Fuseology

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

application details within circuit design.

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

considerations and standards.

The application guidelines and product data in this guide The application guidelines and product data in this guide are intended to provide technical information that will are intended to provide technical information that will help with application design. The fuse parameters and help with application design. The fuse parameters and application concepts presented should be well understood application concepts presented should be well understood in order to properly select a fuse for a given application. in order to properly select a fuse for a given application. Since these are only a

Since these are only a few of the contributing parameters,few of the contributing parameters, application testing is strongly recommended and should be application testing is strongly recommended and should be used to verify performance in the circuit / application. used to verify performance in the circuit / application. Littelfuse reserves the right to make changes in product Littelfuse reserves the right to make changes in product design, processes, manufacturing location and information design, processes, manufacturing location and information without notice. For current

without notice. For current Littelfuse product infomation,Littelfuse product infomation, please visit our web site a

please visit our web site a t www.litt www.littelfuse.com.telfuse.com.

---AMBIENT

AMBIENT TEMPERATURETEMPERATURE:: Refers to the temperatureRefers to the temperature of the air immediately surrounding the fuse and is not to of the air immediately surrounding the fuse and is not to be confused

be confused with “room temperature.with “room temperature.” T” The fuse he fuse ambientambient temperature is appreciably higher in many cases, because temperature is appreciably higher in many cases, because it is enclosed (as in a panel mount fuseholder) or mounted it is enclosed (as in a panel mount fuseholder) or mounted near other heat producing components, such as resistors, near other heat producing components, such as resistors, transformers, etc.

transformers, etc. BREAKING CAPACITY:

BREAKING CAPACITY: Also known as interrupting ratingAlso known as interrupting rating or short circuit rating, this is the maximum approved or short circuit rating, this is the maximum approved current which the fuse can safely break at

current which the fuse can safely break at rated voltage.rated voltage. Please refer to the interrupting rating definition of this Please refer to the interrupting rating definition of this section for additional information.

section for additional information. CURRENT RATING:

CURRENT RATING: The nominal amperage value of theThe nominal amperage value of the fuse. It is

fuse. It is established by the manufacturer established by the manufacturer as a value ofas a value of current which the fuse can carr

current which the fuse can carr y, based on a controlled sety, based on a controlled set of test conditions (See RERATING).

of test conditions (See RERATING).

Catalog Fuse part numbers include series identification Catalog Fuse part numbers include series identification andand amperage ratings. Refer to the FUSE SELECTION GUIDE amperage ratings. Refer to the FUSE SELECTION GUIDE section for guidance on making the

section for guidance on making the proper choice.proper choice. RERATING:

RERATING: For 25ºC ambient temperatures, it isFor 25ºC ambient temperatures, it is recommended that fuses be operated at no more than recommended that fuses be operated at no more than 75% of the nominal current rating established using the 75% of the nominal current rating established using the controlled test conditions. T

controlled test conditions. These test conditions are part hese test conditions are part ofof UL/CSA/ANCE (Mexico) 248-14 “Fuses for Supplementary UL/CSA/ANCE (Mexico) 248-14 “Fuses for Supplementary Overcurrent Protection,

Overcurrent Protection,” whose ” whose primary primary objective isobjective is

protection against fire, etc. Some common variations of protection against fire, etc. Some common variations of these standards include: fully enclosed fuseholders, high these standards include: fully enclosed fuseholders, high contact resistances, air movement, transient spikes, and contact resistances, air movement, transient spikes, and changes in connecting cablesize (diameter and length). changes in connecting cablesize (diameter and length). Fuses are essentially temperature-sensitive devices. Even Fuses are essentially temperature-sensitive devices. Even small variations from the controlled test conditions can small variations from the controlled test conditions can greatly affect the predicted life of a fuse when it is

greatly affect the predicted life of a fuse when it is loadedloaded to its nominal value, usually expressed as

to its nominal value, usually expressed as 100% of rating.100% of rating. The circuit design engineer should clearly understand The circuit design engineer should clearly understand that the purpose of these controlled test conditions is to that the purpose of these controlled test conditions is to enable fuse manufacturers to maintain unified performance enable fuse manufacturers to maintain unified performance standards for their products, and he must account for standards for their products, and he must account for the variable conditions

the variable conditions of his of his application. Tapplication. To compensateo compensate for these variables, the circuit design engineer

for these variables, the circuit design engineer who iswho is designing for trouble-free, long-life fuse protection in his designing for trouble-free, long-life fuse protection in his equipment generally loads his fuse not more than 75% equipment generally loads his fuse not more than 75% of the nominal

of the nominal rating listed by the rating listed by the manufacturermanufacturer,keeping,keeping in mind that overload and short circuit

in mind that overload and short circuit protection must beprotection must be adequately provided

adequately provided forfor..

The fuses under discussion are temperature-sensitive The fuses under discussion are temperature-sensitive devices whose ratings have been established in devices whose ratings have been established in a 25ºCa 25ºC ambient.

ambient. The fuse temperature generated The fuse temperature generated by the currentby the current passing through the fuse increases or decreases with passing through the fuse increases or decreases with ambient temperature change.

ambient temperature change.

The ambient temperature chart in the FUSE SELECTION The ambient temperature chart in the FUSE SELECTION GUIDE section illustrates the effect that ambient

GUIDE section illustrates the effect that ambient

temperature has on the nominal current rating of a fuse. temperature has on the nominal current rating of a fuse. Most traditional Slo-Blo

Most traditional Slo-Blo®®Fuse designs use lower meltingFuse designs use lower melting

temperature materials and are, therefore, more

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

ambient temperature changes. DIMENSIONS:

DIMENSIONS: Unless otherwise specified, dimensionsUnless otherwise specified, dimensions are in inches.

are in inches.

The fuses in this catalog

The fuses in this catalog range in size from the approx.range in size from the approx. 0402 chip size (.041”L x .020”W x .012”H) up to the 5 0402 chip size (.041”L x .020”W x .012”H) up to the 5 AG, also commonly known as a”MIDGET” fuse (13/32” AG, also commonly known as a”MIDGET” fuse (13/32” Dia. x 11/2” Length). As new products were

Dia. x 11/2” Length). As new products were developeddeveloped throughout the years, fuse sizes evolved to fill the various throughout the years, fuse sizes evolved to fill the various electrical circuit protection needs.

electrical circuit protection needs.

The first fuses were simple, open-wire devices,

The first fuses were simple, open-wire devices, followedfollowed in the 1890’s by Edison’s enclosure of thin wire in a lamp in the 1890’s by Edison’s enclosure of thin wire in a lamp base to make the first plug fuse. By 1904, Underwriters base to make the first plug fuse. By 1904, Underwriters Laboratories had established size and rating specifications Laboratories had established size and rating specifications to meet safety standards. The renewable type fuses and to meet safety standards. The renewable type fuses and automotive fuses appeared in

automotive fuses appeared in 1911914, and in 4, and in 1927 Littelfuse1927 Littelfuse started making very low amperage fuses for the budding started making very low amperage fuses for the budding electronics industry.

electronics industry.

The fuse sizes in following chart began with the early The fuse sizes in following chart began with the early “

“AutomobiAutomobile le Glass” Glass” fuses, fuses, thus thus the the term “term “AG”AG”. The. The numbers were applied

numbers were applied chronologically as differentchronologically as different manufac

manufacturers stturers started maarted making a new size: “3AG,king a new size: “3AG,” for” for

Fuse Characteristics, Terms and Consideration Factors

Fuse Characteristics, Terms and Consideration Factors

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length or diameter dimensions of the

length or diameter dimensions of the glass fuses. Tglass fuses. Theirheir designation was modified to AB in place of AG, indicating designation was modified to AB in place of AG, indicating that the outer tube was constructed from

that the outer tube was constructed from Bakelite, fibre,Bakelite, fibre, ceramic, or a similar

ceramic, or a similar material other than glass. The largestmaterial other than glass. The largest size fus

size fuse shown ie shown in the chart n the chart is the 5is the 5AG, or “MIDGETAG, or “MIDGET,,”” a name adopted from its use by the electrical industry a name adopted from its use by the electrical industry and the National Electrical Code

and the National Electrical Code range which normallyrange which normally recognizes fuses of 9/16” x 2” as the smallest standard recognizes fuses of 9/16” x 2” as the smallest standard fuse in use. fuse in use. FUSE SIZES FUSE SIZES S SIIZZEE DDIIAAMMEETTEER R ((IInncchheess)) LLEENNGGTTH H ((IInncchheess)) 1AG 1AG 1/41/4 .250.250 5/85/8 .625.625 2 2AAGG –– ..117777 –– ..558888 3AG 3AG 1/41/4 ..225500 111/41/4 1.251.25 4AG 4AG 9/329/32 ..228811 111/41/4 1.251.25 5AG 5AG 13/3213/32 ..440066 111/21/2 1.501.50 7AG 7AG 1/41/4 .250.250 7/87/8 .875.875 8AG 8AG 1/41/4 ..225500 11 11 TOLERANCES:

TOLERANCES: The dimensions shown in this catalogThe dimensions shown in this catalog are nominal. Unless otherwise specified, tolerances are are nominal. Unless otherwise specified, tolerances are applied as follows. T

applied as follows. Tolerances do not olerances do not apply to lead apply to lead lengths:lengths: ± .01

± .010” for dimensions to 0” for dimensions to 2 decimal places.2 decimal places. ± .005” for dimensions to 3 decimal places. ± .005” for dimensions to 3 decimal places. Contact Littelfuse should you have questions regarding Contact Littelfuse should you have questions regarding metric system and fractional tolerances.

metric system and fractional tolerances. FUSE CHARACTERISTICS:

FUSE CHARACTERISTICS: This characteristic of a fuseThis characteristic of a fuse design refers to how rapidly it responds to

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

Fuse.

Fuse. The distinguishing The distinguishing feature of Slo-Blofeature of Slo-Blo®®fuses is thatfuses is that

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

FUSE CONSTRUCTION: Internal construction may varyInternal construction may vary depending on ampere rating. Fuse photos in this catalog depending on ampere rating. Fuse photos in this catalog show typical construction of a particular ampere rating show typical construction of a particular ampere rating within the fuse series.

within the fuse series. FUSEHOLDERS:

FUSEHOLDERS: In many applications, fuses areIn many applications, fuses are

installed in fuseholders. These fuses and their associated installed in fuseholders. These fuses and their associated fuseholders are not intended for operation a

fuseholders are not intended for operation a s a “switch”s a “switch” for

for turnturning ing power power ““on” anon” and “d “off “off “.. INTERRUPTIN

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

(no explosion or body rupture) and clear the circuit.circuit. Interrupting ratings may vary with fuse design and range Interrupting ratings may vary with fuse design and range from 35 amperes for some

from 35 amperes for some 250VA250VAC metric size (5 x C metric size (5 x 20mm)20mm) fuses up to 200,

fuses up to 200, 000 amperes for the 000 amperes for the 600VA600VAC KLK series.C KLK series.

Information on other fuse series can be obtained from the Information on other fuse series can be obtained from the Littelfuse

Littelfuse

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

excess of the short circuit currents available. NUISANCE OPENING:

NUISANCE OPENING: Nuisance opening is most oftenNuisance opening is most often caused by an incomplete analysis of the

caused by an incomplete analysis of the circuit undercircuit under consideration.

consideration.

Of all the “Selection Factors” listed in the FUSE Of all the “Selection Factors” listed in the FUSE SELECTION GUIDE, special attention must be given SELECTION GUIDE, special attention must be given to items 1, 3,

to items 1, 3, and 6, namely, normal operating current,and 6, namely, normal operating current, ambient temperature, and pulses.

ambient temperature, and pulses.

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

consider the fuse’s nominal melting I22t rating. The fuset rating. The fuse

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

fuse’s nominal s nominal melting Imelting I22t rating must also meet the inrusht rating must also meet the inrush

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

power supply’s smoothing filter.

The procedure for converting various waveforms into I The procedure for converting various waveforms into I22tt

circuit demand is given in the FUSE SELECTION GUI circuit demand is given in the FUSE SELECTION GUI DE.DE. For trouble -free, long-life fuse protection, it is good

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

practice to select a fuse such that the I22t of the t of the waveformwaveform

is no more than 20% of the nominal melting I

is no more than 20% of the nominal melting I22t rating oft rating of

the fuse. Refer to the section on PULSES in the FUSE the fuse. Refer to the section on PULSES in the FUSE SELECTION GUIDE.

SELECTION GUIDE. RESISTANCE:

RESISTANCE: The resistance of a fuse is usually anThe resistance of a fuse is usually an insignificant part of the total circuit resistance. Since the insignificant part of the total circuit resistance. Since the resistance of fractional amperage fuses can be several resistance of fractional amperage fuses can be several ohms, this fact should be considered when using them ohms, this fact should be considered when using them in low-voltage circuits. Actual values can be obtained by in low-voltage circuits. Actual values can be obtained by contacting Littelfuse.

contacting Littelfuse.

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

somewhere in between.

Cold resistance is the resistance obtained using a Cold resistance is the resistance obtained using a measuring current of

measuring current of no more than no more than 1010% of % of the fuse’the fuse’ss nominal rated current. Values shown in this publication nominal rated current. Values shown in this publication forfor cold resistance are nominal

cold resistance are nominal and representative. Tand representative. The factoryhe factory should be consulted if this parameter is critical to the should be consulted if this parameter is critical to the design analysis.

design analysis.

Hot resistance is the resistance calculated from the Hot resistance is the resistance calculated from the stabilized voltage drop across the fuse, with current equal stabilized voltage drop across the fuse, with current equal to the nominal rated current flowing through it.

to the nominal rated current flowing through it. ResistanceResistance data on all Littelfuse products are available on request. data on all Littelfuse products are available on request. Fuses can be supplied to specified controlled resistance Fuses can be supplied to specified controlled resistance tolerances at additional cost.

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SOLDERING

SOLDERING RECOMMENDATIRECOMMENDATIONS:ONS: Since most fuseSince most fuse constructions incorporate soldered connections, caution constructions incorporate soldered connections, caution should be used when installing those fuses intended to should be used when installing those fuses intended to be soldered in

be soldered in place. Tplace. The application of he application of excessive heat canexcessive heat can reflow the solder within the fuse and

reflow the solder within the fuse and change its rating.change its rating. Fuses are heat-sensitive components similar to Fuses are heat-sensitive components similar to semi-conductors, and the use of heat sinks during soldering is conductors, and the use of heat sinks during soldering is often recommended.

often recommended.

Lead-Free

Lead-Free Soldering Soldering Parameters (most Parameters (most instances):instances): Wave S

Wave Solder — older — 260ºC, 10 secon260ºC, 10 seconds maxds max Reflow Solder — 260ºC, 30 seconds max Reflow Solder — 260ºC, 30 seconds max TEST SAMPLING PLAN:

TEST SAMPLING PLAN: Because compliance with certainBecause compliance with certain specifications requires destructive testing, these tests are specifications requires destructive testing, these tests are selected on a st

selected on a st atistical basis for each lot manufactured.atistical basis for each lot manufactured. TIME-CURRENT CURVE:

TIME-CURRENT CURVE: The graphical presentation ofThe graphical presentation of the fusing characteristic, time-current curves are generally the fusing characteristic, time-current curves are generally average curves which are presented as a

average curves which are presented as a design aid butdesign aid but are not generally considered part of the fuse specification. are not generally considered part of the fuse specification. Time-current curves are extremely useful in defining Time-current curves are extremely useful in defining a fuse, since fuses with the same current rating can a fuse, since fuses with the same current rating can be represented by c

be represented by c onsiderably different time-currentonsiderably different time-current curves. T

curves. The fuse specification typically he fuse specification typically will include a lifewill include a life requirement at 100% of rating and maximum opening requirement at 100% of rating and maximum opening times at overload points (usually 135% and 200%

times at overload points (usually 135% and 200% of ratingof rating depending on fuse standard

depending on fuse standard characteristics). A time-currentcharacteristics). A time-current curve represents average data

curve represents average data for the design; how ever,for the design; how ever, there may be some differences in the values for any one there may be some differences in the values for any one given production lot. Samples should be tested

given production lot. Samples should be tested to verifyto verify performance, once the fuse has been selected.

performance, once the fuse has been selected. UNDERWRITER

UNDERWRITERS S LABORATORIESLABORATORIES:: Reference to “ListedReference to “Listed by Underwriters Laboratories” signifies that the fuses by Underwriters Laboratories” signifies that the fuses meet the requirements

meet the requirements of UL/CSA/ANCE 248-14 “Fusesof UL/CSA/ANCE 248-14 “Fuses for Supplem

for Supplementarentar y Overcurrey Overcurrent Protectint Protection”on”. Some 32. Some 32 volt fuses (automotive) in this catalog are listed under volt fuses (automotive) in this catalog are listed under UL Standard

UL Standard 275. Reference to “Recognized under the275. Reference to “Recognized under the Component Program of Underwriters Laboratories” Component Program of Underwriters Laboratories” signifies that the item is recognized under the component signifies that the item is recognized under the component program of Underwriters Laboratories and application program of Underwriters Laboratories and application approval is required.

approval is required.

VOLTAGE RATING:

VOLTAGE RATING: The voltage rating, as marked on aThe voltage rating, as marked on a fuse, indicates that the fuse can be relied upon to safely fuse, indicates that the fuse can be relied upon to safely interrupt its rated short circuit current in a circuit where the interrupt its rated short circuit current in a circuit where the voltage is equal to, or less than, its rated voltage.

voltage is equal to, or less than, its rated voltage. This system of voltage rating is covered by N.E.C. This system of voltage rating is covered by N.E.C. regulations and is a requirement of Underwriters regulations and is a requirement of Underwriters Laboratories as a protection against

Laboratories as a protection against fire risk. Tfire risk. The standardhe standard voltage ratings used by fuse manufacturers for most voltage ratings used by fuse manufacturers for most small- small-dimension and midget fuses are 32, 63, 125, 250 and 600. dimension and midget fuses are 32, 63, 125, 250 and 600. In electronic equipment with relatively low output power In electronic equipment with relatively low output power supplies, with circuit impedance limiting short circuit supplies, with circuit impedance limiting short circuit currents to values of less than ten times the current rating currents to values of less than ten times the current rating of the fuse, it is common practice to specify fuses with of the fuse, it is common practice to specify fuses with 125 or 250 volt ratings for secondary

125 or 250 volt ratings for secondary circuit protection ofcircuit protection of 500 volts or

500 volts or higher.higher.

As mentioned previously (See RERATING), fuses are As mentioned previously (See RERATING), fuses are sensitive to changes in current, not voltage, maintaining sensitive to changes in current, not voltage, maintaining their “status quo

their “status quo” at any voltage up ” at any voltage up to the maximum ratingto the maximum rating of the fuse. It is not until the fuse element melts and of the fuse. It is not until the fuse element melts and arcing occurs that the

arcing occurs that the circuit voltage and available powercircuit voltage and available power become an issue. The safe interruption of the circuit, become an issue. The safe interruption of the circuit, as itas it relates to circuit voltage and

relates to circuit voltage and available poweravailable power, is discussed, is discussed in the section on INTERRUPTING RATING.

in the section on INTERRUPTING RATING. T

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

at voltages greater than the voltage rating. DERIVATION OF NOMINAL MELTING I

DERIVATION OF NOMINAL MELTING I22t:t: LaboratoryLaboratory

tests are conducted on each fuse design

tests are conducted on each fuse design to determine theto determine the amount of energy required to

amount of energy required to melt the fusing element. Thimelt the fusing element. Thiss energy is described as nominal melting I

energy is described as nominal melting I22t and is expressedt and is expressed

as “Ampere Squared Seconds” (A

as “Ampere Squared Seconds” (A22Sec.).Sec.).

A pulse of current is applied to the fuse, and a time A pulse of current is applied to the fuse, and a time measurement is taken for melting to

measurement is taken for melting to occur. If meltingoccur. If melting does not occur within a short duration of about 8 does not occur within a short duration of about 8 milliseconds (0.008 seconds) or less, the level of pulse milliseconds (0.008 seconds) or less, the level of pulse current is increased. T

current is increased. This test procedure is repeated his test procedure is repeated untiluntil melting of the fuse element is confined to within about 8 melting of the fuse element is confined to within about 8 milliseconds.

milliseconds.

The purpose of this procedure is to assure that the heat The purpose of this procedure is to assure that the heat created has insufficient time to thermally conduct away created has insufficient time to thermally conduct away from the fuse element. That is, all of the heat energy ( from the fuse element. That is, all of the heat energy ( II22t)t)

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

to calculate melting I22t. When the melting phase t. When the melting phase reachesreaches

completion, an electrical arc occurs immediately prior to completion, an electrical arc occurs immediately prior to the “

the “opening” of opening” of the fuse the fuse element.element. Clearing I

Clearing I22t = Melting It = Melting I22t + arcing It + arcing I22tt

The nominal I

The nominal I22t values given in this publication pertain t values given in this publication pertain toto

the melt

the melting phaing phase porse portion of tion of the “clearthe “clearing” or “opening”ing” or “opening”.. Alternatively the time can be measured

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Fuse Selection Guide

Fuse Selection Guide

The application guidelines and product data in this guide  The application guidelines and product data in this guide  are intended to provide technical

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

the circuit/application.

Many of the factors involved with fuse selection are listed Many of the factors involved with fuse selection are listed below

below. For additional assist. For additional assist ance with choosing fusesance with choosing fuses appropriate to you requirements, contac

appropriate to you requirements, contac t your Littelfuset your Littelfuse products reprentative.:

products reprentative.: Selection Factors Selection Factors 1.

1. Normal Normal operating operating currentcurrent 2.

2. ApplicatApplication ion voltage voltage (AC (AC or or DC)DC) 3.

3. Ambient Ambient temperaturetemperature 4.

4. Overload current Overload current and length of and length of time in which time in which thethe fuse must open

fuse must open 5.

5. Maximum avMaximum available failable fault ault currentcurrent 6.

6. Pulses, Surge Pulses, Surge Currents, Currents, Inrush CurInrush Currents, rents, Start-upStart-up Currents, and

Currents, and Circuit TCircuit Transientsransients 7

7. . Physical size Physical size limitationlimitations, s, such as such as length, diameterlength, diameter,, or height

or height 8.

8. Agency Agency Approvals required, Approvals required, such as UL, such as UL, CSA, CSA, VDE,VDE, METI, MITI or Military

METI, MITI or Military 9.

9. Fuse features Fuse features (mounting type/form factor(mounting type/form factor, , ease ofease of removal, axial leads, visual indication, etc.)

removal, axial leads, visual indication, etc.) 10. Fuseholder features, if applicable and associated 10. Fuseholder features, if applicable and associated rerating (clips, mounting block, panel mount, PC rerating (clips, mounting block, panel mount, PC board mount, R.F.I. shielded, etc.)

board mount, R.F.I. shielded, etc.)

11. Application testing and verification prior to 11. Application testing and verification prior to

production production

1. NORMAL OPERATING CURRENT:

1. NORMAL OPERATING CURRENT: The current ratingThe current rating of a fuse is typically derated 25% for operation at 25ºC to of a fuse is typically derated 25% for operation at 25ºC to avoid nuisance blowing. For example, a fuse with a

avoid nuisance blowing. For example, a fuse with a currentcurrent rating of 10A is not usually recommended for operation at rating of 10A is not usually recommended for operation at more than 7

more than 7.5A in a .5A in a 25ºC ambient. For additional 25ºC ambient. For additional details,details, see RERATING in the previous section and AMBIENT see RERATING in the previous section and AMBIENT TEMPERATURE below.

TEMPERATURE below. 2. APPLICATION VOLTAGE:

2. APPLICATION VOLTAGE: The voltage rating of theThe voltage rating of the fuse must be equal to, or greater than, the available circuit fuse must be equal to, or greater than, the available circuit voltage. For exceptions, see VOLTAGE RATING.

voltage. For exceptions, see VOLTAGE RATING. 3. AMBIENT TEMPERATURE:

3. AMBIENT TEMPERATURE: The current carryingThe current carrying capacity tests of fuses are performed at 25ºC and will be capacity tests of fuses are performed at 25ºC and will be affected by changes in ambient

affected by changes in ambient temperature. Ttemperature. The higherhe higher the ambient temperature, the hotter the fuse will operate, the ambient temperature, the hotter the fuse will operate, and the shorter its

and the shorter its life. Converselylife. Conversely, operating at a , operating at a lowerlower temperature will prolong fuse life. A fuse also runs hotter temperature will prolong fuse life. A fuse also runs hotter as the normal operating current

as the normal operating current approaches or exceeds theapproaches or exceeds the rating of the selected fuse.

rating of the selected fuse. Practical experience indicatesPractical experience indicates fuses at

fuses atroom temperatureroom temperature should last indefinitely, ifshould last indefinitely, if operated at no more than 75% of catalog fuse rating. operated at no more than 75% of catalog fuse rating. Ambient temperature effects are in addition to the normal Ambient temperature effects are in addition to the normal re-rating, see example. Example: Given a normal operating re-rating, see example. Example: Given a normal operating current of 2.25 amperes in an application using a 229 current of 2.25 amperes in an application using a 229 series fuse at room temperature, then:

series fuse at room temperature, then:

Normal Operating Current Normal Operating Current Catalog Fuse Rating = ———————————— Catalog Fuse Rating = ————————————

0.75 0.75 or or -2.25 Amperes 2.25 Amperes ———————

——————— = 3 = 3 Amp FAmp Fuse (at 25ºC)use (at 25ºC) 0.75

0.75

This charts shows typical

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

(www.littelfuse.com) or ctelfuse.com) or c ontact a ontact a Littelfuse representative.Littelfuse representative.

Curve A:

Curve A: Thin-Film Fuses and 313 Series (.01Thin-Film Fuses and 313 Series (.010 to .150A)0 to .150A) Cur

Curve B: ve B: FLATFLAT-PAK-PAK®®, TeleLink, TeleLink®®, Nano, Nano2®2®, PICO, PICO®®, Blade, Blade

T

Terminal and other erminal and other leaded and catridge leaded and catridge fusesfuses Curve C:

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4. OVERLOAD

4. OVERLOAD CURRENT CONDITION:CURRENT CONDITION: The current levelThe current level for which protection is required. Fault conditions may be for which protection is required. Fault conditions may be specified, either in terms of current or, in terms of both specified, either in terms of current or, in terms of both current and maximum time the fault can be tolerated current and maximum time the fault can be tolerated before damage occurs. Time-curren

before damage occurs. Time-current curves should t curves should bebe consulted to try to match the fuse characteristic to the consulted to try to match the fuse characteristic to the circuit needs, while keeping in mind that the curves are circuit needs, while keeping in mind that the curves are based on average data.

based on average data. 5. MAXIMUM FAULT

5. MAXIMUM FAULT CURRENT:CURRENT: The Interrupting RatingThe Interrupting Rating of a fuse must

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

of the circuit. 6. PULSES:

6. PULSES: The general term “pulses” is used in thisThe general term “pulses” is used in this context to describe the broad category of wave shapes context to describe the broad category of wave shapes referred

referred to as to as “surge “surge currentcurrents”s”, “sta, “start-up rt-up currentcurrents”s”, “inrush, “inrush current

currents”s”, and “t, and “transienransients”ts”. Elec. Electrical trical pulse cpulse c onditioonditions canns can vary considerably

vary considerably from one application from one application to another. Differeto another. Differentnt fuse constructions may not react the same to a given fuse constructions may not react the same to a given pulse condition. Electrical pulses produce thermal

pulse condition. Electrical pulses produce thermal cyclingcycling and possible mechanical fatigue that

and possible mechanical fatigue that could affect the lifecould affect the life of the fuse. Initial or start-up pulses are normal for some of the fuse. Initial or start-up pulses are normal for some applications and require the characteristic of a

applications and require the characteristic of a Slo-BloSlo-Blo®®

fuse. Slo-Blo

fuse. Slo-Blo®®fuses incorporate a thermal delay designfuses incorporate a thermal delay design

to enable them to survive normal start-up pulses and still to enable them to survive normal start-up pulses and still provide protection against prolonged

provide protection against prolonged overloads. overloads. The start-The start-up pulse should be defined and then compared to the up pulse should be defined and then compared to the time-current curve and I

current curve and I22t rating for the fuse. Application testingt rating for the fuse. Application testing

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

to withstand the pulse conditions. Nominal melting I

Nominal melting I22t is a measure of the energy requiredt is a measure of the energy required

to melt the

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

Squared Seconds” (A22Sec.). ThiSec.). This nominal s nominal melting Imelting I22t,t,

and the energy it represents (within a time duration of and the energy it represents (within a time duration of 8 milliseconds [0.008 second] or less and 1 millisecond 8 milliseconds [0.008 second] or less and 1 millisecond [0.001 second]or less for thin film fuses),

[0.001 second]or less for thin film fuses), is a value that isis a value that is constant for each different fusing element. Because constant for each different fusing element. Because everyevery fuse type and rating, as well as its corresponding part fuse type and rating, as well as its corresponding part number

number, has a , has a diffdifferent fusing element, it is erent fusing element, it is necessary tonecessary to determine the I

determine the I22t for each. This It for each. This I22t value is a parameter oft value is a parameter of

the fuse itself and is controlled by the element material the fuse itself and is controlled by the element material and the configuration of the fuse element. In addition and the configuration of the fuse element. In addition to selecting fuses on the

to selecting fuses on the basis of “Normal Operatingbasis of “Normal Operating Curr

Currentents”s”, “Rer, “Reratiating”ng”, a, and “nd “AmbAmbienient Tt Tempeemperaturature” asre” as discussed earlier, it is also necessary to apply the I discussed earlier, it is also necessary to apply the I22tt

design approach. T

design approach. This nominal his nominal melting Imelting I22t is not only at is not only a

constant value for each fuse element design, but

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

nominal melting I22t method of fuse selection is applied tot method of fuse selection is applied to

those applications in which the fuse must sustain large those applications in which the fuse must sustain large current pulses of

current pulses of a short duration. These high-energya short duration. These high-energy currents are common in many applications and are critical currents are common in many applications and are critical to the design analysis.

to the design analysis.

The following example should assist in

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

understanding of the application of I22t.t.

EXAMPLE:

EXAMPLE: Select a 125VSelect a 125V, very , very fast-acting PICOfast-acting PICO®®II fuse thatII fuse that

is capable of withstanding 100,000 pulses of current (I) of is capable of withstanding 100,000 pulses of current (I) of the pulse

the pulse waveform shown in Figure waveform shown in Figure 11..

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

temperature of 25ºC. Step 1

Step 1 — Refer to Chart 1 and select the appropriate— Refer to Chart 1 and select the appropriate pulsewavef

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

Place the applicable value for peak pulse current (icurrent (ipp) and) and time (t) into the

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

and calculate the result, as shown: 1 1 11 II22t= t= — — (i(i P P) = I) = I 2 2t t = = — — (i(i P P)) 2 2tt 5 5 55 1 1 — x 8 — x 822x .004 = x .004 = 0.0512 A0.0512 A22Sec.Sec. 5 5

This value is referred to

This value is referred to as the “Pulse Ias the “Pulse I22t”t”..

Step 2 

Step 2 — Determine the required value of — Determine the required value of Nominal MeltingNominal Melting II22t by referring to Chart 2. A figure t by referring to Chart 2. A figure of 22% is shown inof 22% is shown in

Chart II for 100,000 occurrences of the Pulse I

Chart II for 100,000 occurrences of the Pulse I22t calculatedt calculated

in Step 1. This Pulse I

in Step 1. This Pulse I22t is converted to its required value oft is converted to its required value of

Nominal Melting I

Nominal Melting I22t as follows:t as follows:

Nom. Melt I

Nom. Melt I22t = Pulse It = Pulse I22t/.22t/.22

0.0512/.22 = 0.2327 A 0.0512/.22 = 0.2327 A22Sec.Sec.

Step 3 

Step 3 — Examine the I— Examine the I22t rating data for the PICOt rating data for the PICO®®II, 125V,II, 125V,

very fast-acting

very fast-acting fuse. Tfuse. The part number he part number 251251001, 1 ampere001, 1 ampere design is rated at 0.256 A

design is rated at 0.256 A22Sec., which is the minimumSec., which is the minimum

fuse rating that will accommodate the 0.2327 A fuse rating that will accommodate the 0.2327 A22Sec.Sec.

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

ampere rating, as previously described. 7. PHYSICAL SIZE

7. PHYSICAL SIZE LIMITATIONS:LIMITATIONS: Please refer to thePlease refer to the product dimensions presented in current Littelfuse product product dimensions presented in current Littelfuse product data sheets for specific information.

data sheets for specific information. 8. AGENCY APPROVALS:

8. AGENCY APPROVALS: For background informationFor background information about common standards,

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

site (http://www(http://www.littelfuse.com/design-support.html)..littelfuse.com/design-support.html). For specific agency approval information for

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

and information presented on www.litwww.littelfuse.com. Astelfuse.com. As agency approvals and standards may

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

on the information presented on www.littelon www.littelfuse.com asfuse.com as current information.

current information. 9. FUSE FEATURES:

9. FUSE FEATURES: Please consult the specific productPlease consult the specific product features presented within this catalog and on

features presented within this catalog and on our web siteour web site (http://www

(http://www.littelfuse.com). For additional information .littelfuse.com). For additional information andand support contact your Littelfuse product representative. support contact your Littelfuse product representative.

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FIGURE 1 FIGURE 1 ..000011 ..000022 ..000033 ..000044 ..000055 ..000066 Time (Seconds) Time (Seconds)    C    C  u  u   r   r   r   r   e   e   n   n    t    t    (    (    A    A  m  m   p   p   e   e   r   r   e   e   s   s    )    ) 2 2 4 4 6 6 8 8 10 10

Normal Operating Current Normal Operating Current l l tt Pulse Pulse Energy Energy 2 2 CHART 1 CHART 1 W WAAVVEESSHHAAPPEESS FFOORRMMUULLAASS tt t t tt 11 iipp iipp iipp iipp iipp iibb iipp F F E E D D C C B B A A OR OR tt tt tt tt i = k i = k II22t = it = i p p 2 2tt i = i i = ipp-kt-kt II22t = (1/3)(it = (1/3)(i p p 2 2+ i+ i p piibb+ i+ ibb 2 2) t) t i = i i = ippsin tsin t II22t = (1/2) it = (1/2) i p p 2 2tt II22t = (1/3) it = (1/3) i p p 2 2tt i = kt i = kt22OR i = iOR i = i p p(1-kt)(1-kt) 2 2 II22t = (1/5) it = (1/5) i p p 2 2tt i = i i = ippee–kt–kt)) II22t t (1/2) (1/2) ii p p 2 2tt11 CHART 2 CHART 2 PULSE

PULSE CYCYCLE WITHSTAND CAPABILITYCLE WITHSTAND CAPABILITY

1

10000,,00000 P0 Puullsseess PuPullsse Ie I22t = 22% of Nominal Melting It = 22% of Nominal Melting I22tt

1

100,,00000 0 PPuullsseess PuPullsse e II22t = 29% of Nominal Melting It = 29% of Nominal Melting I22tt

1

1,,00000 0 PPuullsseess PuPullsse e II22t = 38% of Nominal Melting It = 38% of Nominal Melting I22tt

1

1000 0 PPuullsseess PPuullsse Ie I22t = 48% of Nominal Melting It = 48% of Nominal Melting I22tt 100000 100000 10000 10000 1000 1000 100 100 1 100%% 110000%%    N    N  u  u   m   m    b    b  e  e  r  r   o   o    f    f    P    P  u  u    l    l  s  s  e  e   s   s Pulse I

Pulse I22t / Average Melting It / Average Melting I22tt

Note: Adequate time (10 seconds) must exist between pulse Note: Adequate time (10 seconds) must exist between pulse eventsevents to allow heat from the previous event to dissipate.

to allow heat from the previous event to dissipate.

10. FUSEHOLDER FEATURES AND RERATING:

10. FUSEHOLDER FEATURES AND RERATING: ForFor

information about the range of Littelfuse fuseholders and information about the range of Littelfuse fuseholders and specific features and characteristics, please consult with specific features and characteristics, please consult with a Littelfuse products representative or visit our

a Littelfuse products representative or visit our web siteweb site (http://www.littelfuse.com).

(http://www.littelfuse.com).

For 25ºC ambient temperatures, it is recommended that For 25ºC ambient temperatures, it is recommended that fuseholders be operated at no more than 60% of the fuseholders be operated at no more than 60% of the nominal current rating established using the controlled test nominal current rating established using the controlled test conditions specified by Underw

conditions specified by Underw riters Laboratories. Theriters Laboratories. The primary objective of these UL test conditions is to specify primary objective of these UL test conditions is to specify common test standards necessary for the continued common test standards necessary for the continued control of manufactured items intended for protection control of manufactured items intended for protection against fire, etc. A copper dummy fuse is inserted in against fire, etc. A copper dummy fuse is inserted in the fuseholder by Underwriters Laboratories, and then the fuseholder by Underwriters Laboratories, and then the current is increased until a certain temperature rise the current is increased until a certain temperature rise occurs.

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

resistance of the fuseholder clips. fuseholder clips. This value of currentThis value of current is considered to be the

is considered to be the rated current of the fuseholder,rated current of the fuseholder, expressed as 100% of rating. Some of

expressed as 100% of rating. Some of the more common,the more common, everyday applications may differ from these UL test everyday applications may differ from these UL test conditions as follows: fully enclosed fuseholders, high conditions as follows: fully enclosed fuseholders, high contact resistance,air movement, transient spikes, and contact resistance,air movement, transient spikes, and changes in connecting cable size (diameter and length). changes in connecting cable size (diameter and length). Even small variations from the controlled test

Even small variations from the controlled test conditionsconditions can greatly affect the ratings

can greatly affect the ratings of the fuse-holder. Forof the fuse-holder. For this reason, it is recommended that fuseholders be this reason, it is recommended that fuseholders be derated by 40% (operated at no more than 60% of the derated by 40% (operated at no more than 60% of the nominal current rating established using the Underwriter nominal current rating established using the Underwriter Laboratories test conditions, as previously stated). Laboratories test conditions, as previously stated). 11. TESTING:

11. TESTING: The factors presented here should beThe factors presented here should be considered in selecting

considered in selecting a fuse for a fuse for a given application. Thea given application. The next step is to verify the

next step is to verify the selection by requesting samplesselection by requesting samples for testing in the actual circuit. Before evaluating the for testing in the actual circuit. Before evaluating the samples, make sure the fuse is properly mounted with samples, make sure the fuse is properly mounted with good electrical connections, using adequately

good electrical connections, using adequately sized wiressized wires or traces. The testing should include life tests

or traces. The testing should include life tests under normalunder normal conditions and overload tests Under fault conditions, to conditions and overload tests Under fault conditions, to ensure that the fuse will operate properly in the circuit. ensure that the fuse will operate properly in the circuit.

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

temperature coefficient) device.

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

are common and automatic resettability desired. Littelfuse off

Littelfuse offers PTCs with ers PTCs with the following general forms andthe following general forms and features, and come in a variety of

features, and come in a variety of sizes and capacities:sizes and capacities: Surface Mount Devices:

Surface Mount Devices:

Radial Leaded Series: Radial Leaded Series:

Battery Strap Devices: Battery Strap Devices:

If your application requirements fall outside of

If your application requirements fall outside of our productour product range, in certain

range, in certain instances we can offer customizedinstances we can offer customized solutions. Please contact Littelfuse for more information. solutions. Please contact Littelfuse for more information.

T

T

raditiona

raditiona

l Fuse

l Fuse

s

s

Vs. PTCs

Vs. PTCs

Fuses and PTCs are both overcurrent protection Fuses and PTCs are both overcurrent protection devices, though each offer their own unique operating devices, though each offer their own unique operating characteristics and benefits. Understanding the

characteristics and benefits. Understanding the diffdifferenceserences between the two technologies should make the choice in between the two technologies should make the choice in selection easier, depending on the application.

selection easier, depending on the application.

The most obvious difference is that PTCs are automatically The most obvious difference is that PTCs are automatically resettable whereas traditional Fuses need to be replaced resettable whereas traditional Fuses need to be replaced after they they are tripped. Whereas a fuse will completely after they they are tripped. Whereas a fuse will completely stop the flow of current (which may be desired in c

stop the flow of current (which may be desired in c riticalritical applications) after most similar

applications) after most similar overcurrent event, PTCsovercurrent event, PTCs continue to enable the equiment to

continue to enable the equiment to function, except infunction, except in extreme cases.

extreme cases.

Because they reset automatically, many circuit

Because they reset automatically, many circuit designersdesigners choose PTCs in instances where overcurrent events are choose PTCs in instances where overcurrent events are expected to occur often, and where maintaining low expected to occur often, and where maintaining low warranty and service costs, constant system uptime, and/  warranty and service costs, constant system uptime, and/  or user transparency are

or user transparency are at a premium. Theat a premium. They are also ofteny are also often chosen in circuits that are difficult to access in

chosen in circuits that are difficult to access in or remoteor remote locations, were fuse replacement would be difficult. locations, were fuse replacement would be difficult. There are several other operating characteristics to be There are several other operating characteristics to be considered that distinguish PTCs and fuses, and it is also considered that distinguish PTCs and fuses, and it is also best to test and

best to test and verify device performance before useverify device performance before use within the end application.

within the end application.

Littelf

Littelf

use

use

PTC

PTC

Character

Character

istics

istics

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

"tripping." The graph at right shows the typical right shows the typical response of a PTC to response of a PTC to temperature. temperature. Littelfuse Polymer PTCs Littelfuse Polymer PTCs are made chiefly of high are made chiefly of high density polyethylene density polyethylene mixed with graphite. mixed with graphite. During an overcurrent During an overcurrent event, a Polymer PTC event, a Polymer PTC will heat and expand, will heat and expand, which in turn causes the which in turn causes the

conducting particles to break contact and stop the current. conducting particles to break contact and stop the current. The general procedure for res

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

device to cool down.

PTC Products

PTC Products

    L     L   o   o   g   g    r    r    e    e    s    s     i

    i   s   s   t   t   a   a

   n    n    c    c    e    e     (     (   o   o     h     h   m   m    s    s     )     )  Temperature (C)  Temperature (C)  Trip Point  Trip Point

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Leakage Current:

Leakage Current:

When a PTC is in a "tripped state" itWhen a PTC is in a "tripped state" it protects the circuitry by limiting the current flow to a low protects the circuitry by limiting the current flow to a low leakage level. Leakage current can range from less leakage level. Leakage current can range from less thanthan a hundred milliamps (mA) at rated voltage up to a few a hundred milliamps (mA) at rated voltage up to a few hundred milliamps (mA) at

hundred milliamps (mA) at lower voltages. Fuses on thelower voltages. Fuses on the other hand completely interrupt the current flow when other hand completely interrupt the current flow when tripped, and this open circuit results in no leakage current tripped, and this open circuit results in no leakage current when subjected to an overload current.

when subjected to an overload current.

Interr

Interr

upting

upting

Rating:

Rating:

PTCs are rated for a maximumPTCs are rated for a maximum short circuit current at rated voltage also known as short circuit current at rated voltage also known as "breaking capacity" or

"breaking capacity" or Imax. Imax. This fault current level is theThis fault current level is the maximum current that the device can

maximum current that the device can withstand safely,withstand safely, keeping in mind that the PTC w

keeping in mind that the PTC w ill not actually interruptill not actually interrupt the current flow (see Leakage Current above). A typical the current flow (see Leakage Current above). A typical Littelfuse PTC short circuit rating is 40A;

Littelfuse PTC short circuit rating is 40A; or for the batteryor for the battery strap PTCs, this value can

strap PTCs, this value can reach 1reach 100A. Fuses do in fact00A. Fuses do in fact interrupt the current flow in response to the overload interrupt the current flow in response to the overload and the range of interrupting ratings, vary from tens of and the range of interrupting ratings, vary from tens of amperes (A) up to 10,000 (A) amperes at rated

amperes (A) up to 10,000 (A) amperes at rated voltage.

voltage.

Operating Voltage Rating:

Operating Voltage Rating:

General useGeneral use

Littelfuse PTCs are not rated above 60V while Littelfuse PTCs are not rated above 60V while fuses are ra

fuses are ra ted up to ted up to 600V600V..

Hold Current Rating:

Hold Current Rating:

The hold (operating)The hold (operating) current rating for PTCs can be

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

Resistance

Resistance

:: Reviewing productReviewing product

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

resistance of fuses.

Agency Approvals:

Agency Approvals:

Littelfuse PTCs areLittelfuse PTCs are Recognized under the Component Program Recognized under the Component Program of Underwriters Laboratories to UL Standard of Underwriters Laboratories to UL Standard 1434 for Thermistors.

1434 for Thermistors. The devices The devices have alsohave also

been certified under the CSA Component Acceptance been certified under the CSA Component Acceptance Program. Approvals for fuses include Recognition under Program. Approvals for fuses include Recognition under the Component Program of Underwriters Laboratories and the Component Program of Underwriters Laboratories and the CSA Component Acceptance Program. In addition, the CSA Component Acceptance Program. In addition, many fuses are available with full "Listing"

many fuses are available with full "Listing" in accordancein accordance with the new Supplementary Fuse Standard UL/CSA/ANCE with the new Supplementary Fuse Standard UL/CSA/ANCE (Mexico) 248-14. Supplemental fuses.

(Mexico) 248-14. Supplemental fuses.

Time-Current Characteristic

Time-Current Characteristic

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

time delay of a Slo-Blo® fuse.

Temperature Rerating:

Temperature Rerating:

The useful upper limit for aThe useful upper limit for a PTC is generally 85ºC, while the maximum operating PTC is generally 85ºC, while the maximum operating temperature for fuses is

temperature for fuses is 125ºC.125ºC.

Ambient temperature effects are in addition to the normal Ambient temperature effects are in addition to the normal rerating. PTCs hold and trip rating must be rerated when rerating. PTCs hold and trip rating must be rerated when applied at conditions other than room

applied at conditions other than room ambient. Forambient. For

example, any rise in ambient temperature will decrease the example, any rise in ambient temperature will decrease the hold current rating as well as

hold current rating as well as the trip current. A reduction inthe trip current. A reduction in ambient temperature will increase the trip current

ambient temperature will increase the trip current as wellas well as the hold current.

as the hold current.

The temperature rerating curves in the

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

required for a PTC at a given temperature.given temperature.

Temperature Rerating Curves Comparing PTCs to Fuses

Temperature Rerating Curves Comparing PTCs to Fuses

Chart Key Chart Key Curve A

Curve A

Thin-Film Fuses and 313

Thin-Film Fuses and 313

Series (.010 to .150A)

Series (.010 to .150A)

Curve B

Curve B

FLAT-PAK

FLAT-PAK®®, Nano, Nano2®2®,,

PICO

PICO®®, Blade Terminal,, Blade Terminal,

Special Purpose and

Special Purpose and

other leaded and

other leaded and

cartridge fuses cartridge fuses (except 313.010 – .150A) (except 313.010 – .150A) Curve C Curve C Resettable PTCs Resettable PTCs -60°C -60°C -76°F -76°F 20 20 40 40 60 60 80 80 100 100 120 120 140 140 -40°C -40°C -40°F -40°F -20°C -20°C -4°F -4°F 25°C 25°C C C C C A A B B AMBIENT TEMPERATURE AMBIENT TEMPERATURE     P     P     E     E     R     R     C     C     E     E     N     N     T     T     O     O     F     F     R     R     A     A     T     T     I     I     N     N     G     G A A B B 0°C 0°C 32°F 32°F 20°C 20°C 68°F 68°F 40°C 40°C 104°F 104°F 60°C 60°C 140°F 140°F 80°C 80°C 176°F 176°F 100°C 100°C 212°F 212°F 120°C 120°C 248°F 248°F

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P P A A R R T T S S Y Y R R E E T T T T A A B B D D E E D D A A E E L L L L A A I I D D A A R R T T N N U U O O M M E E C C A A F F R R U U S S Series Name Series Name 00660033LL 00880055LL 11220066LL 11221100LL 11881122LL 22001166LL 22992200LL 225500SS UUSSBBRR 1166RR 3300RR 6600RR 7722RR 225500RR 660000RR LLRR LLTT SSTT VVLL VVTT    T    T  e  e    l    l  e  e  c  c  o  o

  m   m

Application

Application

Ul60950 ,TIA-968-A, GR-1089 Req’s

Ul60950 ,TIA-968-A, GR-1089 Req’s XX XX XX

ITU-T Recoomendations

ITU-T Recoomendations XX XX XX

CPE (Customer Premises Equipment)

CPE (Customer Premises Equipment) XX XX XX

Analog Line Card

Analog Line Card XX XX XX

T1/E1/J1 And HDSL T1/E1/J1 And HDSL XX XX XX ISDN ISDN XX XX XX ADSL ADSL XX XX XX Cable Telephony Cable Telephony XX XX XX

PBX/KTS And Key Telephone System

PBX/KTS And Key Telephone System XX XX XX

   C    C  o  o  m  m   p   p   u   u    t    t  e  e  r  r CPU CPU XX XX XX USB USB XX XX XX XX XX XX XX

IEEE1284 Parallel Data Bus

IEEE1284 Parallel Data Bus XX XX XX XX XX

IEEE 802.3 IEEE 802.3 XX XX XX XX IEEE 1394 IEEE 1394 XX XX XX I/O Ports I/O Ports XX XX XX XX XX XX PC Card PC Card XX XX XX XX XX XX XX XX SCSI SCSI XX XX XX XX XX XX Video Port Video Port XX XX XX XX XX XX LCD Monitor LCD Monitor XX XX XX XX XX XX XX    C    C  o  o  n  n   s   s   u   u   m   m   e   e   r   r    E    E    l    l  e  e  c  c    t    t  r  r  o  o  n  n    i    i  c  c  s  s

Set Top Box

Set Top Box XX XX XX XX

Loudspeaker

Loudspeaker XX

Smart Card Reader

Smart Card Reader XX

Mobile Phone

Mobile Phone XX XX XX XX

Linear AC/DC Adapter

Linear AC/DC Adapter XX XX XX XX XX XX XX XX XX

Portable Electronic Input Port

Portable Electronic Input Port XX XX XX XX XX XX

Electromagnetic Loads, Motor

Electromagnetic Loads, Motor XX XX XX XX XX

Solenoid Protection Solenoid Protection XX XX XX XX XX    B    B  a  a    t    t    t

   t  e  r  r  y  y  e Lithium CellLithium Cell XX XX XX XX XX

Battery Pack Battery Pack XX XX XX XX XX    M    M  e  e    d    d    i    i  c  c  a  a    l    l   e   e    l    l  e  e  c  c    t    t  r  r  o  o  n  n    i    i  c  c

Voltage / Current Input Terminal

Voltage / Current Input Terminal XX XX NOTE:

NOTE: The application summary is for reference only. Determination of suitability for a specific application is the responsibility of the customer.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 PTCs are typically used as circuit protection in applications where sensitive components are at constant risk of

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

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

and motor controls.

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

a Littelfuse PTC device that may be appropriate to certaincertain end applications.

end applications.

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

design center athttp://www.littelfuse.com/designcenterhttp://www.littelfuse.com/designcenter.. For detailed product specifications, please

For detailed product specifications, pleaseconsult theconsult the Littelfus

Littelfuse PTC datasheets within this e PTC datasheets within this Catalog or visitCatalog or visit http://www.littelfuse.com/PTCs.

http://www.littelfuse.com/PTCs.

PTC Product Applications

PTC Product Applications

Figure

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References

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