Hardware Reference
In-Depth Information
OHM'S LAW AND THE POWER EqUATION
ThemostimportantequationforanyelectricalengineertoknowisOhm's
law.Ohm'slawdictatestherelationshipbetweenvoltage(measuredinvolts),
current(measuredinamps),andresistance(measuredinohmsorΩ)inacir-
cuit.Acircuitisaclosedloopwithasourceofelectricalenergy(likea9Vbat-
tery)andaload(somethingtouseuptheenergy,likeanLED).Beforedelving
intothelaw,itisimportanttounderstandwhateachtermmeans,atleastata
basiclevel:
■
Voltage
representsthepotentialelectricaldifferencebetweentwopoints.
■
Current
lowsfromapointofhigherpotentialenergytolowerpotential
energy.Youcanthinkofcurrentasalowofwater,andvoltageaseleva-
tion.Water(orcurrent)alwayslowsfromhighelevation(highervolt-
age)tolowerelevation(ground,oralowervoltage).Current,likewater
inariver,willalwaysfollowthepathofleastresistanceinacircuit.
■
Resistance
,inthisanalogy,isrepresentativeofhoweasyitisforcur-
renttolow.Whenthewater(thecurrent)islowingthroughanarrow
pipe,lesscanpassthroughinthesameamountoftimeasthrougha
largerpipe.Thenarrowpipeisequivalenttoahighresistancevalue
becausethewaterwillhaveahardertimelowingthrough.Thewider
pipeisequivalenttoalowresistancevalue(likeawire)becausecur-
rentcanlowfreelythroughit.
Ohm'slawisdeinedasfollows:
V=IR
WhereVisVoltagedifferenceinvolts,IisCurrentinamps,andRisthe
Resistanceinohms.
Inacircuit,allvoltagegetsusedup,andeachcomponentoffersupsome
resistancethatlowersthevoltage.Knowingthis,theaboveequationcomes
inhandyforthingslikeiguringoutwhatresistorvaluetomatchupwithan
LED.LEDshaveapredeinedvoltagedropacrossthemandaredesignedto
operateataparticularcurrentvalue.Thelargerthecurrentthroughthe
LED,thebrightertheLEDglows,uptoalimit.ForthemostcommonLEDs,
themaximumcurrentdesignedtogothroughanLEDis20milliamps(amil-
liampis1/1000ofanampandistypicallyabbreviatedasmA).Thevoltage
dropacrossanLEDisdeinedinitsdatasheet.Acommonvalueisaround2V.
ConsidertheLEDcircuitshowninFigure2-3.
Figure 2-3:
Simple LED circuit
Continues
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