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DuluthMedic

12 volt fan question

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I'd like to run 3 12 volt computer fans in my ice house. There called "muffin fans." I'd like to slow or control the speed of the fan. They are just too loud. I was told I needed a rheostat to slow the speed of the fan. I was also told to try an old switch from a car dashboard heater. They will all be hooked up to a boat battery. Any ideas?...I know I'm not the only one with these fans. Please help!

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Hmmm all the salvaged computer fans I've used were quiet enough as-is. Any way I think a regular 110V dimmer switch from Menards would work ok.

Another approach would to be to hook up the fans in series (+ wire of the first fan to the battery, - wire to the + wire of the 2nd fan, - wire of the 2nd to the plus of the 3rd and - of the 3rd back to the battery this would cut the speed of all three down to 1/3 (only 2 fans hooked in series would cut the speed of both in half).

The fan control from a car will work, the resistor block that goes with the switch is designed for a much heavier current draw motor than a muffin fan. What you'll get is full speed and some other speeds that may be perfect or still to fast.

A more elegant solution would be a variable resistor (AKA a potentiometer) from Radio shack or Axeman, but will require some calculations to get the right size. On the fan there should be a lable with the current draw of just the fan, probably in Amps and probably less than 1 (.05 to .3 is normal)some are listed in milliamps (.001 Amp) My example will use Amps. Divide 12 volts by the current listed on the fan let's say .1 amp and you get 120 which will be the resistance of the fan in ohms. The next step is to divide 13.2 volts (nominal battery voltage)by 120 ohms gives us .11 amps which we multiply 13.2 volts to get 1.452 watts. Potentiometers (Pots) will be rated for ohms and watts, you will need a pot with more watt rating than the fan and 1 to 3 or 4 times as much resistance - 1x the resistance will give roughly full to half fan speed. You wont find a 120 ohm pot but you might find a 500 of 1000 rated at 1.5 or better yet 2 watts. either of which would make this example fan variable from full down to probably stall speed. Run the power from the battery through a fan then throught the pot and back to the battery - Don't forget a fuse near the battery.

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ok...was it a regular 12 volt "light" dimmer switch? Or was it designed for a fan? Or won't it matter. I spoke with a person on hsolist about a dimmer switch they were selling and they said they hooked it up to a 12 volt fan and it got really hot really fast.

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I also wanted to be able to control the fan speed in my shack so I went with 120v fans and control them with a fan speed control that is normally used for a ceiling fan in a house. It works very well. I power the fans from an inverter.

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DuluthMedic, I think I'd have to agree with the person you spoke with. The suggestions of the house dimmer switch are good, BUT... those switches aren't heavy enough to handle the DC amps from the battery. I'd go to an Auto parts store, explain what you're trying to do, and get a headlight switch with a rheostat in/on it. That won't heat up so fast and you should be able to control the fan speed from full out to being able to count the blade revolutions. Phred52

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Hey thanks fellas, I knew i'd get a good answer here! I'm thinking of trying a junk yard for a dimmer switch out of a car/truck. I'll let ya know how it goes.....thanks again!

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Junk yard is a good idea - I went to every radio shack, auto parts store, EVERYWHERE and could not find one. The junk yard would be a good option.

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I would also suggest when you hook up the power sources for the fan to run the power wire to the fan, and use the rheostat to control the ground. This will make the rheostat last a lot longer because you are running it after the load(fan) in the circuit. By doing this with regular toggle switches, you can get by with the lower amperage rated ones because the load is before the switch and the switch doesn't have to carry the full amperage to the load source.

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Another option would be to use a few diodes in series. Or if you experimented with a variable power supply to find the optimum voltage you could use a single zener diode. Diodes pass DC current with one polarity and block it with the reverse. When the positive is connected to the anode (the wide part of the arrow symbol on the diode >| )current will pass but each diode will drop .6Vdc. So if you needed about 10Vdc you could put (3) diodes in series with the load. 12-.6-.6-.6=10.2 The .6Vdc voltage drop will be constant regardless of the load so you could put multiple fans to this circuit with out having to re-adjust the rheostat in the previous posts. The diodes won't heat up like a rheostat (which is a potentiometer with the wiper terminal hooked to one of the other two terminals to make a variable resistor)and they could be soldered to one of the power leads and not needing anything to mount the rheostat in. This would be much cheaper than a rheostat, also not dangerous if the rheostat wattage is undersized.

(Pos+)------->|-->|-->|-------FAN--------(Neg-)

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Well thanks for the input fellas! So far I only installed 2 fans. Picked up a rheostat from radio shack and it was 3 watts and I was pushing 9..soooo she was toasty. Anyway...I thought to myself..they are in fact "computer fans" You can by computer fan speed controllers for computers to run the fans. They are only 8 bucks. I'm going to try that. Thanks again!

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