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elec11 Electrical Circuit Theory.pdf

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ELEC11 ELECTRICAL CIRCUIT THEORY
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Body Electrical Diagnosis - Course L652 1 1. Reviewthebasicelectricalconceptsofvoltage,amperage,and resistance. 2. Reviewthecomponentsofabasicautomotiveelectricalcircuit. 3. Introducebasicwiringdiagramsymbols. 4. Reviewthecharacteristicsofvoltage,amperage,andresistancein seriesandparallelcircuits,showinghowtheyareusedwhen diagnosingelectricalproblems. 5. Showhowseries-parallelcircuitsareusedonthevehicle. 6. Introducethebasicelectricaldiagnostictools. Section 1 Electrical Circuit Theory Learning Objectives:Section 1 2 TOYOTA Technical TrainingElectrical Circuit Theory Body Electrical Diagnosis - Course L652 3 Thefirststepinbeingabletoeffectivelydiagnoseelectricalproblemsis tohaveagoodunderstandingofbasicelectricalprinciples.These conceptswerecoveredindepthintheL623course.Inthiscourse,we willbereviewingeachofthemwithemphasistotheiron-car applicationsandhowtheseconceptsareappliedwhenyouare diagnosinganelectricalproblem. Electrical Circuit Diagnosis Course L623 Course L623 covered the “basics” of electrical. This course will expand on those concepts and apply them directly to the on-car diagnostic process. Simplyput,thinkofvoltageas electricalpressure orpressure differential.Thedifferenceinpressurethatmakesanyhydraulicpump workisvirtuallythesameforelectrons.Thepressuredifferential providedbythepositiveandnegativeterminalsofthebatterycauses theelectronsinaconductortomovewhenthetwoterminalsare connectedtogether. Thismovementorflowofelectronsisusedtoperformusefulwork. Wheneverworkisdone,pressureisusedup.Wecanmeasurewherework isdoneinanelectricalcircuitbymeasuringwherevoltageisusedup. Voltage as Pressure Like the pressure/suction that a pump provides a fluid, a battery or alternator provides for an electrical circuit. Introduction VoltageSection 1 4 TOYOTA Technical Training Whenworkingwithon-carelectricalproblems,therearetwodifferent methodsofvoltagemeasurement.ATechniciancaneithermeasurean opencircuitvoltageoravoltagedrop. Opencircuitvoltage ismeasured whenthereisnocurrentflowthroughthecircuit.A voltagedrop is measureddynamicallywhilethereiscurrentflowthroughthecircuit. Bothopencircuitvoltageandvoltagedroptestinghavetheirplaceinthe diagnosticprocess.Informationfromeachmeasurementcanbehelpfulif usedappropriately.Thesetwomeasurementswillbediscussedinmore detailinthe ElectricalDiagnosticTools sectionofthiscourse. Measuring Open Circuit Voltage This measurement tells you if there is voltage present at a terminal. It cannot tell you if there is any circuit resistance up to that point. Measuring Voltage Drop This measurement can only be done if there is current flow in the circuit. It accurately tells you how much voltage is actually available at the load, or the amount of voltage lost across connections or wiring on either the ground or power side of the circuit.Electrical Circuit Theory Body Electrical Diagnosis - Course L652 5 Whenevercurrentpassesthroughanycomponent,voltageisusedup. ThevoltageusedupiscalledVoltageDrop( D V). is the Greek letter delta. The symbol delta means “change in”. When we use the abbreviated DV it indicates “change in voltage” or V oltage Drop. Thevoltagedrop( D V)ofacomponentisdirectlyproportionaltothe resistanceofthecomponent.Thegreatertheresistancethegreaterthe voltagedrop( D V).Lowresistancecomponentslikefuses,switches, wiresandconnectorsshouldhaveverylow D V.Asageneralrulethe maximum D Vallowedforthesecomponentsislessthan0.1Vper componentorconnector. Higherresistancecomponentsareusuallyreferredtoasloads.Loads usetheirresistancetoconvertcurrentintowork(light,heat,motion). Thisconversioncausesvoltagedrop( D V)astheelectricalpressureis usedup.Typicalloadsincludelamps,motors,relaycoilsandmost sensors.Voltagedrop( D V)isalwaysproportionaltoresistance.The highertheresistance,thehigherthe D V. Inanyelectricalcircuit,allthevoltagewillalwaysbeusedup.Adding upallthevoltagedropsinacircuitwillalwaysequalsourcevoltage. Currentisthetermusedtodescribethe flowofelectrons through thecircuit.Itisthisflowofelectronsthatdoesthework“inthe circuit.Theunitformeasuringtheamountofcurrentflowisthe AmpereorAmp(A).OneAmpequals628billionbillionelectronsper secondflowingthroughacircuit. Currentwillonlyflowifthereisacompletecircuitbetweenasourceof highervoltage(power)andalowervoltage(ground).Voltageisthe pressurethatpushestheelectronsthroughthecircuitandAmperesis ameasureofthenumberofelectronsflowing. Thecombinationofamperagewithvoltagedeterminestheamountof powerthatisbeingusedattheloadinthecircuit.Powerismeasured inWatts(W).Theamountofpowerthatisbeingusedbyaloadcanbe determinedbymultiplyingtheamperagethroughtheloadbythe voltagedropacrosstheload. Current Flow Measured in Amps (A) or milliamps (mA), current flow is caused when a voltage or pressure differential exists at both ends of a conductor. Voltage Drop NOTE Current FlowSection 1 6 TOYOTA Technical Training Measuring Amperage Note that the ammeter is connected in series. The circuit must be “broken” so that the meter could be placed “in-line” with the circuit. The wattage of the light bulb can be determined by multiplying the amperage and the voltage drop. Electrical resistancedescribeshowmuchsomethingopposescurrent flow.ThisoppositiontocurrentismeasuredinOhms( W )orin thousandsofOhms(k W ).Ineverycircuit,resistance regulatescurrent. Inaperfect“circuit,theonlyresistancewouldbetheloadyouare operatingwiththecircuit.Buteventhebestofconductors(materials whichallowtheflowofelectrons)haveacertainamountofelectrical resistance.Materialswhichhaveanextremelyhighresistanceare called insulators. Resistance Resistance is the opposition to current flow. Resistance determines the amount of current flow as long as the voltage stays constant. In theory, the load should be the only resistance in the circuit. ResistanceElectrical Circuit Theory Body Electrical Diagnosis - Course L652 7 Using an Ohmmeter When using an ohmmeter, disconnect the component from the circuit to isolate the measurement from other current flow paths or voltage sources. In this case, the ohmmeter does not measure the open circuit at the burned out light bulb. Inshort,Ohm’slawstatesthatcurrentinacircuitwillalwaysbe proportionaltothevoltageandresistancepresent.“Voltage,amperage, andresistanceinacircuitworkin proportiontoeachother . Mathematically,wecanalways predictwhatelectricityisgoingtodoin acircuit,aslongasweknowwhatanytwoofthethreevaluesare. So,forexample,ifyouknewwhatthevoltageandresistancewereina circuit,youcouldeasilydetermineexactlyhowmuchcurrentthere wouldbeinthecircuit. Ohm’s LawSection 1 8 TOYOTA Technical Training Ohm’s Law Equation The important fact about Ohm’s law is that electrical systems and electrical problems are predictable. There is no magic to how electricity works! Ohm’s Law Formula Basic Principles of Series/Parallel CircuitsElectrical Circuit Theory Body Electrical Diagnosis - Course L652 9 OneofthemoredifficultconceptsconcerningOhm’sLawhastodowith calculatingresistanceinacircuit.Seriescircuitresistanceisprobably theeasiestconcepttounderstandbecauseitissimplyanaddingupof alltheresistancesinacircuitinordertogetthetotalresistance. Figure1-11illustratesthemethod. Series Resistance Thetotalresistanceisthesumofallresistancesinthecircuit.This resistancewillaffectthenumberofampswhichcanflowthroughthe circuit.Ohm’sLawsaysthatampscanbefoundbydividingohmsinto voltage.Inthiscase:12Vdividedby4 W equals3A. InFigure1-12,wehaveaparallelresistanceruleatwork.Thetotal circuitresistancewillalwaysbelessthanthesmallestresistance.To findtotalresistanceyoumusttreateachbranchasanindividualseries circuit.12Vdividedby2 W equals6Acurrentperbranch.Addingthe branchcurrents,6Aplus6Aequals12Atotalcurrent.Dividing12Vby 12Aequalsatotalparallelcircuitresistanceof1 W Noticethatthis numberissmallerthanthesmallestbranchresistance. Parallel Resistance Series Resistance Parallel ResistanceSection 1 10 TOYOTA Technical Training Series/parallelcircuitsarethemostdifficultcalculation. Series/Parallel Resistance Thefirststepistocalculatethetotalresistanceoftheparallelportion ofthecircuit.Theformulais: Usingvaluesfromtheexample: Addingthetotalresistanceoftheparallelportion,2 W ohms,tothe2 W fromtheseriesportionequals4 W totalcircuitresistance. KnowingtheformulaswhichariseoutofOhm’sLawisnotnecessarily helpfulforrepairingautomobiles.Apersoncanknowtheory,butnot knowhowtoapplyitintherealworld.However,aknowledgeofthe relationshipsbetweentheseelementsisessentialtoaTechnician.You needtobeabletopredictwhat shouldbe asopposedto whatis ina problemvehicle.Wearegoingtousetheconceptofvoltagedropinthis classtomakethiskindofdiagnosisquickerandeasier.Later,asyou becomemorefamiliarwithelectricaltheoryandconcepts,theremaybe anopportunitytodomoreadvancedcalculationsliketheoneswehave discussedhere. Series/Parallel Resistance Why Does It Matter?Electrical Circuit Theory Body Electrical Diagnosis - Course L652 11 Themath“sideofOhm’sLawisimportantifwearedesigningacircuit. Butbecauseweareinthebusinessofrepairingelectricalproblems, whatweneedtoknowaboutOhm’slawcanbesummarized.Sincemost circuits(andbasicallyallbodyelectricalcircuits)workonwhatiscalled 12Vpower(whichisactuallymorelike12.6-13.6V)wewilllookat Ohm’slawwiththevoltageheldconstant.Assumingthisfixedvoltage, wecansummarizeOhm’slawas: Whenresistancegoesup,current goesdown.Whenresistancegoesdown,currentgoesup.“ Thisis theheartofOhm’slawwhenitcomestoservicingavehicle. KnowingtheprincipleofOhm’slawplaysanimportantrolewhen diagnosinganelectricalproblem.However,aTechnicianwillrarelyuse acalculatortofixanelectricalproblem.TheimportanceofOhm’slaw isthatitprovidesthefoundationforbeingableto understandand predicthowanelectricalcircuitwillrespond. w Ifacircuitis inoperative,andthereisnocurrentinthecircuit,it meansthatthereisaninfiniteamountofresistanceoran open somewhereinthecircuit. w Ifthecircuitis partiallyworking (suchaswhenabulbisdim), andtheloadisnotreceivingfullbatteryvoltage,itisprobablethat thereis excessiveresistance inthecircuit.Theexcessive resistancecanbelocatedeitherinthecircuititselforinseries somewhereinthecircuit.Or,thebatterymaynotbedelivering12V. Ifthebatteryvoltageislow,morethanonecircuitwillbeaffected. w Ifa20A fuseisblown ,weknowthatalotofcurrentmusthave beenflowinginthecircuit.Inordertogeta20Afusetoblowat12V, theremustbeverylittleresistance(0.6 W )inthecircuit.This conditioniscausedbytoomanyloadsconnectedtothefuse, aftermarketaccessoriesorapossible short-to-ground. Shorts-to-groundprovideanoorverylowresistancetopathto ground beforetheload ,allowingcurrenttoflowunregulated.In thiscase,theresistanceoftheloadhasbeenremoved. BecauseofOhm’slawtheactualcircuitdoesnotneedtooperatein orderforustoknowhowmuchcurrent(ifany)therewillbeorwhereit isgoingtoflow.Ohm’slawmakescurrentpredictable.Soinsteadof checking everycomponent,connectororwire onthevehicle,a Techniciancanusethe SystemCircuitDiagram todeterminewhere currentflowshouldbe,andwhichspecificareacouldbecausingthe problem.Thistypeofdiagnosiseliminatesunnecessarychecksduring therepairprocess. ItsavestheTechniciantime . On-Car Applications of Ohm’s Law HINTSection 1 12 TOYOTA Technical Training w Currentcanonlyflowwhenthereisacompletepathbetweenpower andground(orbetweenahighervoltageandalowervoltage).No completepath=open“circuit=0current. w Theresistanceoftheloadlimitscurrentinthecircuitandconverts currentintowork.Inaperfectcircuittheonlyresistancewouldbe theload. w Whenevercurrentflowsinacircuitvoltagedrops( D V)willhappen. Voltagedropshappensanywherethereisresistance.Thelargerthe resistancethelargerthevoltagedrop. w Everycircuitwilluseupallthesourcevoltage(total D Vwillalways equalsourcevoltage). Ina seriescircuit ,regardlessofthenumberofloadsinthecircuit, thereisonlyonepathforcurrent.Therearenotmanycircuitsonthe vehiclethatpreciselyfitthisdescription.TheLexusradiatorfan circuit,showninfig.1-14,isagoodexampleofatrue“seriescircuit. Ingeneral,thetermseries“isusedtodescribeany in-line connectionbetweenelectricalcomponents.Controlcomponentsinthe circuit,suchasaresistororaswitch,aswellas circuitproblems are alwaysinseries.Thetermseries“isalsousedwhendescribingthe connectionoftestequipment,suchasan ammeter. General Electrical Rules Series Circuit PrinciplesElectrical Circuit Theory Body Electrical Diagnosis - Course L652 13 An Application of Series Circuit Principles On a typical Lexus Electric Radiator Fan Circuit, the fan motors are connected in series to get a low speed. Note all of the connections, switches and relay contacts that are connected in series with each other. Reference the “Radiator Fan and Condenser Fan” circuit in the EWD, System Outline, “Low Speed Operation” for a complete description. Thevoltageprovidedbythebatteryorpowersourcewilldivide proportionallybetweenalloftheresistancesinthecircuit.Theresistor withthelargestvalue,relativetotheotherresistanceinthecircuit, willhavethelargestvoltagedrop.Ifwemeasureallofthevoltage dropsinthecircuitandaddthemtogether,wewillalwaysfindthatthe sumofalldropswillequalsourcevoltage. Theresistanceinaseriescircuit addup tothetotalcircuitresistance. Assumingafixed12Vpowersupply,thecurrentinthecircuitis determinedby totalcircuitresistance .Sincethereisonlyonepath, currentwillbethesameateverypointinthecircuit.Theplacementof excessiveresistanceinthecircuit,eitherbeforeoraftertheload,d
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