Resistor size info needed - The Planted Tank Forum
 
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post #1 of 8 (permalink) Old 03-01-2005, 05:51 PM Thread Starter
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Resistor size info needed

Can someone suggest a resistor size for slowing down my cooling fans? I have two 80ma 12v fans running on a 12v 500ma supply.

TIA.
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post #2 of 8 (permalink) Old 03-01-2005, 08:15 PM
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I am not an electrician, so don't listen to me...

You have a fan 12V and 0.08A, that would tell me it's 150 Ohms, so if you use a resistor of 150 Ohms the fan would get only 6V and turn really slow, maybe try and see what you get with 75 Ohms?

I have a fan running on 110V and not sure maybe 0.3 Amps, and I put a light bulb in series so it runs slower and quieter as well. Not really high-tech, but works.


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post #3 of 8 (permalink) Old 03-01-2005, 08:56 PM
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Maybe a trip to this site would help:

http://www.twoengineers.com/Richard/leds.html

Maybe not

It has an Ohm's law calc on it as well...


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post #4 of 8 (permalink) Old 03-01-2005, 09:49 PM
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Why not try a variable resistor in a low range (like a volume knob) so you could change the speed of the fans without having to get out the soldering iron.
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post #5 of 8 (permalink) Old 03-01-2005, 11:14 PM
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I would use a potentiometer (sp) for controlling your fans, built custom baybus for a year or so using these. good luck!
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post #6 of 8 (permalink) Old 03-02-2005, 01:33 PM
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I also agree on a pot. Hopefully find a linear (non-audio) 12v model with at least 5 watts power handling.

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post #7 of 8 (permalink) Old 03-03-2005, 03:06 AM
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Your fan is going to draw no more current than it is designed to pull. Think about this. You plug in a night light into your wall socket which is usually feed off a 15 amp breaker. The light doesn't draw 15 amps, right. If you hook up the resistors in series with your cooling fan, all you are doing is cutting down the voltage. I'm using a 10 ohm power resistor to cut the voltage to my 12 volt soleniod to 9 volts so it won't run so hot. The current in this sort of circuit is the same at every point. A 150 ohm resistor will cut the voltage almost 85%. If you get a pot, you won't need anything bigger than 1K (which is overkill) and you will need to short two of the three adjacent terminals together.

If you attach a resistor in parallel with the fan, you will reduce the current through it regardless of the resistors resistance. In this instance, the voltage across the fan and the resistor is the same. I don't think this is the approach you want. That my understanding and it's worth about what you paid for it

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post #8 of 8 (permalink) Old 03-04-2005, 07:03 AM
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I vote for the potentiometer too, because then you can then adjust your fan speed.

BUT that's not what you asked for

I assume you're trying to reduce fan noise. I don't know how much you're trying to reduce noise, or how slow your fans can run without your fixture overheating, but I'd guess two 47 ohm, 1/2W resistors in parallel per fan are a good starting point. Cheap and available at Radio Shack.

If that's not enough, use two 100 ohm 1/2W resistors instead.

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