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Whats the electric part i need?

  1. Sep 16, 2005 #1
    [SOLVED] whats the electric part i need?

    I'm trying to put 2 x 12 volts fans in my pc. I found the right wires on the psu,
    so i can plug them in and they work great. However, i need to be able to control the speed they're going at, because they're too noisy. I bought a couple Potentiometers, and they didnt work well. They all fried exept for the 1 mega ohm one, who worked but the fan would either go very fast or off, i couldnt have full control. I dont know what i need to make this work.


  2. jcsd
  3. Sep 16, 2005 #2


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    Staff: Mentor

    Hi Ben, A variable voltage regulator would be a good way to go. Use a LM317T (in the TO-220 package and add a heat sink as necessary). Look up the 317 datasheet, and it will show you how to vary the output voltage. Use your fan's input current (versus voltage) and the LM317T datasheet to figure out how big of a heat sink you need. If you want to bolt the LM317T to the metal chassis of your PC as a heat sink, you need to use a mica insulating washer between the TO-220 tab and the grounded metal of your PC. The metal tab on the adjustable voltage regulator is not grounded.

    Depending on the specs for your motors, you may need to start them up at the full 12V, and then turn down the input voltage after they are running to get the slower speed. Have fun! -Mike-
  4. Sep 16, 2005 #3


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    Using a variable voltage regulator sounds like a cool idea!! As berkeman indicates the LM317 is a handy device and lends itself to this application.
    Their http://www.fairchildsemi.com/ds/LM/LM317.pdf shows useful configuration details. I've used them a few times myself.
    Last edited by a moderator: Apr 21, 2017
  5. Sep 17, 2005 #4
    Computer fan motors are usually dc brushless. changing the voltage does not change the speed. The speed of the fan is dictated by a little controller attached to the fan motor inside the fan housing. Variable speed computer fans have a third wire that you can use to control the speed of the motor, but simply changing the voltage of the standard fan itself will only lead to burned out components.
  6. Sep 18, 2005 #5
    i do have a 3rd wire for the speed control on my fans, but i dunno what circuit im supposed to build.What current do i use to supply that 3rd wire? what potentiometer should i use with the LM317T?
    Last edited: Sep 18, 2005
  7. Sep 20, 2005 #6


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    If you do a little reading around the web, you will find that the third wire returns a periodic pulse (voltage), and may be used to control RPM (or speed). I suspect the little controller that faust9 mentioned, uses this pulse as a feedback signal to achieve the desired speed. If you don't have a controller, you could add one.. see this http://www.pccasegear.com/prod1618.htm [Broken] as an example.. There are lots of them out there to choose from.

    I differ with faust9's opinion, voltage is used to control speed on brushless DC fans.. See graph on this ref)

    Some folks in UK have already done some legwork for your application.. Try this http://www.cpemma.co.uk/reg.html [Broken]. They even give you a layout and parts list. :smile:

    To enlist more of the computer hardware gurus on our forum you might ask under the Hardware section of Technology. (though I find the labelling scheme our forum chose; is ambiguous, perhaps a better name for that section could be computer technology) :smile:
    Last edited by a moderator: May 2, 2017
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