Bed heater constantly on

  • Hello!

    I just got this really weird problem, the bed heater turns on as soon as I turn on the printer and I can't control it from the web interface. The bed heater led on the borad is on as well.

    This started when I installed a second print cooling fan, everything was working fine until the print fans were supposed to turn on at layer 2, then everything stalled and there were banging noises coming from the fans. I unplugged the printer and unplugged the fans to see if there was a short anywhere but I couldn't find any. I installed the fans again and everything seems to be working fine except the bed.

    Has anyone experienced something like this? Is it fixable or did I fry the board somehow?

    Best regards,

  • administrators

    First disconnect the bed heater from the Duet and check whether the bed heater light still comes on at power up. If it does, then unless the underside of your Duet is shorting against some metalwork below it, then I suspect you had a short in your bed heater wiring and the bed heater mosfet has failed short-circuit. The bed heater mosfet can be replaced, but SMD rework equipment is needed. It may be necessary to remove the bed heater terminal block first. The input and output of the driver chip should be measured first in case the mosfet isn't the problem.

  • So the light comes on without the cables attached as well. There's nothing close to the back of the board at all. And I can't detect any short in the bed wiring either, cables and connections look fine and resistance is 1.8 Ohms, which I guess is normal. Also , I didn't touch anything related to the bed when installing the second fan. Oh, and I run the fans on the internal 5V supply, if that might be a clue?

    I have a multimeter but I don't know what to look for if I should check the mosfet and driver chip.

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