PS_ON not working as expected

Hi everyone,

I'm currently rebuilding my printer's electronics with a RADDS/Due combo and I'm running into some troubles here. First and foremost the power control does not seem to be working for me and I would like to figure out why.

My power supply is driven by a relay that is to be controlled by the PS_ON pin (on the servo header of the radds). The relay is an off the shelf breakout board (one of these). VCC and GND are connected to the LCD pin header because at a first glance it looks like the only suitable 5V connector. I'm not sure about the servo connectors though, seems they need an extra 5V power supply. Anyway, the thing is, it appears like the relay is alway "on", i.e. power is always enabled. I cannot disable it via M81. The firmware says ok but nothing happens.

Power control is enabled in the configuration. I've also tried to disable auto power control to be able to switch the PSU on and of as I desire. It doesn't help. Power is always on.

You help would be greatly appreciated.



  • The relay seems low activated and the FET powering the coil seems not to work with 3.3 volts. Can anyone confirm this theory?
  • At least you can easily test if

    - M80  - Turn on power supply

    - M81  - Turn off power supply

    toggle the 3.3v at the ps on pin you used. Then you know at least you are toggling the right pin and it then must be a hardware problem. If nothing happens then PS_ON is not pointing to that pin.
  • It does. So it's definitely a hardware issue. I'm pretty sure it's the MOSFET that drives the relay. Likely does not detect 3.3v as H.
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