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I have a 2-pin 40 mm axial fan as part cooling fan. I don't post a link because I would like a general answer.

It runs on 12 V and when I start it at less than 40% it doesn't start. Even if I start at 100% and then I reduce the speed, at about 30% it stops.

When I read reviews online, I see people use fan speeds down to 10%, which no fan I have ever used can achieve.

How can I pick and connect a correct fan for part cooling, so that I can freely set the speed down to 10%?

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  • $\begingroup$ If it works fine (and actually changes speed) from 40% to 100%, it shouldn't be the MOSFET or controller per se. You may want to enable "FAN_KICKSTART_TIME" to give it a boost when starting, or use "FAN_MIN_PWM" to change the scale of the commands, so that even asking for 10% results in fan movement (at ~40%). $\endgroup$
    – towe
    Commented Feb 6, 2020 at 16:21
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    $\begingroup$ What is the AMP usage at 12V? Forget all this PWM stuff and get a fan that has a speed controller built in, which you can then control via the 3d printer's pwm. $\endgroup$
    – user77232
    Commented Feb 6, 2020 at 16:39

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If the fan you are using is not defective and if you test another fan that behaves in a similar manner, it's possible your firmware or hardware are the root cause of the problem.

The controller directs the driver to vary the power provided to the fan. It's a method called pulse wave modulation, aka PWM. Full voltage is applied to the fan one hundred percent of the time, for full speed operation.

For fifty percent performance, half of the time full voltage is sent, while half of the time no voltage is sent. The time period is rather short, but I didn't determine that aspect of this answer.

One can find a clear explanation of PWM online but one word is worth one one-thousandth of a picture:

pwm graph

If your fans are not performing properly, either the program embedded into the controller is buggy, or the driver module for that unit is bugging out.

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  • $\begingroup$ I know PWM, but are all fans supposed to PWM down to so low values? 2-pin fans are not required to go this low, while PWM fans do. $\endgroup$
    – FarO
    Commented Feb 6, 2020 at 17:07

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