Tutorial

7/27/2015 UnregulatedPowerSupplyTutorialSparkFunElectronics
https://www.sparkfun.com/tutorials/103 4/9
FromthisyougetanKVLloopof:
V =V –(I *R )
V isconstant,asthereisaspecificvoltageinducedbythetransformer.However,theloadcurrentisnot
constant.
Anopencircuit(noload)measurementhasverylittlecurrentflowingandthepowersupplyinherently
hasasmallresistance(about4Ohms),sotheloadcurrent,I multipliedwiththeThevenin
resistance,R ,isclosetozeroandtheloadvoltageisnearlyequivalenttotheTheveninvoltage,V .You
getabout10Vfromtheaboveequation.
Letsayyoushortcircuitthepowersupplyoruseaverysmallresistanceload,abunchofcurrentwillnow
flowthroughthecircuit.Fromtheaboveequation,therewillbeasignificantvoltagedropdetectedonthe
loadvoltage.
Hereisachartofvaryingloadsonour6V/300mAexamplewallwartandtheirmeasuredpeakvoltage,
peakcurrent,peaktopeakripple,andpercentageofrippleineachoutput(youcanignoretheripple,for
now):
Atsmallloads(largeresistances),thereisnotasubstantialvoltagedrop,becausethereisnotmuch
currentflowing.
Ataverylargeload(smallresistance),thereisabunchofcurrentflowingandasignificantvoltagedrop.
Also,thepowersupplyisrunningwaypastitsratedlimitwhentheloadisbelow20Ohmsandthebox
thatplugsintothewallwillbegintoheatup.Atacertainpoint,thethermalcutoffwillbreakthe
connectionintheprimarycoilandnotuntiltheboxcoolswillthethermalcutoffallowcurrenttopass.
ButIhaven'tactuallyaddressedwhythevoltagedropsandwhatarethemechanismsthatcausethe
voltagedrop?
Intheaboveequation,ifyousubstitutethenumbersforthepowersupplyinregulation(witha20ohm
load)youwillget.
V =V –(I *R )=10V–(300mA*4Ohms)=8.8V
Yetwemeasure6.2V(seechartabove).Weknowthereisa1.2Vdrop(–I *R )fromthecopper
losses(resistanceinthecoil)andtheloadisdrawingthecurrent,butthereareothermechanismsthat
contributetotheremainingvoltagedrop.Thereisavoltagedropduetochangesinthemagneticfield
withinthetransformercalledironlosses.Theaboveequationcannowread:
V =V –copperlosses–ironlosses
Load 0 Load 0
0
Load,
0 0
Load 0 Load 0
Load 0
Load 0