4/26/2016 12Volt20AmpPowerSupply
12 Volt 20 Amp Power Supply
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Circuit: AndyCollinson
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Description
A12Volthighcurrent20Amppowersupply.Theoutputvoltageisvariablefrom12.2Voltto14.4V
socanbesetforanydevicerequiringvoltageandcurrentinthatrange.ThisPSUusesanLM723as
theregulator,4parallelconnectedoutboardpasstransistorsandhascurrentlimitingabove25
amps.
Notes
TheinputtransformerT1hasaprimaryratingof240VfortheUkdomesticsupply.ForNorth
Americatheprimaryneedstoberatedat120V.Thesecondarymustbecableofsupplying20VAC
RMSat25Amp.F1istheinputfuseandmustbeaslowblowtyperatedat4amp.Thesupplyis
rectifiedbyBR1,a35AmpbridgerectifiertypeMB356.C1andC2arethesmoothingcapacitors
andtheunregulatedDCvoltagewillnowbeabout26Vwithnoload.1
ThebulkoftheworkisperformedbyanLM723regulator.ThepresetRV1setstheoutputvoltage,
R1andR3settingupperandlowervoltagelimits.Theerrorsignalatpin10feedsadarlington
combinationmadewithQ1toQ5.TheBD131boostscurrentfromtheregulatoratpin10and
sourcesbasecurrenttotheparallelconnected2N3055powertransistors,Q2toQ5.Eachtransistor
suppliesupto5Ampsofcurrentintotheload.Asdeviceparametersvary,inparticular,forward
currentgainhFEofapowertransistor,R3toR6aidcurrentsharing.
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4/26/2016 12Volt20AmpPowerSupply
WithRV1atabout10%outputvoltageisabout12.3Volts.Theregulationisexcellent,thegraph
Pageissafe TotalSecurity2015
belowsimulatesavaryingloadresistanceof0.5to1measuredacrosstheoutput(Vs)inthe
circuitdiagram.Theoutputvoltageissolidat12.3Volt,evenat20Amp.Onlywhentheload
increasesto21Ampdoestheoutputstarttodropby50mV.
Currentregulationisalsobuiltintothecircuit.R7toR10areaparallelcombinationof0.1ohm5
wattpowerresistors.Theeffectiveresistorvalueis0.025ohmandisconnectedbacktocurrent
sensepins2and3oftheLM723.Oncethevoltageacrosspins2and3reaches0.6VolttheICwill
gointocurrentlimit,limitingoutputcurrent.Thisstartstohappenat23Amp,increaseinload
currentnowfurtherthecurrentlimitingoftheLM723.Loadregulationat14.4Visshownbelow.
Withprogressivelyhighercurrentstheoutputwillbereducedfurther,butthisdesignisfor20Amps
nothigher.C5andC4areforhighfrequencydecoupling,D7istopreventhighvoltagebackEMF
fromdamagingtheICandsemiconductors.LED1showsthesupplyishealthy,LED2showsthatthe
regulatedoutputisworking.
SimulationModel
IhavemadeasimulationmodelinLTspice2.Thesimulationcircuitdoesnotincludethe
transformerorbridgerectifierandthecircuitisshownbelow.
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4/26/2016 12Volt20AmpPowerSupply
Thefile12v20a.zip containsthecircuit,symbolsforLM723andpotentiometer,subcircuitsfor
theLM723andpotentiometermodelandspicedataforthesemiconductors.Oncedownloadedto
yourdesktoporhomefolderandunzipped,itshouldworkwithoutproblems.Youcanalsomeasure
individualdevicecurrentsandvoltagesormodifythecircuit.
1PowerSupplyDesign
2LTspicesimulatorsection
Circuit Exchange International Return to Power Circuits http: www.zen22142.zen.co.uk
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