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    RC Low-Pass Filter for Blower Fan

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    Duet Hardware and wiring
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    • wilrikerundefined
      wilriker @SnowCrash
      last edited by

      @snowcrash said in RC Low-Pass Filter for Blower Fan:

      I was reading through this thread and was hoping someone could clarify the value of the inductor in the diagram above? I'm familiar with 1uH (micro-Henry) and 1nH (nano-Henry), but what does 1mH stand for? mili-Henry (1000uH)? for such a small amount of current?

      Yes, mH is milli-Henry.

      Manuel
      Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
      with probably always latest firmware/DWC (incl. betas or self-compiled)
      My Tool Collection

      SnowCrashundefined 1 Reply Last reply Reply Quote 0
      • SnowCrashundefined
        SnowCrash @wilriker
        last edited by SnowCrash

        Thanks @wilriker!

        But that's HUGE just for a fan that draws maybe 0.15mA, no? (ok, the fan will draw more current at startup but still...). Is there some calculation behind this value?

        wilrikerundefined dc42undefined 2 Replies Last reply Reply Quote 0
        • wilrikerundefined
          wilriker @SnowCrash
          last edited by

          @snowcrash Actually I have no clue about all this stuff. I just took those numbers for granted (even though I would also like to learn about where they come from).

          But just as an update: I dropped all this anyway since connecting the signal line showed that the fan actually turns at the speed I set it to, it just feels/sounds a lot lower - but then my ears and fingers are probably the worst way of measuring fan speed in the first place. 😂

          Manuel
          Duet 3 6HC (v0.6) with RPi 4B on a custom Cartesian
          with probably always latest firmware/DWC (incl. betas or self-compiled)
          My Tool Collection

          SnowCrashundefined 1 Reply Last reply Reply Quote 0
          • SnowCrashundefined
            SnowCrash @wilriker
            last edited by SnowCrash

            @wilriker happy you got your fan working quietly (though perhaps not in the way you were hoping to achieve this 🙂 )

            Anyway, at another discussion, @dc42 insisted that an inductor-capacitor circuit is necessary for preventing a non-PWM fan from potentially being damaged while being run with a high frequency PWM signal, and just I started looking into the idea of making a little add-on module that would provide this circuitry for all 3 PWM outputs on the Duet (plus the ability to choose between 12V and 24V for each fan which is the main reason for the module).

            So hopefully @dc42 or someone else familiar with this subject would chime in and clarify this point about the inductor's value.

            1 Reply Last reply Reply Quote 1
            • dc42undefined
              dc42 administrators @SnowCrash
              last edited by

              @snowcrash said in RC Low-Pass Filter for Blower Fan:

              Thanks @wilriker!

              But that's HUGE just for a fan that draws maybe 0.15mA, no? (ok, the fan will draw more current at startup but still...). Is there some calculation behind this value?

              Yes 1mH. The fan draws 0.15A (150mA), not 0.15mA.

              Duet WiFi hardware designer and firmware engineer
              Please do not ask me for Duet support via PM or email, use the forum
              http://www.escher3d.com, https://miscsolutions.wordpress.com

              SnowCrashundefined 1 Reply Last reply Reply Quote 0
              • SnowCrashundefined
                SnowCrash @dc42
                last edited by

                Thanks for the confirmation, @dc42! But what's the calculation behind it?

                1 Reply Last reply Reply Quote 0
                • dc42undefined
                  dc42 administrators
                  last edited by dc42

                  The calculation is L = V * t/I where V is the voltage across the inductor, I is the maximum ripple current current you want to allow, and t is the time involved. Or, turning it around, I = V * t/L. If we take the case of a 24V fan running at 50% PWM and 50kHz PWM frequency, then for L=1mH and 12V across the fan, we have I = 12V * 10us/1mh = 120mA. This is around the same as the fan current, so a capacitor is needed after the inductor to reduce the ripple current in the fan. For the capacitor, C=I*t/V where V is the allowed peak ripple voltage. So V=I * t/C and 1uF will give 120mA * 10us/1uF = 1.2V. This is 10% of the 12V average voltage at 50% PWM, so it is likely to be low enough.

                  Duet WiFi hardware designer and firmware engineer
                  Please do not ask me for Duet support via PM or email, use the forum
                  http://www.escher3d.com, https://miscsolutions.wordpress.com

                  1 Reply Last reply Reply Quote 2
                  • fmaundefined
                    fma
                    last edited by

                    I recently added a Berd air pump for cooling the part, and I noticed that increasing the PWM frequency increased the efficiency: for the same PWM ratio, the higher was the frequency, the higher was the rotation speed. I went up to 100kHz.

                    Is it normal? Could such low -pass filter improve things?

                    Frédéric

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                    • dc42undefined
                      dc42 administrators
                      last edited by

                      That doesn't necessarily mean that the efficiency was increased. The increased rotation speed may have been caused by a EMI suppression capacitor within the motor having a smoothing effect.

                      Duet WiFi hardware designer and firmware engineer
                      Please do not ask me for Duet support via PM or email, use the forum
                      http://www.escher3d.com, https://miscsolutions.wordpress.com

                      1 Reply Last reply Reply Quote 0
                      • fmaundefined
                        fma
                        last edited by

                        Yes, efficiency was not the appropriate word (I didn't measure the current).

                        What about the low-pass filter? Could it improve things?

                        BTW, I also noticed that adding the flyback diode increases the speed for the same PWM ratio.

                        Frédéric

                        1 Reply Last reply Reply Quote 0
                        • dc42undefined
                          dc42 administrators
                          last edited by

                          if you are driving the brushed DC motor from a heater output, then a flyback diode is essential. Without it, energy from the back EMF of the motor inductance is dissipated in the mosfet by avalanche breakdown, leading to reduced efficiency and maybe eventually a blown mosfet.

                          Duet WiFi hardware designer and firmware engineer
                          Please do not ask me for Duet support via PM or email, use the forum
                          http://www.escher3d.com, https://miscsolutions.wordpress.com

                          1 Reply Last reply Reply Quote 0
                          • fmaundefined
                            fma
                            last edited by

                            I was aware of potentially blowing the FET, but I didn't know it changes the efficiency. Thanks for the explanation!

                            Frédéric

                            1 Reply Last reply Reply Quote 0
                            • vfjrundefined
                              vfjr
                              last edited by

                              I know this is a very old topic, but it is the first one that comes up on Google when searching for fixing fans that do not work with PWM, so I hope to be able to share a solution with people who need help.

                              I found a good and cheap solution and I made a new topic about it here.

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