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Controling DC Fan Speed via Voltage

Discussion in 'General Electronics Discussion' started by Tanha, Oct 14, 2016.

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  1. Tanha

    Tanha

    6
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    Oct 14, 2016
    Hello,
    I want to control a 12 volt DC fan speed via its voltage. I have a potentiometer which is 50 ohms, 5 watts.
    I connected 12 volt from power supply to the left terminal of potentiometer, and the negative from power supply to the right terminal of potentiometer. I connected the middle terminal of the potentiometer to the positive wire of the fan. (I connected negative wire of the fan to the negative of power supply.)

    The fan's speed is controlled successfully from 0 up to its full speed.

    But the problem is that the potentiometer heats up.

    1. What can be done to prevent this heating up?
    2. Does this heat-up show that I have done something wrong?
    3. How much heat is ok?

    Good luck
    Ghobar
     
  2. Minder

    Minder

    2,916
    613
    Apr 24, 2015
    Buy one of the $5.00 PWM controllers on ebay.
    M.
     
  3. Tanha

    Tanha

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    Oct 14, 2016
    Let's confine the debate to what has been posed in the first post.
     
  4. JerkStore

    JerkStore

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    3
    Oct 14, 2016
    Nothing really wrong, just inefficient. You are using the pot as a variable voltage divider to vary the voltage at the motor and control speed. When the motor is running at lower speeds most of the voltage drops across your pot and since it's just resistance most of your power is being converted to heat. A more efficient system would use PWM as the prior message says. A simple one can be made with a 555 timer and a few discrete components
     
    Last edited: Oct 14, 2016
    Tanha likes this.
  5. BobK

    BobK

    7,671
    1,681
    Jan 5, 2010
    Then the answer is no, you cannot reduce the heat produced. You could use a larger potentiometer which would not rise in temperature as much, but the same amount heat would still be produced. This is only one of the reasons a potentiometer (actually a variable resistor) is not the way to do motor speed control. The second is that changing voltage changes the torque and does not work well for low speeds. And the third is that you are wasting energy.

    A PWM controller solves all three of these problems.

    Bob
     
    Tanha likes this.
  6. Tanha

    Tanha

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    Oct 14, 2016
    Can you refer to a really simple tutorial for building a pwm controller?
     
  7. JerkStore

    JerkStore

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    3
    Oct 14, 2016
    That's about as simple as it gets. This will drive a small DC Motor but if the motor requires too much current you will have to drive it with a transistor at the output.
     
    Tanha likes this.
  8. Tanha

    Tanha

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    Oct 14, 2016
    It is worth noting that at present I am controlling the fan with the potentiometer as a variable resistor. It reduces one third of the speed.
     
  9. Tanha

    Tanha

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    Oct 14, 2016
    Is there different kinds of 555 timer (for example, NE555N, CMOS, etc.)?
    If yes, what kind do I need?
    (The web page you referred me says nothing that I can find the answer.)
     
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    There are many suppliers of the venerable 555, and there are many variants.

    I can't see any particular reason that any of them wouldn't be suitable, excluding any power supply voltage issues (just make sure you have a power supply compatable with the 555..
     
  11. Tanha

    Tanha

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    Oct 14, 2016
    Please explain more: what are power supply issues? what are the specifications of a power supply that is compatible with the 555?

    (My power supply is desktop computer's PSU which has both 12 volt and 5 volt.)
     
  12. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    Jan 21, 2010
    There is a maximum and a minimum supply voltage for any device. Ensure that the 555 you get is ok for that voltage.

    In your case, 12V will be fine for any normal 555.
     
    Tanha likes this.
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