Tutorial

7/27/2015 UnregulatedPowerSupplyTutorialSparkFunElectronics
https://www.sparkfun.com/tutorials/103 1/9


SkillLevel:Beginner
UnregulatedPowerSupplyTutorial
bya1ronzo|August08,2008|23comments
Intro
Hey!Whyismy9Vwallwartoutputting14V?!
Wehavewrittenthistutorialtoshowyoutheinnardsofawallwartandexplainwhyyoucansometimes
measureahighervoltagethanwhatisspecifiedonthelabelofyourwallwart.
Onanunregulatedpowersupply,wallwart,ACadapter,powerbrick,orwhateveryouwanttocallit,you
usuallyseetwospecifications:
INPUT:AC120V60Hz6W
OUTPUT:DC6V300mA
TheinputspecisstraightforwardaslongasyouareintheUSusingpowerfromaregularwalloutlet.
Whatabouttheoutput?
Ifyoumeasuretheopencircuitoutputwithamultimeteronadevicethatspecstheoutputasshown
above,whatdoyouthinkyoushouldget?
DumbAnswer:about6Vandatleast300mA.Right?......WRONG!
Actually,youwilloftengetamuchhighervoltage.Thehighervoltagecanwreakhavocwithyoursystem
ifyouarenotplanningforit.Thisbehaviorfromanunregulatedsupplyisnormalandinthistutorialyou
willfindoutwhytheoutputspecisnowhereneartheactualopencircuitoutput.Inaddition,Iwillshow
howunregulatedthesupplyreallyis.TheoutputsaysDC,butinsomecircumstances,theoutputcanbe
farfrompureDC.