UNSOLVED Bed Heater Output for Air Pump?

  • I'm hooking up a Berd-Air part cooling setup on my delta printer. Am I safe hooking up the air pump directly to the bed heater output on my Duet 3? The Berd-Air comes with a PWM relay to prevent a fan header from being damaged, but my bed heater output happens to be free anyways and I'd rather use that if I can rather than add yet another relay to my setup.

    The air pump is rated at 1200ma@24v, which is well within the 18A limit for the bed heater output. I'm just not sure if I need to do anything special to prevent damage. Is this a brushed motor? I have no idea! The people who sold me the kit can't even tell me if the PWM relay they provided can handle a 24v signal... 🙄

    edit: actually, if I'm not mistaken, I could connect the air pump directly to output 7, 8 or 9, right? Those can each handle up to 2.5 amps and have a built-in flyback diode to protect the headers from inductive loads? I happen to have 2 of those outputs free.

  • administrators

    There are two potential issues with connecting brushed DC motors to output on the Duet:

    1. Brushed DC motors are inductive, so a flyback diode is needed.
    2. Brushed DC motors draw a much larger current when they start, or if they stall. This may exceed the peak rated current of the mosfet.

    I recommend that you use out1, out2 or out3 on the Duet 3 if any of those are free. Those all have large mosfets (rated at more than 50A) and flyback diodes built in.

    Outputs out4-out9 on the Duet 3 have a higher rated current (more than 11A) than on other Duets so they would probably be safe to use too, but I without knowing the stall current of the Berd Air, I can' t be sure.

  • @dc42 Now you have me very confused, because the documentation here:
    states that outputs 1 to 3 do not have flyback diodes built in and require an external one for inductive loads. Are you saying the documentation has it backwards?

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