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Cheap pc fan controller

Discussion in 'General Electronics Discussion' started by Jirish82, May 19, 2011.

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  1. Jirish82


    May 19, 2011
    Hi all,

    Just bought a new pc and the cpu fan is annoyingly loud, unfortunately I don't have the money to buy a better one or a fan controller at the moment so I thought I might just buy a variable resistor and wire it in as a temporary solution.

    Problem is it's been ages since I did any electronics so I'm not really sure what sort of range the resister would need, don't want to get back and find that even at max it barely makes a difference, or that at min the fan just stops. Can anyone suggest a range that sounds about right?

    also, are there any possible problems I'm overlooking here? Cpu runs cool even under load so not too worried about that.

  2. poor mystic

    poor mystic

    Apr 8, 2011
    Hi Jirish :)
    There is no way to calculate what resistor you would want, because we know neither the power nor the supply voltage of the fan. A variable resistor or rheostat for a job like that would cost way more than a decent fan, too!
    I think a resistor is a poor way to do the job.
    I would look for a 'dead' PC with a quieter fan, and I'd get the proper software to control the fan from the PC manufacturer. I'm not sure, is fan control part of the BIOS?
    Some people put their machines in another room and operate them through the wall, with monitor, mouse and keyboard brought through a little hole, so that fan noise doesn't bother them.
  3. Resqueline


    Jul 31, 2009
    A resistor is not well suited to regulating the fan speed (it tends to either run fast or stop) and you'll need to put a big capacitor in parallell with the fan to somewhat counteract this.

    A zener in series with the +12V to the fan is a much better solution to get a fixed speed. Most fans starts & runs as low as 5V (so you could just hook it up to the +5V instead).
    Beware that some fans can actually make more noise at 5V than at 7V, but at 9V the wind noise usually starts to increase a lot.

    If you want the safety of a temperature regulated fan speed then here's a cheap & simple yet effective circuit that I designed some 20 years ago.
    With the 560 ohm it'll start out at 6.7V, starting to increase at 27 deg. C and ending up at 11V at 45 deg. C.
    With the 300 ohm it'll start out at 5V, starting to increase at 28 deg. C and ending up at 11V at 50 deg. C.
    Of course different thermistors have different steepness of their slopes and so may give different results.

    Attached Files:

  4. poor mystic

    poor mystic

    Apr 8, 2011
    Resqueline you are the top tech.
    Much kudos to you!
  5. Resqueline


    Jul 31, 2009
    Oh, well, thanks, but I forgot to mention that you had some pretty good advice above there too!
  6. nbw


    May 8, 2011
    Once you've mastered that... then think about a temperature sensor to adjust the fan speed automatically...

    Another day maybe ;)
    Last edited by a moderator: May 24, 2011
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