# Thread: Proper resistor to reduce 24v DC to 12v DC

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## Proper resistor to reduce 24v DC to 12v DC

I know that this is an easy question for the electrical gurus on this board, and I knew how to calculate this at one time but can no longer remember. I have a 24v DC power supply that I am using to power a small gear motor. I have a PWM speed controler to vary the speed. Since this is mounted in a small aluminum project box, I find that the unit is building up heat inside the box. I want to take a 12 volt fan pulled out of a PC to cool the unit and so need to drop the voltage on this leg to 12 volt. What resistor will I need, or is this more complicated than I think? Thanks for your help

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Resistors limit current, not voltage.

You need a voltage regulator or dc/dc converter to do the job properly.Use a 7812 voltage regulator. There are schematics how to wire online. Very simple.

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The resistance across the motor will not usually be able to be used with modern fan motors due to the technology used, run the fan on 12vdc, off of a PC P.S. connector for example and measure the current, then calculate a resistor value from R=E/I, where E=12v & I is your measured current.
The voltage drop across the resistor will depend on the current.
Or You could always run two fans in series!
Max.
Last edited by MaxHeadRoom; 08-20-2012 at 04:36 PM.

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That would be fine but he has a PWM speed controller he wants to use. So nothing is going to be a constant there.

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I assumed he meant the 24v supply FOR the PWM (before the PWM!)?
Max.
Last edited by MaxHeadRoom; 08-20-2012 at 08:03 PM.

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Thanks for the replies guys. I was planning to run fan off of another set of leads from the power supply seperate from the PWM. I guess that it will be easier to just scrounge up a 12v wall transformer that I should have laying around.

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The resistor would be cheaper but maybe not so efficient, although those fans don't draw much usually, plus not all that critical..
Max.

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Originally Posted by ftownroe
Thanks for the replies guys. I was planning to run fan off of another set of leads from the power supply seperate from the PWM. I guess that it will be easier to just scrounge up a 12v wall transformer that I should have laying around.
Maybe.
Post the current requirement of the fan and you'll get an answer on the resistor quickly.

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I checked a couple of new ones I had on the shelf, and they show 12v @ just under .3amps, this would mean a 40ohm 3.6w (5w) resistor, so for these it would be rather inefficient, if you have the room, two fans in series may be the answer?
Max.

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I'd go for the two fans in series suggestion, but you will want to get fans that draw the same current (or very close), since the same amount of current will be going through each. Dead PC power supplies are plentiful, and I've yet to see a non-working fan in one. The higher end ones even have two fans. I have a number of them (the fans that is) in my junk box.

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