relays

[M Wehrman]

Anyone know the size of resistor I would need to drop 24v to 12v. It's so I can use one of the many Bosch relays I have. Coil resitance reads .25 on the 2M scale. I think there are Members out there with this "secret" knowledge that can enlighten me!
Mark

[Jim LaGuardia]

Mark, here is a simple solution for 12v relay trip wire on 24v source. use 2 1000 ohm resistors.



(Drawings by Jim Mettler)

[M Wehrman]

Thanks Jim,just what I wanted!

[Jimm391730]

Ah, but the drawings I did for Jim L. were only for a low current signal for a brake controller, not for driving relays.

Your measured 0.25 k ohms should mean 250 ohms. On a 12V system, that relay would draw 12V / 250 ohms = 0.048 amps.

To get the same current through the relay when driven from 24 volts, we need this resistance: 24V / 0.048A = 500 ohms. Since the relay already has 250 ohms, we need about 250 more ohms in series to make 500 ohms.

The power of the resistor is calculated by: 0.048A x 0.048A x 250 ohms = 0.576 watts, so we should use a 1W resistor (and it will get hot when running for more than a minute).

Radio Shack has these 100 ohm resistors: Catalog #: 271-152 and you can put two in series for 200 ohms (probably close enough for the relay's needs). Radio Shack also has these 270 ohm resistors: Model: 1W127-10 | Catalog #: 55049605 but they may need to be ordered online, but it will take only one of these.

Have fun,

[M Wehrman]

####and it will get hot when running for more than a minute####

I want to use it for a fuel pump relay,I think I will just get the proper one and call it good! Thoughts are that if the resistor burned out,I have no go.I really didn't think there was that much math,Thank you! I think its was decimel points that get me,they are better suited for the end of this sentence. Thanks again,Mark

[Jimm391730]

"Hot" meaning to the touch, but well within its designed rating; that is why I said to get a 1W rated resistor. It really is only makng 1/2 a watt of heat.