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Ok, question for an EE. How to set up potentiometer for adjustable speed cooling fan.

C

Creig

Diamond Member
My 2 terms of beginners EE back in 1987 aren't QUITE adequate for me to do this on my own. Hell, actually I can barely remember anything from it. 🙂

What I'm trying to do is set up a variable speed control for the CPU cooling fan on my system. It's a fairly high RPM fan so it makes a bit of noise. Doesn't bother me all that much, but it bothers my wife who, in turn, bothers me. So, I need a way to slow it down when I'm not playing a 3D game.

If I remember right, you need to match the resistence of the fan to the potentiometer somehow. I don't want to be able to shut the fan off at all, just lower the RPMs by about 50%. Since I have a 4 year old daughter running around, I don't want her to be able to shut the fan off by playing the the knob, thus frying my new TBird.

Can anyone give me some instructions, or a link to some directions?
 
Creig: First a warning. Any attempt to modify stuff in your case could cause undesired problems or fry something. I am just giving you the steps to do it. I don't recommend it. I suggest buying a slower or less loud fan. Or even closing the case. With that said...

You will need to measure the resistance of the fan. As you said, you want it to spin 50% slower so you need 50% less current. Avoiding the equations/math, doubling the resistance and applying the same voltage will reduce the current by 50%.

Once you know the resistance, you can add a potentiometer with that resistance in series. I must warn you, you will never get the resistance to 0 Ohms (never get the full amount of current like before to your fan).

If you need more details, just ask.
 
http://www.fanbus.com

check Cliffs faqs for some very good information.

1)Use the 7V/12v switch

2) If you want to use a pot. but keep a miniumum speed.
Set the speed with a pot to the minimum speed you want to run. Measure the resistance of the pot with an ohm meter(take it out of circuit before measuring). Get a resistor of that value and put in parallel with the pot. Then the minimum speed is set by the resistor and the pot will allow you to speed up the fan. You will need a fairly high ohmage pot for this to work. Calculate the wattage of the resistor and pot, 1/8 watt components will not work.
 
Use an external DC power supply like he ones that Wal-mart sells for $8.99 to use in place of the batteries for your boombox. Get one with a sliding voltage adjustment from 12 volts down to 3 volts. Then you can move the slidder up and down and control your fan speed accordingly.
 
Decision, decision... 🙂

Thanks for all the good suggestions, guys. Now all I have to do is decide which to use. Right now I have it on a 5v/12v switch. 12v=7200rpm=38cfm 5v=2900=@22-25cfm I figure that with the lower rpm comes decreased air resistence. This should equate to about 22-25cfm just guessing. This will do fine until I can figure out which route I want to take for a more flexible set of adjustments.
 
Keep it simple...go to Radio Shack, pick up a 25 ohm, 3w Rheostat (271-265b). Cut red(12v) wire going from MB fan header 2 CPU fan. Run it 2 middle tab on rheostat, then run another wire back from either outside tab 2 CPU fan wire. This will allow you 2 cut back fan bout 1000rpms, you can still monitor RPMs, & fan will go into suspend if ya like. QED 😀
 
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