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Old 02-15-2007, 02:29 AM   #1
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Spot Lamps


I have two 55w spots on the front of my car, I would like to rewire them so they sit on reduced power like a side light when not on main beam. Do I use a resistor in series/parallel and if so what size?

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Old 02-19-2007, 12:03 PM   #2
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Spot Lamps


does your vehicle have daytime running lights? Either running lights or headlights come on with ignition?
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Old 02-19-2007, 06:13 PM   #3
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good 'ol ohm's law...

wattage = current * voltage
Voltage = current * resistance

cars are 12vdc.
current (amps) is dependant on the load (resistance) so you can't change that directly
all you can really do is play with the resistance values (load 'er down to reduce current!)

so, 55W lights will draw 4.5 amps, and be a 2.6ohm load.

You can play with the figures and sub them back into the two above equations and recalculate current, wattage, and resistance.

your resistor will have to be a big one, that sucker will get red hot too... again, just use ohms law. once you figure out what resistance you need, you'll have to determine the power it can take before it burns up, which is of course wattage.... current * voltage, its gonna be around 50watts or so...

yes that's a hugggee power rating on a resistor, why? because incadescent light bulbs are a waste! it's pretty much like a dead short, thats why it glows red hot and emits light! something like 90% of the energy turns into heat and is wasted, just 10% or so is visible light


why not just get lower rated bulbs?


(unrelated)....
an x-ray machine is only about 1% efficient in producing x-rays . you don't see many energy-star compliant x-ray machines!

Last edited by elementx440; 02-19-2007 at 06:23 PM.
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Old 02-20-2007, 02:16 AM   #4
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Thanks for you help - You know what it's like I got an Idea but it sounds like it's not worth the trouble.

Cheers.
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