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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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JustXtreme (author)2010-02-02

 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. 

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frollard (author)2010-01-25

forward power 80mw
forward current = 24mA

divide = 3.33 volts.

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steveastrouk (author)frollard2010-01-25

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

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frollard (author)steveastrouk2010-01-25

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

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steveastrouk (author)frollard2010-01-25

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

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frollard (author)steveastrouk2010-01-25

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


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frollard (author)2010-01-25

...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.

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steveastrouk (author)frollard2010-01-25

Oops. Picked wrong line !

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lemonie (author)2010-01-25

We're writing a FAQ for questions like this. Please look at this page and see if it answers the question
www.instructables.com/id/EG07LH2G4PBSPE2/step2/Questions-on-the-subject-of-LEDs-and-resistors/

L

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