Safe to add capacitor to smooth LED powered by PWMed bed output?



  • So I'm using klipper with a duet ethernet and because of the recommended limits on the PWM frequency in Klipper, it's a bit irritating to look at. Would it be safe to add a capacitor to the LED bar (i'm using the bed output pins)? If so what spec capacitor is recommended for a 24v system?


  • administrators

    If you add a capacitor then there will be high peak currents every time the bed heatert mosfet turns on and the capacitor charges. So I don't advise doing this.

    It seems odd to me that Klipper should care about the PWM frequency. Maybe it doesn't use the hardware PWM that all Duet heater and fan outputs provide.



  • OK, kind of thought it would would not be that simple. Though I was thinking I could maybe get around that issue by using a smaller cap.

    Klipper does have option for hardware PWM but maybe just not on the heater bed port. Is it only the fan ports that support hardware PWM?



  • @dc42 said in Safe to add capacitor to smooth LED powered by PWMed bed output?:

    ..all Duet heater and fan outputs provide.

    Sorry missed the first time around.

    I ended up moving to E0 pin and klipper allowed me to enable hardware PWM there. Not sure why the bed didn't work but this works fine too.

    Thank you for the info.


  • administrators

    @norbs12 said in Safe to add capacitor to smooth LED powered by PWMed bed output?:

    OK, kind of thought it would would not be that simple. Though I was thinking I could maybe get around that issue by using a smaller cap.

    Klipper does have option for hardware PWM but maybe just not on the heater bed port. Is it only the fan ports that support hardware PWM?

    All Duets (right from version 0.6) support hardware PWM on all heater and fan ports. It's part of our "no compromises" design philosophy. RRF for Duets has no software PWM function.

    Duet 3 boards support PWM on some of the general purpose I/O ports too.



  • @norbs12 said in Safe to add capacitor to smooth LED powered by PWMed bed output?:

    ........................Not sure why the bed didn't work but this works fine too.

    Depending on which board/firmware version you are using, it used to be the case that the bed heater default PWM frequency was around 10Hz (to suit SSRs and the like), whilst other heater and fan outputs would default to much higher values. So that might explain why heater 0 acted differently.



  • I'll have to toy around with it more later. I actually found out today I had a faulty PSU causing most of my headaches. On any load change it was causing my lighting to flicker a little. It was not enough to cause printing issues but it was noticable in the lighting and the main reason I was trying to get hardware PWM working (so I could do higher PWM frequencies and avoid the communication from klipper). But now that I got hardware PWM on the lights and the issue persisted, I realized it could be a bad PSU. I finally tried a different PSU and everything is perfect. Working on RMA now.

    Either way, I appreciate all the info and quick responses as always.


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