltage up to a few
hundred milliamps (mA) at lower voltages. Fuses on the
other hand completely interrupt the current flow when
tripped, and this open circuit results in no leakage current
when subjected to an overload current.

Interrupting Rating:

PTCs are rated for a maximum

short circuit current at rated voltage also known as
"breaking capacity" or Imax. This fault current level is the
maximum current that the device can withstand safely,
keeping in mind that the PTC will not actually interrupt
the current flow (see Leakage Current above). A typical
Littelfuse PTC short circuit rating is 40A; or for the battery
strap PTCs, this value can reach 100A. Fuses do in fact
interrupt the current flow in response to the overload
and the range of interrupting ratings, vary from tens of
amperes (A) up to 10,000 (A) amperes at rated
voltage.

Operating Voltage Rating:

General use

Littelfuse PTCs are not rated above 60V while
fuses are rated up to 600V.

Hold Current Rating:

The hold (operating)

current rating for PTCs can be up to 14A, while
the maximum level for fuses can exceed 30A.

Resistance

Reviewing product

specifications indicates that similarly rated
PTCs have about twice (sometimes more) the
resistance of fuses.

Agency Approvals:

Littelfuse PTCs

are Recognized under the Component Program of
Underwriters Laboratories to UL Standard 1434 for
Thermistors. The devices have also been certified under
the CSA Component Acceptance Program.

Time-Current Characteristic

Comparing the time-

current curves of PTCs to time-current curves of fuses
show that the speed of response for a PTC is similar to the
time delay of a Littelfuse Slo-Blo® fuse.

Temperature Rerating:

The useful upper limit for a

PTC is generally 85ºC, while the maximum operating
temperature for fuses is 125ºC.

Ambient temperature effects are in addition to the normal
rerating. PTCs hold and trip rating must be rerated when
applied at conditions other than room ambient. For
example, any rise in ambient temperature will decrease the
hold current rating as well as the trip current. A reduction in
ambient temperature will increase the trip current as well
as the hold current.

The temperature rerating curves in the table below
compare PTCs to fuses and illustrate that more rerating is
required for a PTC at a given temperature.

Temperature Rerating Curves Comparing PTCs to Fuses

Chart Key

Curve A

Thin-Film Fuses and 313

Series (.010 to .150A)

Curve B

FLAT-PAK

, Nano

2®

PICO

, Blade Terminal,

Special Purpose and

other leaded and

cartridge fuses

(except 313.010 – .150A)

Curve C

Resettable PTCs

-60°C

-76°F

100

120

140

-40°C

-40°F

-20°C

-4°F

25°C

A
B

AMBIENT TEMPERATURE

PERCENT OF RA

TING

0°C

32°F

20°C

68°F

40°C

104°F

60°C

140°F

80°C

176°F

100°C

212°F

120°C

248°F

PtC Characteristics and terms (continued)

Littelfuse_Fuseology_Selection_Guide-html.html

Fuseology Selection Guide

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Surface Mount

raDIaL LeaDeD

Battery Strap

Series name

LoRho 0603L 0805L 1206L 1210L 1812L 2016L 2920L 250s usbR

16R

30R

60R

72R

250R 600R

Telecom

application

ul60950 ,tiA-968-A, gR-1089 Req’s

itu-t Recoomendations

CPe (Customer Premises equipment)

Analog Line Card

t1/e1/J1 And hdsL

isdN

AdsL

Cable telephony

PbX/Kts And Key telephone system

Comput

CPu

usb

ieee1284 Parallel data bus

ieee 802.3

ieee 1394

i/o Ports

PC Card

sCsi

Video Port

LCd Monitor

Consumer

Electr

onics

set top box

Loudspeaker

smart Card Reader

Mobile Phone

Linear AC/dC Adapter

Portable electronic input Port

electromagnetic Loads, Motor

solenoid Protection

Bat

tery

Lithium Cell

battery Pack

Medical

electr

onic

Voltage / Current input terminal

Note: the application summary is for reference only. determination of suitability for a specific application is the responsibility of the customer.

PTCs are typically used as circuit protection in applications
where sensitive components are at constant risk of
damage from overcurrent conditions. The ability of PTCs to
reset themselves after exposure to a fault current makes
them ideal within circuits that are not easily accessible to a
user or technician or where constant uptime is required.

Typical applications include port protection on personal
computers (USB, Firewire, keyboard/mouse, and serial
ports), peripherals (hard drives, video cards, and hubs), cell
phone, battery packs, industrial controls, lighting ballast
and motor controls.

The chart below is meant as a quick guide in narrowing to
a Littelfuse PTC device that may be appropriate to certain
end applications.

For detailed application assistance please contact a
Littelfuse product specialist or visit our new reference
design center at

www.littelfuse.com/designcenter

For detailed product specifications, please consult the
Littelfuse PTC datasheets within this catalog or visit

www.littelfuse.com/PTCs.

PtC Product Applications

Littelfuse Electronics eCatalogs includes the latest standards, products, diagrams and videos in a new interactive
format. View the eBook on your mobile device, tablet or desktop for easy access to Littelfuse products, technologies
and technical resources. Quickly order free product samples, request more information and download datasheets all
within the new user-friendly design.

Littelfuse_Fuseology_Selection_Guide-html.html

www.littelfuse.com

Fuseology Selection Guide

Outside

D+
D -

VBU

Signal

Ground

Shield

Ground

(2) V0402MHS03

1206L150

USB Port

USB Controller

V5.5MLA0603

PTC

Cell Phone

Battery -
Control

Battery +

Signal

Ground

Shield

Ground

Battery Connector

Battery Controller IC

(3) V5.5MLA0402
(Multilayer
Varistor)

PTC

Outside

D+
D -

VBUS

Signal

Ground

Shield

Ground

1206L150

USB Port

USB Controller

V5.5MLA063

(2) PGB1010603
(PulseGuard

ESD Suppressor)

PTC

Telco

Tip

Signal

Ground

RJ11 Connector

Tx/Rx Circuits

SMD/leaded
SIDACtor

Device

Ring

PTC

600R Series
250R Series

Outside W

orld

TPA+
TPA-

VBU

Signal

Ground

Shield

Ground

1812L110/33

1394 Port

1394 Controller

V33MLA1206

(4) PGB1010603
(PulseGuard

ESD Suppressor)

TPB+
TPB-

PTC

Data Pair

Power Source

Equipment

(PSE)

Powered

Device

(PD)

Switch/Hub

Powered End Station

PTC

POWER OVER ETHERNET

LI-ION BATTERY PACK

The following are examples of typical circuits using
Littelfuse PTCs in combination with other Littelfuse circuit
protection devices to provide a comprehensive protection
solution. Contact a Littelfuse application expert for design

assistance or visit

www.littelfuse.com/designcenter

www.littelfuse.com/PTCs

for additional information.

Be sure to to veryify specifications and test device
performance before use in the end application.

USB 1.1

USB 2.0

IEEE 1394 - FIREWIRE

TIP/RING CIRCUIT - METALLIC

typical PtC Circuit Protection designs

Littelfuse_Fuseology_Selection_Guide-html.html

Fuseology Selection Guide

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Littelfuse is at your service to help solve your electrical

protection problems. When contacting Littelfuse sales

engineers, please have all the requirements of your

applications available. Requests for quotes or assistance

in designing or selecting special types of circuit protection

components for your particular applications are also

welcome. In the absence of special requirements,

Littelfuse reserves the right to make appropriate changes

in design, process, and manufacturing location without

prior notice.

Fuse ratings and other performance criteria are evaluated
under laboratory conditions and accptance criteria, as
defined in one or more of the various fuse standards. It is
important to understand these standards so that the fuse
can be properly applied to circuit protection applications.

UL/CSA/ANCE (Mexico) 248-14 FUSES FOR

SUPPLEMENTARY OVERCURRENT PROTECTION (600 Volts,

Maximum) (Previously UL 198G and CSA C22.2, No. 59)

UL LISTED

A UL Listed fuse meets all the requirements of the UL/
CSA/ANCE 248-14 Standard. Following are some of the
requirements. UL ampere rating tests are conducted at
100%, 135%, and 200% of rated current. The fuse must
carry 100% of its ampere rating and must stabilize at a
temperature that does not exceed a 75ºC rise.

The fuse must open at 135% of rated current within one
hour. It also must open at 200% of rated current within 2
minutes for 0-30 ampere ratings and 4 minutes for 35-60
ampere ratings.

The interrupting rating of a UL Listed fuse is 10,000
amperes AC minimum at 125 volts. Fuses rated at 250
volts may be listed as interrupting 10,000 amperes at 125
volts and, at least, the minimum values shown below at
250 volts.

Ampere Rating of

Fuse

Interrupting Rating

In Amperes

Voltage Rating

0 to 1

250 VAC

1.1 to 3.5

100

250 VAC

3.6 to 10

200

250 VAC

0.1 to 15

750

250 VAC

15.1 to 30

1500

250 VAC

Recognized Under the Component Program of

Underwriters Laboratories

The Recognized Components Program of UL is different
from UL Listing. UL will test a fuse to a specification
requested by the manufacturer. The test points can be
different from the UL Listed requirements if the fuse
has been designed for a specific application. Application
approval is required by UL for fuses recognized under the
Component Program.

standards

UL 275 AUTOMOTIVE GLASS TUBE FUSES (32 Volts)

UL LISTED

UL ampere ratings tests are conducted at 110%, 135%,
and 200%. Interrupting rating tests are not required.

CSA Certification

CSA Certification in Canada is equivalent to UL Listing in
the United States.

The Component Acceptance Program of CSA is

equivalent to the Recognition Program at UL.

METI (Japan Ministry of Economy, Trade and Industry)

METI APPROVAL

METI approval in Japan is similar to UL Recognition in the
United States.

METI

has its own design standard and characteristics.

INTERNATIONAL ELECTROTECHNICAL
COMMISSION (IEC)

Publication 60127, Parts 1, 2, 3, 4, 6

The IEC organization is different from UL and CSA, since
IEC only writes specifications and does not certify. UL
and CSA write the specifications, and are responsible for
testing and certification.

Certification to IEC specifications are given by such
organizations as SEMKO (Swedish Institute of Testing
and Approvals of Electrical Equipment) , BSI (British
Standards Institute) and VDE (German Standard Insitute)

, as well as UL and CSA.

IEC Publication 60127 defines three breaking capacity
levels (interrupting rating). Low breaking capacity fuses
must pass a test of 35 amperes or ten times rated current,
whichever is greater, while enhanced breaking capacity
fuses must pass a test of 150 amperes and high breaking
capacity fuses must pass a test of 1500 amperes.

60127 Part 2

Sheet 1

— Type F Quick Acting, High Breaking Capacity

Sheet 2

— Type F Quick Acting, Low Breaking Capacity

Sheet 3

— Type T Time Lag, Low Breaking Capacity

Sheet 4

— Style Fuses 1/4×1 1/4

Sheet 5

— Type T Time Lag, High Breaking Capacity

Sheet 6

— Type T Time Lag, Enhanced Breaking Capacity

The letters ‘F’ and ‘T’ represent the time-current

characteristic

of the fast-acting and time delay fuses. One

of these letters will be marked on the end cap of the fuse.

Littelfuse_Fuseology_Selection_Guide-html.html

www.littelfuse.com

Fuseology Selection Guide

UL/CSA/ANCE (Mexico) 248-14 vs. IEC 60127 Part 2 FUSE
OPENING TIMES vs. METI/MITI

Percent of

Rating

uL & CsA

std 248-14

ieC tYPe F
sheet 1 (*)

ieC tYPe F
sheet 2 (*)

ieC tYPe t

sheet 3 (*)

ieC tYPe t

sheet 5 (*)

Meti/Miti

100

4Hr.Min.

—

130

—

1Hr.Min.

135

60 Minutes

Max.

—

150

—

60 Minutes

Min.

60 Minutes

Min.

60 Minutes

Min.

60 Minutes

Min.

160

—

1Hr.Max.

200

2 Minutes

Max.

—

2 Minutes

Max.

210

—

30 Minutes

Max.

30 Minutes

Max.

2 Minutes

Max.

30 Minutes

Max.

(*) Note: The IEC Specification is written up to

10.0A. Any components above these ratings are
not recognized by the IEC (although the fuses
may have similar opening characteristics).

IEC also has opening time requirements at 275%, 400%
and 1000%; however, the chart is used to show that
fuses with the same ampere rating made to different
specifications are not interchangeable. According to the
IEC 60127 Standard, a one ampere-rated fuse can be
operated at one ampere. A one ampere-rated fuse made to
UL/CSA/ANCE 248-14 should not be operated at more than
.75 ampere (25% derated — See RERATING section).

METI

does not differentiate between fast acting and time

delay characteristics.

Publication IEC 60127-4 (Universal Modular Fuse-Links
[UMF])

This part of IEC 60127-4 covers both PCB through-hole
and surface mount fuses. This standard covers fuses rated
32, 63, 125, and 250 volts. This standard will be accepted
by UL/CSA making it the first global fuse standard. This
specification uses different fusing gates than IEC 60127-2;
the gates used here are 125%, 200%, and 1000%.

The fuses must not open in less than one hour at 125%
of rated current and open within two minutes at 200% of
rated current. The 1000% overload is used to determine
the fuse characteristic. The opening time for each rating is
listed below.

Type FF  : Less than 0.001 sec.
Type F  : From 0.001 - 0.01 sec.
Type T  : From 0.01 - 0.1 sec.
Type TT  : From 0.1 - 1.00 sec.

These characteristics correlate to the terminology used in
IEC 60127-1.

Breaking capacity (interrupting rating) varies based on

voltage rating. Parts rated at 32 & 63 volts must pass a
test of 35 amperes or ten times rated current, whichever
is greater. Parts rated at 125 volts must pass a test of 50
amperes or ten times rated current, whichever is greater.
Parts rated at 250 volts are further defined as either low,
intermediate or high breaking. The low breaking capacity
fuses must pass a test of 100 amperes rated current, while
intermediate breaking capacity fuses must pass a test of
500 amperes and high breaking capacity fuses must pass a
test of 1500 amperes.

MILITARY/FEDERAL STANDARDS

MIL-PRF-15160 and MIL-PRF-23419

These specifications govern the construction and
performance of fuses suitable primarily for military
electronic applications.

MIL-PRF-19207

This specification governs the construction and
performance of fuseholders suitable for military
applications.

DSSC Drawing #87108

This drawing governs the construction and performance
of .177” × .570” (2AG size) cartridge fuses and axial lead
versions suitable for military applications. DSSC #87108
designation is included in the fuse end cap marking.

standards (continued)

A robust web-based tool to help circuit designers

identify the optimal electronic fuses for their products.

To use the Littelfuse iDesign tool, simply register to

create a free online account at the iDesign Login.

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Fuseology Selection Guide

2014 Littelfuse • Fuseology Selection Guide

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MILITARY/FEDERAL STANDARDS

FEDERAL SPECIFICATION W-F-1814

This specification governs the construction and
performance of fuses with high interrupting ratings that are
approved for federal applications. Fuses approved to these
specifications are on the Federal Qualified Products List.

Write to the following agencies for additional information

on standards, approvals, or copies of the specifications.

Underwriters Laboratories Inc. (UL)

333 Pfingsten Road

Northbrook, Illinois, USA 60062-2096

Canadian Standards Association (CSA)

5060 Spectrum Way, Suite 100

Mississauga, Ontario, Canada L4W 5N6

International Electrotechnical Commission (IEC)

3, Rue de Varembe

1211 Geneva 20, Switzerland

Naval Publications and Military StandardsForm

Center (for Military and Federal Standards)

5801 Tabor Avenue

Philadelphia, Pennsylvania, USA 19120

Defense Supply Center Columbus (DSCC)

3990 East Broad Street

Columbus, Ohio, USA 43218-3990

Ministry of Economy Trade and Industry (METI)

1-3-1 Kasumigaseki

Chiyouda-ku, Tokyo 100-8901, Japan

standards (continued)

Littelfuse_Fuseology_Selection_Guide-html.html

www.littelfuse.com

Fuseology Selection Guide

1. Define the circuit operating parameters.

Normal operating current in amperes:

....................................................................................................

Normal operating voltage in volts:

....................................................................................................

Maximum interrupt current:

....................................................................................................

Ambient

temperature: ....................................................................................................

Typical overload current:

....................................................................................................

Required opening time at specified overload:

....................................................................................................

Transient pulses expected:

....................................................................................................

Agency

approvals:

....................................................................................................

Mounting type/form factor:

....................................................................................................

Typical resistance (in circuit):

....................................................................................................

2. Select the proper circuit protection component.

(

Refer to Table on Page 3 and specifications with Data Sheets

)

3. Determine the opening time at fault.

Consult the Time-Current (T-C) Curve of each PTC series to determine if the
selected part will operate within the constraints of your application.

If the device opens too soon, the application may experience nuisance
operation. If the device does not open soon enough, the overcurrent may
damage downstream components.

To determine the opening time for the chosen device, locate the overload
current on the X-axis of the appropriate T-C Curve and follow its line up to its
intersection with the curve. At this point read the time tested on the Y-axis.
This is the average opening time for that device.

If your overload current falls to the right of the curve the device will open.
If the overload current is to the left of the curve, the device will not operate.

4. Verify ambient operating parameters.

Ensure that the application voltage is less than or equal to the device’s rated
voltage and that the operating temperature limits are within those specified
by the device.

5. Verify the device’s dimensions.

Compare the maximum dimensions of the device to the space available in
the application. The dimension of each product is included within each data
sheets on the following pages.

6. Test the selected product in an actual application.

PtC selection Worksheet

A robust web-based tool to help circuit designers

identify the optimal electronic fuses for their products.

Littelfuse iDesign

Online Fuse Design and Selection Tool

Littelfuse_Fuseology_Selection_Guide-html.html

Fuseology Selection Guide

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuse and PtC Products selection guide

Fuse PRoduCts

Fuses provide protection by completely stopping the flow of energy to sensitive circuits. If current exceeds the fuse’s operating range, the metal
wire or strip melts safely within an enclosure. Littelfuse offers the most extensive range of fuses available, and for easy replacement of cartridge
fuses Littelfuse offers a wide selection of fuseholders including panel mount, in-line, and surface and thru-hole circuit board mount devices.

Series

Name

Vie

w Dat

asheet

Order Samples

Size

Time Lag

Fast A

cting

Ver

Fast A

cting

Device Range

(Operating

Current Options

in Amps)

Max. Voltage

Rating

(Volts)

Interrupting

Rating at

Max. Voltage

Rating

(Amps)

Operating

Temperature

Range

Agency

Approvals

Halogen F

ree

oHS Compliant

Lead F

ree

PSE

UMF

Surface Mount:

Ceramic Chip

437

•

1206

•

0.25 - 8

125 / 63 / 32

-55°C to +150°C

•

• • •

438

•

0603

•

0.25 - 6

32 / 24

•

• • •

440

•

1206

•

1.75 - 8

• •

• • •

441

•

0603

•

2 - 6

• •

• • •

469

•

1206

•

1 - 8

24 / 32

24 - 63

• •

• • •

501

•

1206

•

10, 12, 15, 20

150

•

• • •

Thin Film

466

•

1206

•

0.125 - 5

125 / 63 / 32

-55°C to +90°C

• •

429

•

1206

•

• •

468

•

1206

•

0.5 - 3

63 / 32

35 - 50

• •

• • •

467

•

0603

•

0.25 - 5

35 - 50

• •

• • •

494

•

0603

•

0.25 - 5

35 - 50

• •

• • •

435

•

0402

•

0.25 - 5

• •

• • •

Nano

2®

Fuse

448

•

2410

•

0.062 - 15

125 / 65

35 - 50

-55°C to +125°C

• • •

• •

449

•

2410

•

0.375 - 5

125

• • •

• •

451 / 453

•

2410

•

0.062 - 15

125 / 65

35 - 50

• • • •

• •

452 / 454

•

2410

•

0.375 - 12

125 / 72

• • •

• •

456

•

4012

•

20, 25, 30, 40

125

100

•

• •

458

•

1206

•

1.0 - 10

75 / 63

•

• •

443

•

4012

•

0.5 - 5

250

•

• •

464

•

4818

•

0.5 - 6.3

250

100

• • • •

465

•

4818

•

1 - 6.3

250

100

• • • •

462

•

4118

•

0.500 - 5

350

100

-40°C to +80°C

•

485

•

4818

•

0.500 - 3.15

600

100

-55°C to +125°C

•

• •

Telelink

Fuse

461

•

4012

0.5 - 2.0

600

-55°C to +125°C

• •

461E

4012

1.25

600

•

• •

OMNI-BLOK

Fuseholder

154

•

0.062 - 10.0

125

35 - 50

-55°C to +125°C

•

• •

154T

•

0.375 - 5

125

•

• •

Fuse and Clip

Assemblies

157

•

0.062 - 10

125

35 - 50

-55°C to +125°C

•

• •

157T

•

0.375 - 5

125

•

• •

159

•

0.5 - 2

600

•

• •

160

•

0.5 - 5

250

•

• •

PICO

SMF

Fuse

459

•

0.062 - 5

125

50 - 300

-55°C to +125°C

• •

460

•

0.5 - 5

125

• •

Flat Pak

202

•

0.062 - 5

250

-55°C to +125°C

• •

203

•

0.25 - 5

250

• •

EBF

446

•

2.0 - 10.0

350

100

-40°C to +125°C

• •

447

•

2.0 - 10.0

350

100

• •

tAbLe CoNtiNues oN NeXt PAge

(1) Detailed information about most product series listed here can be found on our web site.
(2) Size for these surface mount items refers to common industry length and width dimensions of the device surface area. Example: 0402 = .04” x .02”
(3) In some cases for these categories the ratings, agency approvals and specifications vary by part number and are presented here as ranges representing the whole series.
Please refer to product data on www.littelfuse.com and in our data sheets for detailed information by part number.
* Please refer to data sheet for detailed specifications.

Littelfuse_Fuseology_Selection_Guide-html.html

Series

Name

View Datasheet

Order Samples

Time Lag

Medium Acting

Fast Acting

Very Fast Acting

Device

Range

(Operating

Current

Options in

Amps)

Max. Voltage

Rating

(Volts)

Interrupting

Rating at

Max Voltage

Rating

(Amps)

Operating

Temperature

Range

Agency Approvals

RoHS Compliant

Lead Free

Americas

Europe

Asia

CSA

QPL

UMF

VDE

TUV

BSI

Semko

PSE

CCC

CQC

Radial Leaded / Socket:

Micro

™

Fuse /

TR3 Fuse

262/268

• ••

•

0.002 - 5

125

10,000

-55°C to +125°C

• •

•

269

•

0.002 - 5

125

10,000

-55°C to +125°C

• • •

•

272/278

• ••

•

0.002 - 5

125

10,000

-55°C to +125°C

• •

•

273/279

• ••

•

0.002 - 5

125

10,000

-55°C to +85°C

• •

•

274

•

0.002 - 5

125

10,000

-55°C to +85°C

•

303

• •

•

0.5 - 5

125

-55°C to +70°C •

•

• •

TR5

Fuse

370

• •

•

0.4 - 6.3

250

35 - 50

-40°C to +85°C

•

• •

•

• •

372

• •

•

0.4 - 6.3

250

35 - 50

•

• •

• • • • • • •

373

• •

•

0.5 - 10

250

•

• •

374

• •

•

0.5 - 10

250

•

• •

382

• •

•

1 - 10

250

100

•

• •

• • • • • • •

383

• •

•

1 - 10

300

50 - 100

•

• •

•

• •

TE5

369

• •

•

1 - 6.3

300

-40°C to +85°C

•

• •

385

• •

•

0.35 - 1.5

125

•

• •

391

• •

•

0.125 - 4

•

• •

392

• •

•

0.8 - 6.3

250

25 - 63

•

• •

• • •

395

• •

•

0.05 - 6.3

125

100

•

396

• •

•

0.05 - 6.3

125

100

•

• •

397

• •

•

0.35 - 1.5

125

•

• •

398

• •

•

0.125 - 4

•

• •

399

• •

•

0.125 - 4

•

• •

400

• •

•

0.5 - 6.3

250

130

•

• •

•

• •

804

• •

•

0.8 - 6.3

250

150

-40°C to +125°C

• • •

• •

• • •

808

• •

•

2 - 5

250

100

-40°C to +85°C

•

• •

TE7

807

• •

•

0.8 - 6.3

300

100

-40°C to +125°C • • •

•

• •

Axial Leaded / Cartridge:

PICO

Fuse /

PICO

Fuse Axial

251

• •

•

0.062 - 15

125

300DC / 50AC

-55°C to +125°C

• •

•

253

• •

•

0.062 - 15

125

300DC / 50AC

•

275

• •

•

20 - 30

300DC / 50AC

• •

•

263

• •

•

0.062 - 5

250

• •

•

471

• •

•

0.5 - 5

125

• •

•

472

• •

•

0.5 - 5

125

•

473

• •

•

0.375 - 7

125

• •

•

265/266/267

• •••

•

0.062 - 15

125

300DC / 50AC

• • •

•

3.6x10 mm

874

• •

•

0.1 - 10

250

-55°C to +125°C

•

• •

875

• •

•

0.1 - 10

250

•

• •

876

• •

•

0.125 - 5

250

35 - 50

•

• •

877

• •

•

2 - 6.3

250

35 - 63

•

• •

4.5x14.5

(2AG)

208

• •

•

0.125 - 10

350

100

-55°C to +125°C

•

• •

209

• •

•

0.25 - 7

350

100

•

• •

220

• •

Special Fuse

0.3 - 7

250 / 300 / 350

35 - 100

• • •

•

• •

2205

• •

•

0.25 - 2.5

250

• •

•

• •

224/225

• ••

•

0.375 - 10

250 / 125

35 - 500

• • •

•

• •

229/230

• ••

•

0.25 - 7

250 / 125

35 - 400

• • •

•

• •

5x20

217

• •

•

0.032 - 15

250

35 - 150

-55°C to +125°C

• •

• • • • •

• •

218

• •

•

0.032 - 16

250

35 - 100

• •

• • • • •

• •

213

• •

•

0.2 - 6.3

250

35 - 63

• •

• • •

•

• •

219XA

• •

•

0.04 - 6.3

250

150

• •

• • •

•

• •

216

• •

•

0.05 - 16

250

750 - 1500

• •

• • • • •

• •

215

• •

•

0.125 - 20

250

400 / 1500

• •

• • • • •

• •

232

• •

•

1 - 10

250 / 125

300 / 10,000

•

• •

235

• •

•

0.1 - 7

250 / 125

35 - 10,000

•

• •

233

• •

•

1 - 10

125

10,000

•

• •

234

• •

•

1 - 10

250

100 - 200

•

• •

239

• •

•

0.08 - 7

250 / 125

35 - 10,000

•

• •

285

• •

•

0.125 - 20

250

400 - 1500

•

• •

477

• •

•

0.5 - 16

400DC / 500AC

100 - 1500

• •

•

• •

977

• •

•

0.5 - 16

450DC / 500AC

200 / 100

•

• •

6.3x32 mm

(3AG/3AB)

312/318

• ••

•

0.062 - 35

250 / 32

35 - 300

-55°C to +125°C

• • •

•

• •

313/315

• ••

•

0.01 - 30

250 / 125 / 32

35 - 300

• • •

•

• •

314/324

• ••

•

0.375 - 40

250

35 - 1000

• • •

•

• •

322

• •

•

12 - 30

200 - 1000

•

332

• •

•

1- 10

250

100 / 200

• •

•

• •

325/326

• ••

•

0.01 - 30

250

100 - 600

• • •

•

• •

505

• •

•

10 - 30

450 / 500

20,000 - 50,000

• •

•

• •

506

• •

•

15 - 20

600DC

10,000

• •

•

• •

508

• •

1000VAC / DC

High Voltage

Fuse

0.315 - 1

1000

10,000

• •

•

• •

Littelfuse_Fuseology_Selection_Guide-html.html

Fuseology Selection Guide

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Fuse and PtC Products selection guide (continued)

PtC PRoduCts

PTCs (positive temperature coefficient) increase resistance as temperature increases. They are designed to prevent unsafe levels of current
while allowing constant safe current levels, and their resistance will “reset” automatically when the current and temperature returns to a
safe level. PTCs are typically used in applications where automatic reset is desired.

Series Name

Vie

w Dat

asheet

Order Samples

Size

Hold Current (I

HOLD

)

Max Voltage

MAX

)

Max Fault

current (I

MAX

)

Operating

Temperature

Range

Agency Approvals

Halogen Free

RoHS

Lead Free

cUR

TUV

Surface Mount:

LoRho PTC

(Low Resistance)

• •

0402

0.1 - 0.5A

6 / 12 V

50 A

•

0603

0.5 - 2.0A

0805

0.75 - 3.0A

-40°C to 85°C

1206

0.75 - 4.5A

1812

1.9 - 3.7A

2920

7.0A

0603L

• •

0603

(1608)

0.04 - 0.5A

6 - 15 V

40 A

•

0805L

• •

0805

(2012)

0.10 - 1.10 A

6 - 15 V

100 / 40 A

•

1206L

• •

1206

(3216)

0.10 - 2.0A

6 - 30 V

100 A

•

1210L

• •

1210

(3225)

0.05 - 2.0A

6 - 30 V

10 / 100 A

•

1812L

• •

1812

(4532)

0.1 - 3.0A

6 - 60 V

10 / 20 / 100 A

•

2016L

• •

2016

(5041)

0.30 - 2.00 A

6 - 60 V

20 / 40 A

•

2920L

• •

2920

(7351)

0.30 - 3.00 A

6 - 60 V

10 / 40 A

•

250S

• •

see data sheet

0.13 A

60 V

3 A

•

Radial Leaded:

USBR

• •

see data sheet

0.75 - 2.50 A

6 / 16 V

40 A

-40°C to 85°C

•

16R

• •

2.50 - 14.00 A

16 V

100 A

•

30R

• •

0.90 - 9.00 A

30 V

40 A

•

60R

• •

0.10 - 3.75 A

60 V

40 A

•

72R

• •

0.20 - 3.75 A

72 V

40 A

•

250R

• •

0.08 - 0.18 A

250 V

3 / 10 A

•

600R

• •

0.15 - 0.16 A

600 V

3 A

•

Battery Strap:

(Low Resistance)

• •

see data sheet

1.9 - 3.7 A

6 V

50 A

-40°C to 85°C

•

(1) detailed information about most product series listed here can be found on our web site.
(2)

Size for these surface mount items refers to common industry length and width dimensions of the device surface area. Example: 0402 = .04” x .02”

Littelfuse_Fuseology_Selection_Guide-html.html

www.littelfuse.com

Fuseology Selection Guide

Packaging and Part Numbering

1.-3.

5.-7.

10.

Stelle

3xx

XXX

digit

TR3

303

370

372

382

385

391

950

373

374

Variant

Standard, long leads 18,8 mm

long leads 18,8 mm, TR3

392

395

short leads 4,3 mm

396

T²CP

Version

397

Standard

PIP Surface Mount (TR5 blister tape 2x500 pcs.)

398

Rated Current Specification

3-digit

399

062

= 62mA

example 4.-7. digit

100

= 100mA / 1A / 10A

Pico

125

= 125A

0062 = 62mA

275

0100 = 100mA

< 1A

1100 = 1A

≥ 1 - < 10A

2100 = 10A

≥ 10 - < 100A

3125 = 125A

≥ 100A

tape in bulk 100 pcs., only Picofuse 275

bulk 2.500 pcs., only Picofuse 275

Tape, Ammopack
Tape, Ammopack
Tap, Rolle/ Reel

varying production

customized

short leads 3,3 / 3,5 mm (special model)

bulk 1.400 pcs., only TE5

/ T²CP / MP / IP

bulk, 200 pcs., TR3 long leads / TR

Packaging

Tape, Ammopack 1.000 pcs. TR5

bulk, 1.000 pcs. TR5

bulk, 300 pcs., TR3 short leads

Explanation

TR5

TE5

Littelfuse Wickmann Products Part Numbering System

0 X X X X X X X X X X X X X X X

Packaging Type *

Ammo packed

Bubble packed

Chip packed

Reeled

Filler

A/X

100

200

250

500

1000

1500

2000

2500

3000

4000

5000

10000

12000

Misc.

Options Codes *

RT1

Reel and Tape, 2.062in (52.4mm) lead spacing

RT2

Reel and Tape, 2.50 in (63.5mm) lead spacing

RT3

Reel and Tape, 2.874 in (73mm) lead spacing

Pigtail lead type fuse

Indicating fuse

RoHS compliant

Lead-free

Product Series Code

* Not all options and codes listed here are available for all products.

For information about the specific options available for any Littelfuse product,

please refer to the packaging details information within each product data sheet

or contact your Littelfuse products representative.

Ampere Rating Code

Decimal is to far right for whole number

amp ratings, to far left for ratings less than

one, and within center for fractional amp ratings.
Examples:

10A fuse is “010.”

1/4A or 0.25A fuse is “.250”

1 1/2A or 1.5A fuse is “01.5”

1 1/4A or 1.25A fuse is “1.25”
Refer to the Electrical Characteristics tables

presented in each product data sheet

for specific amp rating codes

Packaging Quantity Code *

Tape and Reel packaging per EIA-296:

Tape width is defined as the width of the tape and reeled fuse (x) as measured from inside tape to inside tape.

Pitch is defined as the space between two tape and reeled fuses (y) as measured from lead to lead.

Example:

437 series fuse is “0437”

Littelfuse Fuse Products Traditional Part Numbering System

Littelfuse_Fuseology_Selection_Guide-html.html

Fuseology Selection Guide

2014 Littelfuse • Fuseology Selection Guide

www.littelfuse.com

Liability

Littelfuse, Inc. its affiliates, agents, and employees,
and all persons acting on its or their behalf (collectively,
“Littelfuse”), disclaim any and all liability for any errors,
inaccuracies or incompleteness contained here or in any
other disclosure relating to any product. Littelfuse disclaims
any and all liability arising out of the use or application
of any product described herein or of any information
provided herein to the maximum extent permitted by law.
The product specifications do not expand or otherwise
modify Littelfuse terms and conditions of purchase,
including but not limited to the warranty expressed therein,
which apply to these products.

Right to Make Changes

Littelfuse reserves the right to make any and all changes to
the products described herein without notice.

Not Intended for Use in Life Support or Life Saving
Applications

The products shown herein are not designed for use
in life sustaining or life saving applications unless
otherwise expressly indicated. Customers using or selling
Littelfuse products not expressly indicated for use in such
applications do so entirely at their own risk and agree
to fully indemnify Littelfuse for any damages arising or
resulting from such use or sale. Please contact authorized
Littelfuse personnel to obtain terms and conditions
regarding products designed for such applications.

Intellectual Property

No license, express or implied, by estoppel or otherwise,
to any intellectual property rights is granted by this
document or by any conduct of Littelfuse. Product
names and markings noted herein may be registered
trademarks of their respective owners. Littelfuse makes
no representations or warranties of non-infringement or
misappropriation of any third party intellectual property
rights unless specifically provided for herein.

Legal disclaimers

Disclaimer

Specifications, descriptions and data contained in this
document are believed to be accurate. However, users
should independently evaluate each product for the
particular application. Littelfuse reserves the right to
change any information contained herein without notice
and may, at its sole discretion, change the design,
manufacture or construction of any product. Visit

www.littelfuse.com

for the most up-to-date information.

Littelfuse’s only obligations for any of its products are
specified in its Standard Terms and Conditions and
Littelfuse shall not be liable for any indirect, consequential
or incidental damages from any sale or use of any of its
products.

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