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3rd Gear Chirper
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I need to relocate some indicator lights. Is it okay to use LED's in place of regular lightbulbs in a computer-controlled circuit? Take the o/d off light for instance. It works with a lightbulb but if I use a LED instead, would there be too little resistance in the circuit which would then cause excessive current flow through the control circuit?

I doubt this, as LED flashlights work fine without resistors, but I figured I'd check before I let the smoke out of my PCM.
 

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To be safe, I would measure the resistance in the bulb then find a resistor (or resistors) to equal or come close (within 10% +/-). Either that or use 12vdc LEDs
 

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Use an led and a dropping resistor...

The led will have an intrinsic voltage; red ones are usually 1.7V, green is about 3V , and blue or white are about 5V. The datasheet for the led will have that voltage.

the car power runs 14.4 when the car is running, so you take that voltage, subtract the led voltage, and the resistor has to drop the rest...

14.4V-1.7V=12.7V (you can use several leds in series, just add the led voltage together.)

the Leds have a maximum current; usually around 20 milliamps, or .02A

12.7V/0.02A=635 ohms; http://www.digikey.com shows 634 ohm resistors are a 1% value, so

12.7V/634 ohms=.02003A; close enough for rock and roll...

If you use two leds in series, the formula is 14.4V-1.7V-1.7V=11V 11V/0.02A=550 ohms

The only problem you can run into is if the voltage regulator goes bad, leds wont last as long as the std bulbs will.
 
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