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Posted by D Sherman [72.47.9.228] on Wednesday, June 08, 2011 at 22:49:07 :
In Reply to: blackout light voltage help - david sherman? posted by mannyc [173.77.206.116] on Wednesday, June 08, 2011 at 20:40:01 :
What's wrong with the original resistor? Otherwise, if you have to pick a new resistor, hook the bulb up to 6 volt, measure how many amps it draws, and then divide 6 volts by that number of amps to get the resistance, which is also the resistance you'll need to put in-line with it to drop 12 volts to 6 volt. For example, if it draws 2 amps, you'll need 6/2=3 ohms. The catch is that just any old resistor won't work. It needs to handle some watts (heat). Watts is volts times amps, so if it draws 2 amps at 6 volt, that's 12 watts. Better pick a 20 or 25 watt resistor to be on the safe side. I would rummage one out of my junk pile, but this isn't the sort of thing a guy can just go buy at radio shack. The only easy-to-find power resistors are ignition ballast resistors. You might be able to find one that's close, though generally they are more like one ohm. Another thing you might consider, if it's the same wattage as a 6 volt brake or turn signal bulb, is to simply use such a bulb, hidden away somewhere, as the "dropping resistor".
The main thing thing, no matter what you do, is to not hook it up to any more than rated voltage. The lifetime of a light bulb goes down very quickly with increased voltage, and those sealed beam blackout headlight bulbs are getting very scarce and expensive. The Army finally did what it should have done 50 years ago and now has a blackout headlight assembly that takes an ordinary tail light bulb inside of it. They look fine on an M35 or HMMWV, but they're obviously wrong for WWII vehicles.
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