RC Low-Pass Filter for Blower Fan

  • @wilriker I'm glad it helped you to understand whats going on.

    With your open question; the PWM cycle does not reset, so the remaining 1ms after the filter would be added to the next cycle.

    cycle 1)
    high for 5ms, low for 4ms

    cycle 2)
    low for 1ms, high for 5ms, low for 3ms

    cycle 3)
    low for 2ms, high for 5ms, low for 2ms

    I agree with @dc42, to get the best output is to use a L-C filter.

  • Thanks @dc42 and @Iamturbo1978 for all your patience with me so far and for your very understandable explanations in this topic! 👍

    Just to make sure I understood everything correct here is a drawing of what it should look like:

    Of course power-source, MOSFET and flyback diode are already on the board and I would have to add the right half of the diagram. Right?

  • administrators

    Yes, that's correct.

  • @dc42 Yesterday I connected the tacho output of the fan to my Duet. Although this fan is obviously not a 4-wire PWM fan it gives me quite stable RPM readings when I set my fan to a specific speed.

    You said it might be that still this would give me bogus values and I found something really strange so maybe you could tell me if that could fall under what you warned me about:

    My fan according to its specs (TFD-B5015M12B) has a top speed of 4500 RPM@12V. If I run it with M106 S1 it will get full Vin which in my case is 13.6V so of course it will run faster. But tacho readings tell me it runs at 6500 RPM. If I then run it with M106 S0.88 it should get very close to 12V and the tacho reads 5500 RPM. Interestingly there is another model of the same family that would have exactly this speed @12V but I checked my order as well as the type label on my fan to not be the faster model.

    Of course there is a chance that my fan is labelled wrong or that they have changed the specs since this specific fan was produced. But I just wonder would it be possible for the RPM reading to be off at around 1000 RPM and scale according to this?

  • @wilriker so this could be the H type with 5500. If you measure the current also, and it is 0.18 A instead of 0.14, then you have the other type.

  • @joergs5 To measure the current I would have to put my multimeter in series, right? Never done that so far.

  • @wilriker directly into one of the main lines of the blower. Disconnecting the line and putting the current meter at those two open contacts. Be aware to set the maximum measure value of the current meter high first (above the maximum ampere, e. g. 1 A).

    If it's a Molex connection, you can turn current off, draw out one of the cables (the pin have a barb, by pressing down you can remove the cable), connect to amperemeter at both sides, turn on and then measure.

  • @joergs5 OK, thanks for confirming. As I already cut my wires to the fan I think I can quite easily insert the multimeter in this circuit using some screw terminals... I need to put ferrules on the wire ends anyway, so that's up next.

    Then again it would be really strange to have the H type when buying the M type that is also labelled as being the M type (including the amperage). I would expect this on a super-cheapo Chinese fan but not from a brand. But let's wait and see. 😀

  • @wilriker maybe labeled by someone who needs better glasses, M and H are similar 😉

    I thought it may be the H type because your 5500 measuring fits exactly to this higher model.

  • @joergs5 I know it would fit perfectly. Also about 6500 RPM @13.6V would match that quite well.


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