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Filtering PWM to smooth DC

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IanB:

--- Quote from: Rapsey on September 22, 2018, 04:15:56 pm ---May I ask why you opted for an RC filter over LC?

--- End quote ---

Mainly for convenience. All though an LC filter is much more efficient*, it is generally the case that resistors are small and cheap while inductors tend to be bigger and more expensive. But by all means use an LC filter if you can calculate the sizes of L and C correctly and have space to install them.

* About efficiency, an inductor stores energy and gives it back to the system, while a resistor just throws energy away.

wraper:

--- Quote from: Rapsey on September 21, 2018, 03:47:56 pm ---Someone once told me brushless motors are always 3-phase and thus have 3 wires (this fan only has 2) but I'm pretty sure it's brushless anyway. It's a Sunon MF50151VX.

--- End quote ---
Brusleless fans have 2 wires, if they have 3rd wire it's tachometric output. They have internal circuit which switches the windings. And MF50151VX is certainly a brusheless fan. PWMing it will certainly cause damage to it (although it likely will keep working). You need to make a proper buck converter.
3 wires for the motor itself is when you have BLDC motor which needs external motor driver which will switch windings.

--- Quote from: blueskull on September 21, 2018, 05:10:42 pm ---Don't filter it. Reduce PWM frequency to ~160Hz, and magic may happen.

--- End quote ---
Like blowing up electrolytic capacitor inside the fan?

wraper:

--- Quote from: IanB on September 22, 2018, 04:20:47 pm ---
--- Quote from: Rapsey on September 22, 2018, 04:15:56 pm ---May I ask why you opted for an RC filter over LC?

--- End quote ---

Mainly for convenience. All though an LC filter is much more efficient*, it is generally the case that resistors are small and cheap while inductors tend to be bigger and more expensive. But by all means use an LC filter if you can calculate the sizes of L and C correctly and have space to install them.

--- End quote ---
Given heat dissipation on that resistor, proper buck converter likely will be smaller in size. Total component count needed is almost the same. Also you won't be able to go near to full voltage unless using very low resistance, which will mean huge current spikes. I don't see any good reason using MOSFET + large resistor + cap vs MOSFET + diode (or second MOSFET) + small inductor + cap. If avoiding buck converter for any reason, instead of using RC filter, just smooth PWM before pass element and go linear.

Rapsey:
I've started by trying a low pass RC filter, mainly because I have plenty of resistors and capacitors lying around but no diodes or inductors. This is my maiden voyage into filter circuits so... brace for newbie mistakes.

I calculated my resistor value based on a cut-off frequency of 10 Hz and a 330 μF capacitor (16V electrolytic). This may be too big a capacitor for the job but it does arrive at a convenient resistor value:

R = 1 / (2×π×fc×C) = 1 / (2×π×10×330×10^−6) = 48.2 Ω => I used a 47 Ω resistor.

The oscilloscope now shows a fairly constant voltage: a zigzag shape (linear up & down ramps) with 10mV peak-to-peak. So far so good I think.

Controlling the fan through this now works without any whining noise. However... It doesn't get up to full speed anymore. Some voltage measurements:

1/8 duty cycle: ~2.8V
1/4 duty cycle: ~4.8V
1/2 duty cycle: ~6.8V
1/1 duty cycle: ~8.7V

So yeah, quite a long way off 12V at "full" speed.

Best guess: I should increase my cut-off frequency to result in a lower resistor value? Maybe 50 Hz for 10 Ω?

MiDi:
This is the problem with rc filter: they have an attenuation dependent of the current through resistor - it is a loaded rc-filter, forming a voltage divider...
So the resistance has to be so low, that negligible voltage drop due to current occurs at dc, which can lead to unreasonable high capacitor.
LC filter would be an option, but has its own problems with resonance and this depends on input and output impedance as well...
You should take the suggestions as serious, but it is always the best to make your own experience  ;)

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