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elec02 Electrical Circuits.pdf

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ELEC02 ELECTRICAL CIRCUITS
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Electrical Circuit Diagnosis - Course 623 2-1 Acircuitisacompletepathforcurrentwhenvoltageisapplied.There arethreebasictypesofcircuits: w Series w Parallel w Series-parallel Allcircuitsrequirethesamebasiccomponents: w Powersource w Protectiondevice w Conductors w Load w Controldevice w Ground Components of a Circuit All circuits have these basic components. Fig. 2-01 TL623f201 Section 2 Electrical Circuits Types of CircuitsSection 2 2-2 TOYOTA Technical Training Powersource- Inautomotivecircuits,thesourceistypicallythe battery. Protectiondevice- Circuitsrequireprotectionfromexcessive current.Excessivecurrentgeneratesheatandcandamagewires, connectors,andcomponents.Fuses,fusiblelinks,andcircuitbreakers protectcircuitsbyopeningthecircuitpathwhenthereistoomuch current. Load- Theloadcanbeanycomponentthatuseselectricitytodowork: w Light w Coil w Motor Controldevice- Thesimplestcontroldeviceisaswitch.Aswitch opensorclosesthepathforcurrent.Closetheswitchandcurrentis presenttooperatetheload.Opentheswitchandcurrentstops.The loadnolongeroperates. Acontroldevicecandomorethanjustturntheloadonoroff.Itcan alsoregulatehowtheloadworksbyvaryingtheamountofcurrentin thecircuit.Adimmerisanexampleofsuchacontroldevice. Thereareothertypesofcontroldevices: w Relays w Transistors w ECUs Ground- Theconnectiontogroundprovidesashortcut“backtothe source.Groundistypicallyanymajormetalpartofavehicle.Youcan thinkofgroundasazerovoltagereference.Groundprovidesacommon connectionthatallcircuitscanusesothattheydonothavetobewired allthewaybacktothebattery. Thecircuittypeisdeterminedbyhowthepowersource,protection devices,conductors,loads,controldevices,andgroundsareconnected.Electrical Circuits Electrical Circuit Diagnosis - Course 623 2-3 Simple Series Circuit This diagram shows a simple series circuit. Battery voltage is applied through the fuse to the control device (switch). When the switch closes, there is current in a single path through the load (lamp) to ground. Fig. 2-02 TL623f202c Aseriescircuithasthesekeyfeatures: w Currentisthesameineverypartofthecircuit. w Thesumofalltheindividualresistancesequalsthetotalresistance inthecircuit. w Thesumoftheindividualvoltagedropsinthecircuitequalsthe sourcevoltage. Aseriescircuithasonlyonepathforcurrent.Thatmeanscurrentis thesamethrougheverypartofthecircuit.Ifanypartofthecircuitis brokenordisconnected,thewholecircuitwillstopworking.Nocurrent ispresentinaseriescircuitunlessthereiscontinuitythroughthe entirecircuit. Key Features Series CircuitsSection 2 2-4 TOYOTA Technical Training YoucanuseOhm’sLawtopredictthebehaviorofelectricityinacircuit. Forseriescircuits,applyOhm’sLawasfollows: w Totalcircuitresistance(R T )equalsthesumoftheindividualload resistances(R 1 +R 2 ). -R T =R 1 +R 2 w Circuitcurrent(I)equalsvoltage(E)dividedbytotalresistance(R). - I=E/R w Voltagedrop(E R1 ,E R2 )acrosseachloadequalscurrent(I)times loadresistance(R 1 ,R 2 ). -E R1 =IxR 1 -E R2 =IxR 2 Inmostmoderntexts,currentisrepresentedasI“andvoltageasE.“ YoumayalsoseetheserepresentedasA“foramperage,insteadofI“ forcurrent,andV“insteadofE“forvoltage.Whenusingthat terminology,theOhm’sLawequationlookslikethis:A=V/R. Ohm’s Law in Series Circuits When troubleshooting, use Ohm’s Law to predict the behavior of a series circuit. Fig. 2-03 TL623f203c Applying Ohm’s Law NOTEElectrical Circuits Electrical Circuit Diagnosis - Course 623 2-5 UseOhm’sLawtotroubleshootseriescircuits: w Poorconnectionsandfaultycomponentscanincreaseresistance. w SinceE/R=I,moreresistancemeanslesscurrent. w Lesscurrentaffectstheoperationoftheloads(dimlamps,slow runningmotors). w Thereisnocurrentifthereisabreak(opencircuit)anywherein thecurrentpath. w SinceE/R=I,lowervoltagealsomeanslesscurrentandhigher voltagemeansmorecurrent. w Highvoltageincreasescurrentandcanalsoaffectcircuitoperation (blownfuses,prematurecomponentfailure).Section 2 2-6 TOYOTA Technical Training Voltage Drops in a Series Circuit Troubleshoot by taking voltage measurements with a digital multimeter. Fig. 2-04 TL623f204c Voltagedropsinaseriescircuit- Everyelementinacircuitthat hasresistancegeneratesavoltagedrop. w Theloadinthiscircuit(lamp)generatesthelargestvoltagedrop. w Thedimmergeneratesasmaller,variablevoltagedroptocontrol thebrightnessofthelamp. w Othercomponentsalsogenerateevensmallervoltagedrops. - Fuseandfuseconnectors - Wiring - Harnessconnectors w Thesumofallthevoltagedropsisequaltothesourcevoltage.Electrical Circuits Electrical Circuit Diagnosis - Course 623 2-7 Current in a Series Circuit When practical, remove the fuse to measure current in a circuit. Fig. 2-05 TL623f205c Currentinaseriescircuit- Currentinaseriescircuitisthesame ateverypointinthecircuit. w Measurecurrentbyopeningthecircuitandinsertingthemeterin series. w ThecircuitnowincludestheDMMinserieswiththecircuit. w Useafusedleadifremovingthecircuitfuse.Section 2 2-8 TOYOTA Technical Training Measuring Resistance in a Series Circuit Remove the fuse before beginning resistance measurements. To test the dimmer, disconnect it from the circuit. Fig. 2-06 TL623f206c Resistanceinaseriescircuit- Tomakeresistancemeasurements: w Removepowerfromthecircuit(turnitofforpullthecircuitfuse). w Isolatecomponentstobetestedfromtherestofthecircuit (disconnectorremovethecomponent). w Testsuspectcomponentsoneatatime. Intheseriescircuitabove,isolatethedimmerforresistancetesting. w Resistancevariesasthedimmerknobturns. w ResistanceishighestwiththedimmerturnedallthewaytoDim.“ w ResistanceislowestwiththedimmerturnedallthewaytoBright.“ EXAMPLEElectrical Circuits Electrical Circuit Diagnosis - Course 623 2-9 Open Circuit This open circuit between the dimmer and the lamp means the lamp does not operate at all (a break in the current path). Fig. 2-07 TL623f207 Opencircuit - Anybreak(open)inthecurrentpathofaseriescircuit makesthewholecircuitinoperative.Opencircuitscanbecausedby: w Brokenorlooseconnections w Cutwire w FaultycomponentSection 2 2-10 TOYOTA Technical Training Find an Open Circuit Look for an open circuit by testing for voltage in the circuit. Start with the point closest to the power source (battery) and move toward the circuit ground. Fig. 2-08 T623f208c Testingforavailablevoltage- Findthefaultinanopencircuitby testingforavailablevoltage. w Beginatthefuse. w Workyourwaypointbypointtowardthecircuitground. w Proceeduntilyoufindapointwherevoltageisnolongerpresent. w Theopencircuitisbetweenyourlasttwotestpoints.Electrical Circuits Electrical Circuit Diagnosis - Course 623 2-11 Split - Half Method Circuits with easy access to components can use the split-half method to isolate the problem. Fig. 2-09 TL623f209c Split-HalfMethod- Youcanusethesplit-halfmethodoncircuits whereaccesstotherelatedcomponentsisgood.Thesplit-halfmethod worksasfollows: w Locatethemiddleareaofthecircuitthathastheproblem. w Determineifthesource(battery+)orgroundsideofthatsectionof thecircuitisbadbythefollowing: - Checkforavailablevoltageonthesourceside. - Checkforcontinuitytogroundonthegroundside. w Splitthebadsectionyoufoundinstep2inhalfandrepeatthe sametests. w Continuesplittingthecircuitintosmallerhalvesrepeatingsteps2 and3untilyouisolatethecauseoftheproblem.Section 2 2-12 TOYOTA Technical Training Continuity Check to Find an Open Circuit Look for an open circuit by testing for continuity. In a logical sequence, check individual segments of the circuit. Fig. 2-10 T623f210c Testingforcontinuity- Thepreferredmethodoftestingacircuitis withpowerappliedandcheckingforvoltagedrop. Whenthatisnotpossible,findthefaultinanopencircuitbytesting forcontinuityasfollows: w Removepowerfromthecircuit(turnitofforpullthecircuitfuse). w Refertothewiringdiagramtochooseindividualsectionsofthe circuitforcontinuitychecks. w UseaDMMtocheckeachsection.Isolatecomponentsandsections asneeded(bydisconnectingorremovingwiresorcomponents). w Proceeduntilyoufindasectionthatdoesnotshowcontinuity(very highresistance).Theopencircuitwillbeinthatsection.Electrical Circuits Electrical Circuit Diagnosis - Course 623 2-13 Short Circuit The short circuit shown in this diagram is before the load. It provides an unwanted path for current to flow to ground. In most cases, a short like this increases current so much that it blows the circuit fuse. Fig. 2-11 TL623f211c Shortcircuit- Ashortcircuitisafaultinthecurrentpath.Ashort canbe: w anunwantedpathbetweentwopartsofacircuit. w anunwantedpathbetweenpartofacircuitandground. w anunwantedcurrentpathinsideacomponent. w anunwantedpathbetweentwoseparatecircuits. Excessivecurrent- Shortcircuitsmaycauseexcessivecurrent. w Thistypicallyblowsthecircuitfuse. w Itmaynotbepossibletotroubleshootthecircuitunderpower. Isolateashortcircuit- Toisolateashortcircuit,disconnectsections orcomponentsofthecircuitoneatatime. w Refertotheelectricalwiringdiagramtodeterminealogical sequenceoftesting. w Usecontinuitycheckstofindandisolateunwantedcurrentpaths.Section 2 2-14 TOYOTA Technical Training Isolating a Short Circuit You can troubleshoot a short circuit with continuity checks, or you can use a sealed beam headlight in the isolation method shown here. Fig. 2-12 TL623f212c Isolatingashortcircuit- Circuitbreakersandshortdetectorsmay damagesomecircuits.Thefollowingmethodworkswellforlocating mostshortcircuits: w Removetherelatedfuse. w Jumperinasealedbeamheadlighttothefuseconnections(the headlightbecomestheloadinthecircuitallowingyoutoisolatethe areawiththeshort). w Applypowertothecircuitandtheheadlightwillilluminate. w Isolatesectionsofthecircuituntiltheheadlightturnsoff.This pinpointswhatsectionofthecircuittheshortisin. w Inspectthatsectionofthecircuittolocatethecauseoftheshort. w Repairthecauseoftheshort. w Removetheheadlampandreinstallthefuse. w Verifypropercircuitoperation.Electrical Circuits Electrical Circuit Diagnosis - Course 623 2-15 Parallel Circuit In this diagram, each lamp is in its own parallel branch of the circuit. This makes it possible for one lamp to operate while the other is inoperative. Fig. 2-13 TL623f213 Aparallelcircuithasthesekeyfeatures: w Totalcurrentequalsthesumofthebranchcurrents. w Resistanceofeachbranchdeterminesthecurrentthrougheach branch. w Ifthebranchresistancesarethesame,branchcurrentswillbethe same. w Ifthebranchresistancesaredifferent,thecurrentineachbranch willbedifferent. w Thevoltagedropacrosseachloadresistanceisthesame.Thisis becausethesourcevoltageisappliedequallytoeachbranch. w Theequivalentresistanceofthecircuitislessthanthesmallest branchresistance. Parallelcircuitoperation- Thecircuitshownaboveresemblesan automotivebrakelightcircuit. w Whentheswitchisopen,voltageisappliedtotheopencontactof theswitch.Nocurrentflows. w Whentheswitchisclosed,currentflowsthroughtheswitchand bothlampstoground.Thelampslight. Key FeaturesSection 2 2-16 TOYOTA Technical Training Parallel Circuit A parallel circuit has a source, protection device, loads with dedicated current path, control device and ground. Fig. 2-14 TL623f214 Aparallelcircuitcontainsalltheelementsofaseriescircuit: w Powersource w Protectiondevice w Load w Controldevice w Ground However,aparallelcircuithasmorethanonepathforcurrent.It typicallyhastwoormoreloads,anditmayhavemultiplecontrol devices. Thecircuitloadsareconnectedinparallelpathscalledbranches.“ Eachbranchoperatesindependentlyoftheothers.Inaparallelcircuit, itispossibleforoneloadtobeinoperativewhileotherloadscontinueto operate. Parallel Circuit ElementsElectrical Circuits Electrical Circuit Diagnosis - Course 623 2-17 Ohm’s law in Parallel Circuits You can use Ohm’s law to predict circuit behavior. Total resistance is less than the smallest branch resistance. Voltage drop in each branch equals source voltage. Fig. 2-15 TL623f215 ApplyingOhm’sLaw- YoucanuseOhm’sLawtopredictthe behaviorofelectricityinacircuit. Forparallelcircuits,applyOhm’sLawasfollows: w Thetotal(orequivalent)resistance(R)islessthanthesmallest branchresistance. R T = R 1 xR 2 R T = R 1 +R 2 - Whenyouaddabranchresistancetoaparallelcircuit,the equivalentresistanceofthecircuitdecreases. - Whenyouremoveabranch,theequivalentresistanceincreases. w Voltagedropacrosseachbranchinthecircuitisthesame.Section 2 2-18 TOYOTA Technical Training UseOhm’sLawtotroubleshootcircuits: w Ifthereisanopencircuitinoneormoreofthebranches,the increasedequivalentresistancewillreducecurrent. w Increasingresistanceinonebranchmayaffectonlythecomponent operationinthatbranch.However,iftheresistancegoeshigh enoughtocreateanopencircuit,thecircuiteffectivelylosesa branch.Inthatcase,equivalentresistanceincreasesandcurrent decreasesfortheentirecircuit. w Increasedresistanceintheseriessegmentofthecircuitcanalso reducecurrent.Lowsourcevoltagecanalsoreducecurrent. w Asinseriescircuits,highsourcevoltageorashortcircuitto groundbeforetheloadcanincreasecurrent,blowfuses,and damagecomponents.Electrical Circuits Electrical Circuit Diagnosis - Course 623 2-19 Current in Parallel Circuits Total current in the circuit equals the sum of current in each branch. Fig. 2-16 TL623f216c Current- Currentinaparallelcircuitbehavesdifferentlythanitdoes inaseriescircuit. w Currentthroughthefuseandtheswitchisthesame. Currentthroughthelampsissplit. w Ifthelampshaveequalresistance,currentthroughthelampsis identical. w Ifthelampshaveunequalresistance,thelampwithlower resistanceconductsmorecurrentthanthelampwithhigher resistance. w Ifonelampfails,theotherlampwillstillworkandconductthe sameamountofcurrentasbefore. w Totalcurrentinthecircuitdoeschangewhenone
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