Bed is Heating Uncontrollably (Duet 2 Wifi)



  • Hi all, went to begin a print yesterday and around 5-10 minutes after switching on, I noticed that my bed temperature was already up to 80 degrees. Thermistor is picking up change in temperature, but heating is switched to 'off'.

    I've a standard Ender 3, and replaced the board with Duet 2 Wifi. Bed heater is plugged directly into the board (no external mosfet in between etc, which I've seen some people doing in forums).

    • Bed begins heating as soon as main power is switched on
    • Mosfet next to heater terminals gets hot very quickly (am leaving it all switched off for time being obviously)
    • Even when board is powered by USB, bed heats up about 5 to 10 degrees hotter than room temperature. Drops again when switching off
    • Board powered off, touching terminals between the two outer terminals of the mosfet (gate & source I believe) with a digital multimeter shows a current flows between the points (when powered off).

    My guess is that the mosfet has died. Could someone confirm this with me, or give some insight as to any more testing/troubleshooting I should do?

    Thanks all, appreciate your help.



  • Certainly sounds like a failed close circuit MOSFET. No need to read across the MOSFET itself, just reading the heater terminals should be fine, many boards have been killed by accidental slips of multimeter probes.

    Disconnect the heater (including thermistor) from the board and measure it's resistance. Make sure there is no continuity between the heater and the thermistor wires.

    Who did you buy the board from?


  • administrators

    @adamlove that certainly sounds like a failed FET. Is the resistance of the bed sensible or is that short circuit as well? I would contact your supplier for a replacement.


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