help with resistor for 24v->12v

[doug]

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

 

[Sjouke Burry]

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

Google for "24 to 12 convertor" (no quotes).
One of them which looks nice:
http://www.powerstream.com/dc1.htm

 

[Homer J Simpson]

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.

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.

 

[John G]

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

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.

 

[jasen]

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

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