Data Sheet

6/30/2015 LightemittingDiodes(LEDs)learn.sparkfun.com
https://learn.sparkfun.com/tutorials/lightemittingdiodesleds?_ga=1.43468261.1900427809.1434378181 12/13
ForwardVoltageDrop
Oh,Ialsopromisedthatwe’dtalkabouttheconceptofForwardVoltageDrop.Rememberwhenwe
werelookingatthedatasheetandImentionedthattheForwardVoltageofallofyourLEDsadded
togethercan’texceedyoursystemvoltage?Thisisbecauseeverycomponentinyourcircuithasto
sharethevoltage,andtheamountofvoltagethateverypartusestogetherwillalwaysequalthe
amountthat’savailable.ThisiscalledKirchhoff’sVoltageLaw.Soifyouhavea5Vpowersupplyand
eachofyourLEDshaveaforwardvoltagedropof2.4Vthenyoucan’tpowermorethantwoatatime.
Kirchhoff’sLawsalsocomeinhandywhenyouwanttoapproximatethevoltageacrossagivenpart
basedontheForwardVoltageofotherparts.Forinstance,intheexampleIjustgavethere’sa5V
supplyand2LEDswitha2.4VForwardVoltageDropeach.Ofcoursewewouldwanttoincludea
currentlimitingresistor,right?Howwouldyoufindoutthevoltageacrossthatresistor?It’seasy:
5(SystemVoltage)=2.4(LED1)+2.4(LED2)+Resistor
5=4.8+Resistor
Resistor=54.8
Resistor=0.2
Sothereis.2Vacrosstheresistor!Thisisasimplifiedexampleanditisn’talwaysthiseasy,but
hopefullythisgivesyouanideaofwhyForwardVoltageDropisimportant.Usingthevoltagenumber
youderivefromKirchhoff’sLawsyoucanalsodothingslikedeterminethecurrentacrossa
componentusingOhm’sLaw.Inshort,youwantyoursystemvoltageequaltotheexpected
forwardvoltageofyourcombinedcircuitcomponents.
ResourcesandGoingFurther
You’vemadeit!Youknow,like,almosteverything…aboutLEDs.NowgoforthandputLEDson
whateveryouplease!
Ifyou’dliketolearnmoreaboutsomeLEDrelatedtopics,visittheseothertutorials:
Light
IRCommunication
RGBPanelHookupGuide
DasBlinkenTopHat
LEDDisplayDriverHookupGuide
InteractiveHangingLEDArray–CreateagiantLEDarraydrivenbytheArduinoProMini.
Andnow…adramaticreenactmentofanLEDbeingoverpoweredandburningitselfout: