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Old 02-05-2005, 07:03 PM   #1
Lachlan
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Default Ohm's law guru's: Help me pick the proper resistor

If I want to stick an LED with 2.5V max potential rating on it into a hole where a 12v light bulb went, what resistor do I need? I'm not sure what the current is, this is for the tiny little fog/cruise type button switches.
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Old 02-05-2005, 07:13 PM   #2
Scoobie Steve
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A 1k ohm will work fine.
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Old 02-05-2005, 07:35 PM   #3
nhluhr
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use this to calc your own resistors:
http://ourworld.compuserve.com/homep...Bowden/led.htm

Keep in mind though that when the vehicle is runner, the alternator keeps the voltage a 13.8-14.2 and not 12V. If you can't find a resistor of the exact value calculated, always use a higher value to be sure you don't overvoltage your LEDs.
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Old 02-05-2005, 07:49 PM   #4
Lachlan
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Thanks, but the problem is I'm not sure what the current is through the illumination stuff.
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Old 02-05-2005, 09:10 PM   #5
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OK.

Assume 14.2v max.
Assume 15mA max for the LED.

V=IxR
I=V/R
R=V/I
14.2v - 2.5v = 11.7v (you need to drop 11.7v through the resistor to get your 2.5v forward voltage for the LED.

So...

R=11.7/0.015
R=780 ohms

Find the closest match to a 780 ohm resistor.

Power rating...

P=I^2 x R
P=0.015 x 0.015 x 780
P=0.1755W

You will need a 1/4W resistor (1/4W = 0.250W)
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