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help with resistor for 24v->12v

Hi All, I drive an old unimog with 24v electrics and would like to put some better/bigger/brighter tail lights on it. The 24v stuff isn't easily available locally so i'd like to use 12v stuff. Can someone throw me a formula to determine what size resisor to use to be able to do this. I know 24v stuff would be better, and LEDs better still, but limited budget and local availability are important. I don't have the wattage of the lamps in front of me but if somebody could help me out with the formula I'm sure I can get one of my kids to do the ciphering. thanks in advance, doug

Reply to
doug
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Google for "24 to 12 convertor" (no quotes). One of them which looks nice:

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Reply to
Sjouke Burry

If a lamp is 40W at 12V you need a 40W resistor to drop 12V.

W = I * V so 40 = I * 12 thus I = 40 / 12 = 3.333A

V = I * R so 12 = 3.333 * R or R = 12/3.333 = 3.6 Ohm

But why not use two identical lamps in series? Simple and no calculations.

Reply to
Homer J Simpson

Have you tried truck shops? Big rigs often use 24 volts. Or as someone else suggests use 2 identical lamps in series because you have to waste that much power and a resistor would just make a lot of heat whereas a lamp is made for that much dissipation.

--
John G

Wot\'s Your Real Problem?
Reply to
John G

one way is to use two similar 12V lamps wired in series.

Are you really worried about people not seeing you in a unimog?

If you use resistors they'll get real hot.

for 21w lamps you'll need 1.46 ohm resistor (1.5 is close enough)

The formula is R=144/W

the power ("wattage") rating of the resistor should exceed that of the 12V lamp

Bye. Jasen

Reply to
jasen

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