I have a 3D printer which has two fans, both running on 24 V. I want them to be very quiet, except for when really needed. I already upgraded to bigger fans, so running them slower would still give good enough airflow.

Fan A is always running on full power, and I reduced its voltage to ~12 V using a DC-DC step-down for it to be quieter. This is working perfectly.

Fan B is PWM controlled, but it is 2 pin fan, so I can get it quieter by setting a lower duty in the software. But it always has an annoying PWM noise. My multimeter cannot measure the frequency, but if i try multiple times, I get values like 4/7/8/13 Hz (not kHz!). I think that it is ~8 Hz using a software PWM, but I am not sure. It is not a high frequency ringing, but an audible sound multiple times per second.

The easiest way I can think of, would be using a 4 pin fan, using the GND and + of fan A for the power, and the + of fan B as PWM signal input. But I cannot find any 4 pin fans which run on 24 V, except for some huge ones (120 mm+). I would need some 40 to 60 mm ones, exact size would not matter - but I don't think they exist.

Fans with 4 pins which run on 12 V would be not problem to acquire, and I was thinking I could take the 12 V from the step-down of fan A as power, and use the fan B PWM as a signal, using a 12 V Zener diode. But I am having some doubts. Would this work, as they do not share a common ground because of the stepdown?

I searched for "PWM to voltage" converters, but all those modules seem to be very specific on the input frequencies and such.

I found tutorials for "low pass filters" using a resistor, a diode and a capacitor to flatten the PWM, but I am not sure which values would be good.

Would my idea work? Would the low pass idea work? Which would be good values for the components? (I found 50 ohm/220 uF) Are there easier/better ways to fix this annoying PWM noise without losing the ability to control the fan? Does not need to be exact, having a "low and silent" and a "full power" mode would be sufficient.

---Edit: Thanks to a comment I found 24 V fans with 4 pins, but the datasheets tell me, that this is not a solution, because they all want a much higher frequency pwm signal (20 khz or more). Probably the same for the 12 V ones I found on consumer shops before, so that is out of the question too (If it would work at all because of other reasons).

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    \$\begingroup\$ Single-digit Hz sounds (no pun intended) really low for any sort of PWM, maybe the replacement fan isn't playing well with the switched voltage that the printer is giving it and you're hearing some kind of beat frequency. The 24V 4-wire fan option sounds good, but if the printer is modulating the 24V then the PWM input isn't really doing anything. Low-pass filtering the power rail could help or not, depending on the fan. Searching for 24V 4-wire 40mm and 60mm fans on Digikey gives 100+ results, so there are options out there. If you don't need to monitor the RPM, there are 3-wire fans too. \$\endgroup\$
    – vir
    Commented Jan 25, 2023 at 20:53
  • \$\begingroup\$ Original fan and 2 other fans also make the same noise, so I dont think it is a problem of the fan I currently use. Will check out digikey :) \$\endgroup\$
    – Flo
    Commented Jan 25, 2023 at 21:02

2 Answers 2


Don't put a resistor in the power path! It's lost power and it's another issue to get rid of the heat.

Filtering a low frequency waveform is not easy, it'll be bulky. If you can "guess" the frequency of the sound you're hearing (It's not 7 Hz because humans can't hear that), it might help you choose a less buky filter to target that frequency and above.

Like Voltage Spike said, an LC filter can work. I might add a diode before it, in case the LC combinaison forces you to use a lower L value, to prevent losing the capacitor's energy in the driving circuit.


What the PWM is doing is limiting the voltage when used on VCC of the fan, it's a not so great way of doing speed control. There are better ways to limit voltage than PWM, like with a voltage controlled voltage regulator

The quick and dirty solution would be to do what you are doing with switched PWM, but use an LC filter


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