Help with slowing fans

Discussion in 'The Projects Forum' started by whoyah, Jul 19, 2009.

  1. whoyah

    Thread Starter New Member

    Jul 19, 2009
    Hi All,

    I need some basic help. I am cooling my salt water aquarium with a 2 120mm computer fans, wired in parallel to a 12v power supply. I would like to slow the fans down some, due to noise. I was thinking a rheostat or pot but I am a real novice out figuring out the right part, Any help would be appreciated. If possible I would like to get the part at RadioShack since there is one here in town but ordering a part is an option.

    Spec for the fans.
  2. bertus


    Apr 5, 2008

    Is the power supply fixed?
    On the webpage you gave it is atated the fans will start at 6 Volts.
    Is the speed of the fans enough when you put two fans in series on your 12 Volts powersupply?

  3. millwood


    you can build a 555-based pwm generator driving a mosfet to control your fans. the pwm can be adjusted with a pot, or through some thermister / thermal switch, all of which can be had at RS.
  4. someonesdad

    Senior Member

    Jul 7, 2009
    A pot would be the simplest solution, but the problem is that it would need to be rated at approximately 5 watts. It probably won't be terribly easy finding a suitable pot. Fortunately, Radio Shack sells 10 W sand resistors for about a buck apiece. You can cobble together the actual resistance you need by combining some sand resistors in series and parallel.

    The trick is to figure out what voltage you want to run the motor at (and, thus, what resistance you want to put in series with the fan). With a lab DC power supply, this is trivial, so I'd look for one of those first. You can cobble together a substitute by putting some AA batteries in series, but the result will jump in voltage increments by about 1.5 volts. Once you know the voltage AND the running current of the motor (measure this with an ammeter), then it's straightforward to calculate and build the resistance you need.

    If you demand adjustability of motor speed, then the pot or a PWM solution is what you'll have to look for.
  5. millwood


    no need to worry about getting the right power resistor: a mosfet + a pot = variable power resistor.

    and I think irf510 will work as well - RS has it. just make sure that you put it on a big heatsink.
  6. someonesdad

    Senior Member

    Jul 7, 2009
    I agree; this is what I'd do too -- but the OP may not be comfortable with using a solid state solution nor have the ancillary needed equipment.
  7. Tahmid

    Active Member

    Jul 2, 2008
    For doing this efficiently, you should use PWM. I have attached an easy circuit using 555 and NPN transistor. Hope it serves your purpose.