Controlling Fans' speed with potentiometers!

Discussion in 'The Projects Forum' started by ElectronicLife, Jul 22, 2014.

  1. ElectronicLife

    Thread Starter New Member

    Jul 19, 2014
    8
    0
    Greetings Guys!

    I have a question on controlling 2 DC Fans' speed with potentiometers!

    I want to wire them up in parallel connection , more details :

    2 similar DC fans : 12 volts and 2.5 AMPS for each
    1 adapter : 12 volts and 5 AMPS
    2 potentiometers for each of the fans!

    But I think something is wrong with it ; as it is obvious (according to Ohm law) R is in contrast with I.It means when I want to change the resistance of one of my fans with potentiometer , then the AMPS of the another fan will be increased due to parallel connection! furthermore , it burns out!!!

    What can I do about it ?!:confused:

    Thanks in advance:)
     
  2. bertus

    Administrator

    Apr 5, 2008
    15,646
    2,345
    Hello,

    The second fan will not burnout due to the fact that the voltage will stay the same.
    The fan just takes the current it needs.

    Bertus
     
  3. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    I smell a smoking potentiometer in someones future..

    12V x 2.5A = 30 watts
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,515
    2,369
    Fans such as those sound similar to automotive types, they were typically ran with high wattage resistors switched in for speed control, now they are often done with PWM.
    You could also put them both in series across 12v!;)
    Max.
     
  5. sivajoginaidu

    New Member

    Jul 23, 2014
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  6. Sensacell

    Well-Known Member

    Jun 19, 2012
    1,128
    266
    If the fans are brushless DC type, they have a built in controller that may not work right if the series resistance is high, i.e. you will have limited control over the speed, only at the upper range.

    The potentiometers will need to dissipate large amounts of power- better to use a rheostat- a fancy name for a power potentiometer, but these are large, expensive and hard to find these days.
     
  7. ElectronicLife

    Thread Starter New Member

    Jul 19, 2014
    8
    0
    Thanks a million ...
     
  8. ElectronicLife

    Thread Starter New Member

    Jul 19, 2014
    8
    0
    What do you mean by that ?!
     
  9. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
    4,769
    969
    That means you CANNOT use a potentiometer as it will go up in smoke almost instantly.. They are rated up to a few watts max and cannot handle the dissipation.
    These aren't uber small computer fans you are talking about here.

    Hence the suggestion above for a rheostat.. But wait till you see the prices for those in the sufficient rating.
     
  10. Dodgydave

    Distinguished Member

    Jun 22, 2012
    4,974
    744
    I would use a pwmcircuit using a 555 timer.
     
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