Dedicated PSU's for hotend and bed



  • So, I'd like to preface this post with a little background. Historically I'm more familiar with cheaper melzi boards, the Duet is higher end and is one of the nicer boards out there, so I'm not as familiar with it. If the wiring diagram below seems funky, just be aware this is almost exactly how it works with a lower end board. Perhaps there's an issue with the signal wires I'm using, maybe not.

    I have a couple of cheap little 360 watt 12v Power Supplies, I'd like to use one to power the hotend and the board, and another to power the bed. Typically how the mosfets work is, when the board sends current to heater 1, the 18 gauge wire triggers the signal on the mosfet, which in turn pulls the voltage from the PSU to power the heating element. This gives a stronger and more stable current, allows for an additional PSU which helps to reduce loads, basically allowing for potentially faster heating, even if it's by fractions. The same thing goes for the bed.

    However, here's the problem. When I signal for the hotend to heat, my signal wires don't seem to be triggering for heater 1, but when I wire the hotend directly to the board, it works fine. I'm thinking at this point perhaps it's just a defective mosfet, but I'd like some input on whether or not what I'm doing here is even supported by the board, and possibly if there's just some issues with it signaling. Let me know what you think.

    Prepare your eyes for my terrible MSPaint skills

    Thanks all 🙂



  • Wiring looks fine. Swap the small connectors on the Mosfets coming from the Duet and see if the bed comes on with your hotend commanded on. That'll let you know if it is the Mosfet or not.


  • administrators

    Why not connect the hot end directly to the Duet? Hot end heaters draw only a few amps, so the external mosfet serves no useful purpose.



  • @dc42:

    Why not connect the hot end directly to the Duet? Hot end heaters draw only a few amps, so the external mosfet serves no useful purpose.

    To bypass the drain directly from the board and go straight to the PSU, using the H1 as the signal for the mosfet to indicate draw. It's not completely necessary, but it's something I'm used to doing. I've always had a preference for running the heating aspect of a printer directly from the supplies for the sake of complete energy efficiency.


  • administrators

    The efficiency saving from doing that is negligible, quite possibly negative because of the energy needed to drive the opto isolator in the mosfet switch.


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