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What Potentiometer should I get to control the speed of a computer fan?

Discussion in 'General Electronics Discussion' started by DiamondsareMine, Jul 19, 2015.

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  1. DiamondsareMine

    DiamondsareMine

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    Jul 19, 2015
    Hi, I am trying to control the speed of a fan I have with a potentiometer but I don't know what to get. I don't know much about electronics and I don't know if I will pick the right one. What kind potentiometer do I have to buy? It would be awesome if you also had a link too. Thank You!

    Fan Specs:
    12V DC
    0.32A
    3.84W
     
  2. AnalogKid

    AnalogKid

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    Jun 10, 2015
    This type of control is the most basic, and the most inefficient. Important question - do you want to just slow down the fan a bit, or dial it all the way down to 0 speed?

    ak
     
  3. DiamondsareMine

    DiamondsareMine

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    Jul 19, 2015
    I would like to slow the fan down a bit if it a "bit" could take it down to about half speed.
     
  4. Colin Mitchell

    Colin Mitchell

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    Aug 31, 2014
    For a start, the pot must be able to take 350mA without burning out.
    Next, you want to drop about 6v to 8v to reduce the speed.
    The effective resistance of the fan is about 38 ohms.
    If you add about 100R pot, the fan will reduce in speed.
    A 100R pot capable of handling 350mA is 12 watts.
    A 10 watt wire-wound pot will just be acceptable.
     
  5. Minder

    Minder

    2,912
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    Apr 24, 2015
  6. DiamondsareMine

    DiamondsareMine

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    Jul 19, 2015
    Do you have a suggestion on what you would get that would match all these requirements?
     
  7. cjdelphi

    cjdelphi

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    Oct 26, 2011
    a 555 under astable to supply pwm to the fan, a pot to adjust the 555, the 555 controls the fan..

    but, as stated the fan may already be pwm controlled from internal curcuutry
     
  8. Alec_t

    Alec_t

    2,729
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    Jul 7, 2015
    Digikey's price for a 100Ω 12W wirewound pot is £77 :eek:. So PWM is the way to go!
    If a conventional PWM control conflicts with the internal commutation of the fan you could try my 'Synchronous Pulse Delay' method as described in this article.
     
  9. Kiwi

    Kiwi

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    Jan 28, 2013
    Are you using the fan in a computer, or are you using it for another application?
     
  10. cjdelphi

    cjdelphi

    1,098
    104
    Oct 26, 2011
    Not bad price for a 5w pot...
     

    Attached Files:

  11. cjdelphi

    cjdelphi

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    Oct 26, 2011
    anyway, a 20c 10-100k pot which controls a 555's astable mode will give you a varied on/off pulse the pot will adjust the firing time, the output pin3 could be then connected to the base via a 1k resistor which can power the fan...

    you may need a flyback diode across the motor in reverse to suppress high voltages from the motor (unless it's brushless and then i'm not entirely sure, do brushless have inductive kick backs? i don't think they do?)
     
  12. AnalogKid

    AnalogKid

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    653
    Jun 10, 2015
    A BLDC (Brush-Less DC) fan is a wierd combination of a constant resistance and constant current device. As the voltage to the fan is decreased, the current does decrease, suggesting constant resistance, but not linearly as it would with a resistor. Still a constant resistance is a reasonable starting point for this type if question.

    I = E / R
    P = I^2 x R

    For a nominally 38 ohm fan, let's start with a 50 ohm pot. At full speed the pot resistance is zero so it dissipates 0 power. At full rotation the total resistance of the pot plus fan is 88 ohms, so the current is only 0.137 mA, so the power dissipated in the pot is 0.92 W. If you limit the pot's rotation to only 38 ohms, the power is 0.86 W. Digikey and Mouser have 2 W critters for $9.

    http://www.digikey.com/product-detail/en/RV4NAYSD500A/RV4N500C-ND/222956
    http://www.mouser.com/ProductDetail...=sGAEpiMZZMtC25l1F4XBU7rZyhV5cpJRj5zllmXo7sI=

    ak
     
  13. Minder

    Minder

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    Apr 24, 2015
    A BLDC motor operates from a DC supply but requires commutation to the 3 stator winding, so There has to be embedded electronics of some kind.
    Picmicro TC653 etc
    M.
     
    Last edited: Jul 19, 2015
  14. DiamondsareMine

    DiamondsareMine

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    Jul 19, 2015
    Would it be easier just to change my fan? If I were, what would i be looking for?
     
  15. Minder

    Minder

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    Apr 24, 2015
    If you want to guarantee a simple form of control, then DC brushed are generally the simplest, the older PC fans were practically all this type.
    M.
     
  16. DiamondsareMine

    DiamondsareMine

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    Jul 19, 2015
    http://comingsoon.radioshack.com/25-ohm-3-watt-rheostat/2710265.html#.VavSRflViko
    Ok then, would this work? Sorry,noob question: Would my fan not be able to spin as fast because it is a 3 watt potentiometer or would the potentiometer break?
     
    Last edited: Jul 19, 2015
  17. Minder

    Minder

    2,912
    610
    Apr 24, 2015
    A potentiometer is not a really recommended way to control a motor directly, it is very wastefull in power also.
    M.
     
  18. DiamondsareMine

    DiamondsareMine

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    Jul 19, 2015
    I have never done anything with embedded electronics before. Do you know of a guide then?
     
  19. Minder

    Minder

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    Apr 24, 2015
    You mean apart from post #5?
    M.
     
  20. DiamondsareMine

    DiamondsareMine

    12
    0
    Jul 19, 2015
    Never mind, thanks for helping me. I'm sorry that I am so bad at this.
     
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