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APPLICATIONNOTEELECTRONICTRANSFORMERFORA12VHALOGENLAMPbyP.Fichera,R.Scollo1.INTRODUCTION·Itispossibletoincludeprotectionagainstshortcircuitofthelampfilament;Lightingthatuseshalogenlampsiscommonlyfoundinresidentialenvironments,whereitoffersawarm·Weightcanbereducedandtheconstructionmadelightincomparisonwiththatgeneratedbyfluorescentmorecompact;andtubes,andgreaterefficiencythanconventional·Acousticnoise(mainshum)iseliminated.filamentbulbs.2.THEELECTRONICSOLUTIONTheselampsareavailablewithvoltageratingsof6,12or24Volts,andsoatransformerisneededinThetopologyofthecircuitistheclassichalf-bridge.ordertoprovidethelampwithalowvoltagesupplyThecontrolcircuitcouldberealisedusinganIC(sofromeither110Va.c.or220Va.c.mains.Theyarefixingtheoperatingfrequency),butthereisamoregenerallyavailablewithpowerratingsof50,75,100economicalsolutionwhichconsistsofaself-and250Watts.oscillatingcircuitwherethetwotransistorsaredriveninopposingphasebyfeedbackfromtheoutputcircuit.The“electronic”transformerrepresentsanalternativemeansofpowerconversiontothemorestandard3.CIRCUITDESCRIPTIONironcore,50Hztransformertechnique.Thelinevoltageisrectifiedbythefull-bridgerectifier,Theadvantagesoftheelectronictransformergeneratingasemi-sinusoidalvoltageatdoublethecomparedwiththeclassicalsolutionare:linefrequency.Thediacstartstoconductduringeachcycle,startingthehalf-bridgeoscillation.The·Theoutputpowerfromtheelectronictransformerturn-onofthediaccanbevariedbymodifyingthetothelampcanbevaried,thusdimmingcontroltimeconstantoftheRCnetworkformedbyRandcanbeadded;Figure1.Electronictransformerfor12VHalogenLampRC2T'1TR1T1DC1C3T''1TR2AN528/09991/4APPLICATIONNOTEC1.Thiscanbevariedtoprovidealampdimming3.1ShortCircuitProtectionfeature.Figure2showsacircuitwithcircuitrytoprotectOncethecyclehasstarted,thediodeDmaintainsagainstashortcircuitoftheloadadded(note-somethevoltageacrossC1atavaluelessthanthediaccomponentshavebeenremovedforclarity).Theconductionthresholdvoltage,sothetransis
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