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# what type of resistor should i use if i have 3 of these LED's in a series with a 12v batter? Answered

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Hello All,
Thank you for your responses, i used a 100 ohm 1/4 watt resistor.
i'm using a 12v battery.
It all seems to be working so far.
Thank You & Go Instructables!
Kinda sounds like a good cartoon superhero group.

forward power 80mw
forward current = 24mA

divide = 3.33 volts.

That was maximum power dissipation, which from your previous comment you would have to infer was at 75mA forward current, so Vf= 80/75 = 1 ish volt....

Too low IMHO - call it 1.2 V instead ?

Steve

ah, touche.  Very few leds are that low...infrared?

Again, like you say - all depends on the voltage :D

b3v-250c googled gives me a first result of a 3-led array:
http://www.bluemelon.com/measureexplorer/me-led-b3v-250c;jsessionid=9F6B2ACAD2F633CED212BB5C38A8436F#photo-92108/T800600

You don't think this could have a resistor in it already do you ?

the 3 pack I found already does surely...otherwise it wouldn't be rated for 12v...

...and you don't wanna run them at 75mA or they will be sad very fast.  12- (3*3.33) = 2 volt drop / .024

83.3 "or next higher' ohms resistor.

it will draw 2v * .024a = .048Watts, or 1/20th watt.  A very small power resistor would do.