Components Pulse&OverloadCapabilityofWirewoundResistors Modernelectroniccircuitsanddevicesaremoresensitivethanevertotransients,andthishasledtoanincreasedneedfortransientprotection.Designersareoftenfacedwithadifficulttaskintermsofthelevelofprotectionrequiredandtheabilityoftheponentstowithstandgiventransientsorpulses.Whilstthereisagreatdealofinformationavailableonthecapabilityofsemiconductors,ponents,suchasresistors,areoftenneglectedormisunderstoodbydesigners. Asresistorsareoftenlocatedinareaslikelytoexperiencetransients,withpowersuppliesorlightingcircuitsfor example,theirtransientorpulsecapabilityisacriticalissue. Themontypeofresistorsusedfortransientprotectionarewirewoundresistors.However,thisisaproductrangeforwhichpulsecapabilitydataisnotoftengivenondatasheets.Toethis,thefollowingpagesdetailthepulsehandlingandoverloadcharacteristicsofTTelectronicsmostpopularwirewoundresistors. Inadditiontoprovidingprehensiverangeofstandardwirewoundresistors,TTelectronicshasworkedcloselywithdesignerstocustombuildresistorsforspecificprotectionapplications. •DatagivenforMetalClad,axialCementandVitreousEnamelledwirewounds •Detailedpulsegraphs •Custompartsavailable SubsidiariesofTTelectronicsplc LeadinginTomorrow’sTechnology
2 OverloadingofWirewoundResistors Inordertolimittheirtemperaturerise,wirewoundresistorshavemaximumcontinuouspowerratings.Howevertheyalsohaveanoverloadrating,oftenreferredtoasShortTermOverloadondatasheets. Thisvariesordingtotheproducttypeandisspecifiedasamultipleoftheratedpowerappliedforaspecifiedtime.Theoverloadratingsforthemostpopularaxialwirewoundresistorsaregivenbelow; Type: W20SeriesWHSeriesWA80Series W31 Overload:(Multipleofratedpower) 105510 Duration: 5Seconds5Seconds5Seconds5Seconds Forexample,theW22isratedasa6Wattdevice,howeveritiscapableofdissipating60Wattsfor5seconds.Intermsofenergy,thiscorrespondsto300Joules. Itmightbethoughtthatthisdeviceiscapableofhandling300Joulesirrespectiveofthepulsewidth.Howeverbecauseittakesafinitetimefortheheatproducedtobeproperlydistributedthroughouttheresistorbody,itisnecessarytoimposelimitsontheappliedpulseenergysoastopreventexcessivestressesduetothermalshockdamagingponent. Forexample,ifweconsideraW220R22subjectedto200Voltsfor1.5milliseconds. UsingE=V2t/R,theenergyis270Joules. Thisiswithinthe300Joulesquotedabove,butbecauseitwouldtakelongerthan1.5msfortheheatproducedinthewiretoflowintothesurroundingmaterials,thetemperatureofthewirewouldrisefarbeyondthatintended. Ifthewireissubjectedtotemperaturesbeyonditsoperatinglimitstheresistancevalueofponentmaychangeexcessively,thecoatingcanbedamagedandinextremecasesthewirecanmelt. Forthesereasonswehaveproducedthe“EnergyCapability”and“OverloadRating”graphswhichshouldbereferredtofor allpulseapplications.(Thetermpulseimpliesasinglepulseappliedtoaresistor,whichisnotalreadydissipatingpowerandisinanambienttemperatureof70°Corless.) Forpulsesofupto100msdurationthe“EnergyCapability”graphshouldbeused.Forlongerpulses(>100ms)the“OverloadRating”graphshouldbeused.Howevertheenergyappliedinthefirst100msshouldnotexceedthatallowedonthe“EnergyCapability”Graph.Forveryshortpulses,<1ms,referenceshouldbemadetothe1.2/50µstable.”OverloadRating”graphsarenotprovidedfortheWA80andW31Series;fortheseproductsthetableaboveshouldbeused. Itisimportanttorealisethatforcedair-coolingand/orheatsinkshavenoeffectonoverloadratingswithinthefirstfewseconds.
3 RepetitiveorSuperimposedPulses Asthereisamaximumtemperaturetowhichthewirecanriseduringanoverload,iftheresistorisalreadydissipatingheat,thenthepulseenergyappliedshouldbelessthanthatallowedbythegraph.Usingtheformulabelowwecanestimatethe“equivalent”energyofapulseappliedtoaresistorthatisalreadydissipatingpower.Eap=Ex(1+Pav/Pr) Itshouldalsobenotedthatforrepetitivepulsestheaveragepowerdissipatedmustnotexceedthecontinuouspowerratingoftheresistor. Where:Eap=EquivalentpulseenergyE=KnownpulseenergyPr=ResistorpowerratingPav=Meanpowerbeingdissipated LimitingElementVoltage(
L.E.V) TheL.E.Visthemaximumcontinuousvoltagethatcanbeappliedtoaresistor. GenerallyforlowervaluesthepowerratingisexceededbeforetheL.E.Visreached.WithhighervaluestheL.E.Vimposeslimitationsontheappliedpower. Forexample,consideraW2343K0.Thevoltagerequiredtodissipatetheratedpowerof9Wattsis622Volts.Howeverthe
L.E.Vofthepartisonly500Volts.Therefore,asthiscannotbeexceedforcontinuousoperation,thepowerislimitedto5.81Watts. WhilsttheL.E.Vcannotbeexceedforcontinuousconditions,wirewoundresistorsarecapableofwithstanding3timestheirL.E.Vforpulseapplications. Graphs Thefollowingpagescontainthe“EnergyCapability”and“OverloadRating”graphsdescribedaboveforthemostpopulartypesofwirewoundresistor.Informationisalsoavailableonrequestfortubularresistorsandotherwirewoundproducts.Inadditiontothis,TTelectronicsareabletooffercustomdesignpartstomeetspecificcustomerrequirements. ForanyadditionalinformationortodiscussyourspecificrequirementspleasecontacttheResistorApplicationsTeam. Energy(Joules) 10001001010.10.1100 ResistiveComponents4 W20Series“EnergyCapability”Graph W21W215W22W23W24
1 10 100 1000 10000 Value(Ohms) W20Series“OverloadRating”Graph 100000 Basedonawiretemperatureriseof750°
C MultipleofRatedPower W21,W21510 W22,W23,W24 Energy(Joules)
1 0.1
1 10 100 OverloadDuration(Seconds) WA80Series“EnergyCapability”Graph 100 10WA82WA83WA84WA85WA87
1 Basedonawire temperaturerise 0.1 of750°
C 0.1
1 10 100 1000 10000 Value(Ohms) Energy(Joules) ResistiveComponents5 W31Series“EnergyCapability”Graph 100 10 1 0.10.01 1000 W31 0.1
1 10 100 1000 Value(Ohms) WHSeries“EnergyCapability”Graph Basedonawiretemperatureriseof750°C 10000 1001010.10.01 100 WH5WH10WH25WH50 0.1
1 10 100 1000 10000 Value(Ohms) WHSeries“OverloadRating”Graph Basedonawiretemperatureriseof750°C 100000 Energy(Joules) MultipleofRatedPower WH5/1010 WH25/50
1 0.1
1 10 100 OverloadDuration(Seconds) 6ResistiveComponents PeakVoltage(Volts)PeakVoltage(Volts) 1.2/50µsVoltageCapability Thegraphsbelowgivethemaximumvoltagethatmaybeappliedintheformofa1.2/50µspulseasdefinedinIEC61000-4-5andANSIC62.41.Thishasa1.2µsrisetimeand decaysexponentiallywitha50%amplitudepulsewidthof50µs.Themaximumpermittedresistancechangeis2%.Voltagesabove6kVhavenotbeentested. WA80Series1.2/50µsGraph 10000 W21Series&W311.2/50µsGraph 10000 1000 100 100.1 WA82WA83WA84WA85WA87
1 10 100 Value(Ohms) 1000 1000 1000.1 W21/31W215W22W23W24
1 10 100 Value(Ohms) 1000 FurtherInformation ForadditionalinformationortodiscussyourspecificrequirementspleasecontactourResistorApplicationsTeamusingthecontactdetailsbelow. Note:Graphsarebasedontheoreticaldata. TTelectronicshasover60yearsexperienceindesigningandmanufacturingponents. WelwynComponentsLimitedWelwynElectronicsPark,BedlingtonNorthumberlandNE227AA,UK Telephone:+44(0)1670822181Facsimile:+44(0)1670829465Email:info@ Website: IRCInc.(AFD)4222SouthStaplesStreetCorpusChristi,Texas78411,USA Telephone:+13619927900Facsimile:+13619923377Email:ircafd@ Website: LeadinginTomorrow’sTechnology GeneralNoteWelwynComponents/IRCreservestherighttomakechangesinproductspecificationwithoutnoticeorliability.AllinformationissubjecttoWelwyn’s/IRC’sowndataandisconsideredurateattimeofgoingtoprint. ©WelwynComponentsLimited SubsidiariesofTTelectronicsplcIssueB·01.03