Electrical math help please

[Huw Powell]

I would recommend trying a larger pair of resistors first, start at 
maybe 10 ohms minimum.

The reason is that if you match the bulb resistance, you are going to 
forever be wasting that much current/power through them.

There's a decent chance that the module "compares" the current through 
each leg, rather than absolutely expecting the 4 amps or so the bulbs 
normally draw.  So if you could get away with wasting, say, half an amp 
(25 ohm resistors) through each side, you'd be better off - and make 
less heat, too.

Drawing the full current would be literally like leaving a 100w light 
bulb on in an empty room any time your headlights are on.

One more thing - especially if you end up stuck with running the full 
50-odd watts through the resistors - put them somewhere their heat can 
be dissipated safely.

LL - NY wrote:
> Since V = IR
> 
> 
> R = V/I
> 
> 
> 
> and P = VI
> 
> I = P/V
> 
> 
> substitute in and we get R = V^2/P  or R = 12V^2/55W = 144V^2/55W = 2.6 Ohms
> per lamp.
> 
> I'd venture if you could find any generic 2 - 3 ohm minimum 24V rated
> resistor (it's going to need to be able to handle 55W's though) it'll not
> only work, but work for a long time. Any lesser Power rated resistor will
> eventually fail. (I don't think I've ever seen retail resistors that are
> rated for lesser voltages, so I'd not worry about breakover voltage, and any
> computer circuit board rated 3 or 5 V resistors wouldn't be capable of
> handling +55W.
> 
> LL - NY a.k.a. the Physics Teacher.
> 
> On 11/4/08, Grant Lenahan <glenahan at vfemail.net> wrote:
>> bulb watts = volts x amps
>> so amps = watts / volts
>>
>> now you have amps
>>
>> set resistor such that volts / ohms = amps  --or-- ohms = volts / amps
>>
>> Grant
>>
>> On Nov 3, 2008, at 10:14 PM, Tom Love wrote:
>>
>>> I built a relayed headlight harness and now it seems I need to fool
>>> the auto check system into thinking there are still bulbs
>>> connected. Using a resistor seems to be the way to go but  how do I
>>> figure the size to fool the system into thinking the bulbs are
>>> still connected? TIA
>>> _______________________________________________
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>>
>> Grant Lenahan
>> glenahan at vfemail.net
>> (201) 602-4702 mobile
>>
>>
>>
>>
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-- 
Huw Powell

http://www.humanspeakers.com/audi

http://www.humanthoughts.org/