Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Resistor Help Needed


JWW

#28598

Resistor Help Needed | 13 May, 2004

Hi Everyone,

I want to use LEDs in an enclosed box for edge lighting etched glass. There are 15 LEDs in a box and they are set up in 5 series of 3 LEDs. The LEDs are 3.5V 20Ma 8000mcd. I've worked out that I need an 82 ohm resistor for each series of three LEDs. My question is this. Will an 82 ohm 1/4W carbon film resistor be big enough without overheating and breaking down in an small enclosed area or will I need a 1/2W or 1W resistor? I'd prefer to use 1/4W resistors if I can get away with it safely.

Thanks for your help.

John

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Dilbert

#28690

Resistor Help Needed | 20 May, 2004

Three LEDs, 3.5volts @.02amps each is 10.5volts. You said you calculated out an 82 ohm resistor so 82 ohms * .02amps is 1.64volts, I guess your going to use a 12volt supply.

P=I*E =.02amps * 1.64volts =.033watts so a .25watt resistor will be fine for a 12v supply

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JWW

#28700

Resistor Help Needed | 20 May, 2004

Hi Dilbert,

Thanks for the reply and information. I forgot to mention I'll be using them with a 12V power supply.

Thanks again.

John

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