Electronics > Beginners

Filtering PWM to smooth DC

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Zero999:
As mentioned above, an RC filter will result in some voltage drop and power wastage. In this case, the current is 0.17A and the resistor is 47R, so that's a loss of 8V. In reality, the motor will just draw less current, so the voltage drop won't be that high, but it will still be significant, so the fan will be unable to run at top speed.

Gyro:
Yes, as IanB said, I used RC for convenience, at that low a frequency (65Hz) I think a decent inductance conveniently small inductor would probably have had >10R DCR anyway. The fan being 24V fan made supply current lower too.

It looks as if you've had a nice quiet first stab. A 47R series resistor is going to result in a fair voltage drop. As your PWM frequency is 500Hz, you have quite a lot of leeway. You only need to get rid of the nasty edges, so, yes, I'd shift to 10R (2V drop or less) and then tune it down further by ear.

EDIT: From your ripple readings, it sounds as if the fan drive is an open collector output, but best to double check that it isn't push-pull. You wouldn't want to blow it.

wraper:

--- Quote from: Gyro on September 22, 2018, 06:32:24 pm ---Yes, as IanB said, I used RC for convenience, at that low a frequency (65Hz) I think a decent inductance conveniently small inductor would probably have had >10R DCR anyway. The fan being 24V fan made supply current lower too.

It looks as if you've had a nice quiet first stab. A 47R series resistor is going to result in a fair voltage drop. As your PWM frequency is 500Hz, you have quite a lot of leeway. You only need to get rid of the nasty edges, so, yes, I'd shift to 10R (2V drop or less) and then tune it down further by ear.

--- End quote ---
But why would you use low frequency for LC (it's just a buck converter really) to begin with? Just use timer in MCU to generate high frequency PWM.

Gyro:
In my case (see my linked thread) I was stuck with a primitive 555 + opamp based thermistor sensing speed control on the Instek PSU board. I suppose I could have messed around with capacitor values to increase the 555 switching frequency, but that would have involved debugging and it was simpler to leave the PCB with heatsinks, thick transformer connections etc. in situ and just mod the fan lead.

Rapsey:
A couple of follow-up observations:

With the 10 Ω resistor the fan is able to get up to 11V at 100% PWM. Evidently this does not scale linearly with the PWM: even at 1/32 duty cycle I was still getting ~4.7V.

The output isn't quite as smooth anymore but still, no whining noise so that's alright. Wave forms are very interesting to see. At low PWM it's a sawtooth pattern (near-vertical up, ramp down) with 500-600mV peak-to-peak. At 50% PWM it's a nice zigzag ramp up & down, and at 100% it's... I don't even know. See attachments.

Kinda surprising to see that, I guess 100% PWM isn't the same as continuous 12V DC after all.

Apologies for the crappy portable scope, I don't have room for a big one right now.

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