新型电子课件第9章triac.ppt

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1、CHAPTERSIXTHETRIAC6.1introductionoftheTriacthetriacisadeviceverysimilarinoperationtotheSCR.WhenanSCRisconnectedintoanACcircuit,theoutputvoltageofSCRisrectifiedtodirectcurrent.thetriacisdesignedtoconductonbothhalvesoftheACwaveform.Theoutputofthetriacisalternatingcurrentinsteadofdirectcurrent.双向可控硅2

2、.theSchematicsymbolofaTriacTheschematicsymbolTheequivalentcircuitThetriacismadeliketwoSCRsconnectedinparallelfacinginoppositedirectionswiththeirgateleadstiedtogether.3.theoperationofTriac(1)NoticethattheschematicsymbolforthetriacissimilartotheconnectionofthetwoSCRs.Thegatemustbeconnectedtothesamep

3、olarityasMT2toturnthetriacon.(2)WhenthevoltageappliedtoMT2increasesinthepositivedirection,thegatefiresthehalfofthetriacwhichisforwardbiasedduringthathalfofthecycle.(3)SincetheotherSCRhalfofthetriacisreversebiasedduringthathalfcycle,itcannotbefired.(4)WhentheappliedACvoltagebecomesnegativeatMT2,the

4、gatefirestheotherhalfofthetriacwhichisnowforwardbiased.4.AddingadiodeIfadiodeisconnectedinserieswiththegate,itpermitsonlyhalfoftheACcycletoprovidegatecurrent:assumethepositivehalfcycle.Ifanoscilloscopeisconnectedacrosstheloadresistor,onlythepositivehalfoftheACwaveformisseen.Thediodeisreversebiased

5、duringthenegativehalfcycleandnogatecurrentflowstoturnthenegativehalfofthetriacon.Ifadiodeisremovedfromthecircuitandreplacedfacingintheoppositedirection,itblocksthepositivehalfcyclesandpassesthenegativehalf.TheoscilloscopeconnectedacrosstheloadresistorshowsthenegativehalfoftheACwaveform.Thisisagood

6、exampleofthetwoSCRarrangementofthetriac.5.Addingavariableresistor(1)Ifavariableresistorisusedtocontrolthegateofthetriac,thesamecontrolproblemsexistasdidwiththeSCR.(2)Asimpleresistorcontrol,asshownbelow,permitsthetriactocontrolonlyhalfofthewaveformjustlikeanSCRwhichhasnotbeenphaseshifted.MT1MT2◆bot

7、hhalvesoftheACcycleprovidegatecurrent◆bothhalvesoftheACwaveformcanbeseenacrosstheload.◆thegatecurrentislockedinphasewiththevoltageappliedtotheMT1andMT2◆thetriaccannotbefiredaftertheACvoltagereached90°and270°6.Tes