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Old 10-11-2007, 01:12 AM   #1
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Cool Solution for intermittent short- light bulb as ballast resistor.

On a recent trip, I had a problem with an intermittent short in the running light circuit that I could not find, and needed to drive home in the dark. Running short on fuses, I hit upon an elegant solution: I wired a 21 watt brake light bulb in series at the umbilical connector. With the filament not glowing, the resistance is very low, and the LED tail lights and low wattage side lights ran near full brightness. As I bounced along the road and the short would occur, instead of blowing yet another fuse, the light on the tongue would glow, causing its resistance to increase so that the lights winked out but the fuse didn't blow; when the circuit un-shorted, the bulb on the toungue would go out, and the running lights would be back. A light bulb in series with a circuit works as a ballast resistor, meaning it increases its resistance with higher load. The wattage of the ballast bulb must be somewhat higher than the sum of all the lights in that circuit to allow close to normal voltage to the light when the short is not present.

This is more protective to the wiring than a circuit breaker, which allows full current until it cycles off. On a trailer circuit with long wire runs, there is a greater risk that the wires could get dangerously hot without cycling the breaker. This is especially true with higher amperage circuit breakers; cars come in to my repair shop all the time with too-big circuit breakers and melty-looking (but not burnt) wires.

I thought I'd pass it along, as it may help someone else out of a bind.
-SilvrSausage
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Old 10-11-2007, 07:57 AM   #2
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When in a bind you gotta' do what you gotta' do.

What a great post! I would never have thought of that.

Did you use an 1156 bulb or one of the newer "wire prong" lights?
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Old 10-11-2007, 08:35 AM   #3
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Now that was a moment of 'enlightenment'

I'll store that one in my bag of get home tricks. The closest I've come to that one was using a spare 12v battery to light one tail light after removing it from the circuit.
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Old 10-11-2007, 04:14 PM   #4
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Thumbs up

87, I used what I had, which was an 1156 bulb robbed from the backup light of my TV, plus the bulb holder which allowed me to use the spade connectors and wire in my toolbox. A 3157 or other wire type would have made the repair that much quicker. Any port in a storm, though, eh?
HiHo, I was fixin' to do just that with my loose deep cycle battery when I had that "flash" of inspiration. Thinking about it, it might have resulted in a fire after all, because I didn't have a fuseholder along. Either that, or it would have tipped in the trailer.
Regards,
-Silvrsausage
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