Tutorial

7/27/2015 UnregulatedPowerSupplyTutorialSparkFunElectronics
https://www.sparkfun.com/tutorials/103 3/9
Next,youhaveasinglephase,shelltypesplitbobbin(shellbobbin),stepdowntransformer.Noticethe
coilofwireintheprimarycoilisthinnerandcontainsmoreturnsthanthesecondarycoil(stepdown
transformer).
Afullwavecentertappedrectifierusingtwodiodes.ThesupplyalsohasmountingholesinthePCBfor
whatseemstobeabridgerectifierforhigheroutputvoltagesusingthesametransformer.Inaddition,
thereisasimplefiltercapacitortosmooththerectifiedsignal.
Whydoyouget10Voltsona6VDCsupply?
Inshort,whenyoumeasuretheopencircuitvoltageofthisexample6Vwallwart,yougetabout10V.In
thissituation,thereisverylittlecurrentflowingthroughthepowersupply'ssecondarycoilandthusnot
muchofavoltagedrop.Whenaloadoryourcircuitisattachedtotheoutputofthesupply,thereisa
voltagedrop.Thetransformerwillhaveavoltagedropfromtheopencircuit10V,accordingtoOhm's
Law.
Inamoredetailedapproach,thepowersupplywillideallyhaveaTheveninequivalentcircuit(avoltage
sourceandsomeoverallresistance),whichisattachedtoaloadresistorthatcausesavoltagedrop.
Hereisideallywhatthecircuitlookslikewithaloadattached: