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
help with resistor for 24v->12v
Oct 23, 2006
4 Replies
Google for "24 to 12 convertor" (no quotes). One of them which looks nice:
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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.
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?
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
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