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I wanted to control the speed of a very fast and noisy brushless dc cooling fan. The fan I have is a 3 pin 12v 6.3w 120mm fan by Delta Electronics. From what I've learned from the internet is that the speed for 3 pin fans are adjusted by reducing the input voltage and 4 pin fans have an inbuilt pwm controller. However, I have seen some videos where the speed of 3 pin fans being easily controlled by an Arduino using mosfets.

What I want to know is whether trying to control the speed of my fan with a pwm output destroy the inbuilt electronic circuitry of the bldc fan or not. I have a small 2 amps pwm motor speed controller with me that might emulate what the Arduino just did. But, can bldc motors simply be controlled by pwm just like regular small motors?

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  • \$\begingroup\$ Another option is to replace the fan with one that has a PWM input (i.e. 4 wire) but otherwise similar specifications. 120mm square, 12V, 6W is pretty common... \$\endgroup\$
    – Supa Nova
    Mar 9, 2022 at 2:11

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You shouldn't damage the fan by modulating the power input but only the manufacturer will be able to give you a definitive answer on that since only they will know what circuitry they have in there.

A problem you might find, however, is that the tachometer or other features (locked rotor protection, etc) may not work correctly. There are controllers such as the TC646 that will measure the fan's speed by looking at the supply current spikes/dips as the windings are commutated. They usually also implement the fan features that could be impaired by themselves too.

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  • \$\begingroup\$ So what are my other options for speed control without damaging the circuitry? A linear driven potentiometer, or an adjustable buck-boost converter. I won't be needing the tachometer since this won't be going into a motherboard. I will always be in front of the fan when controlling speed so if the rotor gets locked I can attend to it. \$\endgroup\$
    – Kokachi
    Mar 8, 2022 at 20:55
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    \$\begingroup\$ I think you should be fine with PWM on the power supply but a variable voltage supply would probably work too. You could still run into the same issues with accessory circuitry but I doubt you'll permanently damage anything either way. \$\endgroup\$
    – vir
    Mar 8, 2022 at 21:39
  • \$\begingroup\$ okay, then I will go ahead with the pwm module. The buck boost converter only has a tiny potentiometer that needs to be turned with a tiny head screwdriver, a real hassle. \$\endgroup\$
    – Kokachi
    Mar 9, 2022 at 19:11

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