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Potentiometer Guidance

Discussion in 'General Electronics Discussion' started by Sunjumper, Sep 28, 2021.

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  1. Sunjumper


    Sep 28, 2021
    Hello Forum, I am making a project with a 12 Volt computer fan. Would you advise me on a suitable switchable potentiometer, ie linear and ohms value to control the speed of the fan? Best regards, Sunjumper.
  2. Bluejets


    Oct 5, 2014
    Potentiometers are commonly only rated at 1/8 to 1/4w and not suitable.
    Anyhow, many computer fans require a pwm signal or similar to run.
    Should be heaps of info out on the web, try a google search for the make , model, type, number of wires that you have.
  3. VenomBallistics


    Aug 30, 2018
    You will encounter two basic to types of fan.
    Brushed and brushless.
    The fastest way to a solution is an RC servo tester and an ESC per motor type.
    If it's a two wire fan, it's brushed. If three, it's brushless.
    12A - 20A ESC will be more than plenty.
  4. Sunjumper


    Sep 28, 2021
    Many thanks
    Many thanks Bluejets for your reply and advice, it's much appreciated. Regards, Sunjumper.
  5. Sunjumper


    Sep 28, 2021
    Thanks for your reply and advice, it's much appreciated. Regards, Sunjumper.
  6. dave9


    Mar 5, 2017
    There's some fairly strange info in the above posts. Practically all computer fans made in the last 25+ years are brushless. I mean "PC" fans not industrial rack who-knows-whats special applications.

    There are lots of two wire brushless fans, and regardless of whether two, three, or even four wires, practically none need a PWM signal, will simply run at the full speed possible from the voltage they're fed.

    A potentiometer capable of significant current is more often called a rheostat and you could use one of those but the ohm and wattage value needed would depend on the particular fan you want to use, and how much you want to slow the fan down. Rheostats capable of much current aren't cheap either (compared to a typical potentiometer), might be as inexpensive to just buy a purpose built computer fan controller.

    However in applications where I'm content with a fixed, slower speed, I far more often just use a series resistor, 2W, 3W, even more depending on what I have lying around then the ohm value is somewhere between 22 and 120 depending on the fan's original RPM and current rating. Further slowing down a fan that was already fairly slow, like under 1800RPM and 0.15A or so, would get a resistor closer to 120 ohms, while one closer to 5000RPM and 0.5A or more, closer to 22 ohms and everything in between, some middling resistor value.

    I have sped up the process using resistors by making a dongle with fan connectors on each end and a rheostat in the middle so I can dial in the speed I want, then measure the set resistance to pick the closest resistor value, or series/parallel resistors if I'm being picky about getting closer to a specific value I didn't have already.

    Another option instead of a purpose built fan controller is a series of diodes to drop voltage, or a switchmode supply module in series adjusted to the appropriate voltage, IF your target fan RPM needs a voltage below the original supply voltage minus the forward drop voltage across the regulator module which for this example is about 3.0V so (for whatever reason I was assuming you're starting with 12V) would be about 9V max output:

    If you just need a fan that runs at around 1000RPM, you could just buy a fan with that spec. instead of using something you already had that spins too fast.
    Last edited: Oct 1, 2021
  7. Sunjumper


    Sep 28, 2021
    Hello Dave9, Many thanks for your advice and guidance in your comprehensive reply. Also thank you for your included link. Very much appreciated. Regards, Skyjumper.
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