Getting rid of PWM fan noise

Question

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).

Answer 1

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.

Answer 2

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