Using PS_ON as "safety switch" on SSR controlled bed

  • I am in the process of gathering the required hardware to convert my I3 clone from Marlin/MKS Gen L to RRF/Duet.
    On my current system I run an ATX power supply and a 240v bed with an SSR.
    As an added safety device in case the SSR fails in the open position, I run a mechanical relay powered by the ATX.
    In the event of a thermal runaway, Marlin shuts down the PS_ON supply which in turn switches of the relay and in turn the 240 volt supply.
    I also have a thermal fuse as I believe in redundant systems.

    Part of my upgrade will include switching to a 24 volt power supply, so I have to rethink this a bit.

    I have searched the forum and found discussion within the last 12 months on how to implement PS_ON shutdown after a fault, but can't find anything that says that it has in fact been implemented and how it can be used.

    In my case I'm hoping to use the PS_ON to switch the relay itself.

    Is this now possible in RRF?

    Thanks in advance...

  • administrators

    When a heater fault occurs, RRF turns the affected heater off, pauses the print, and waits a defined period of time to allow you to reset the fault. If you don't respond within that time, it shuts down and attempts to turn off power via PS_ON.

    The amount of time that it waits is set by the S parameter of the M570 command. See So if you want to replicate the Marlin behaviour, use S0 or a very low value.

  • Thanks for the reply.
    That's exactly what I'm after.

    So I should be able to power a 5v relay from Pin1 of the expansion and run the ground of the relay to PS_ON.

    This will require me to find an alternate power source for my BLTouch.
    If I convert the BLTouch to 3.3v logic, is there any issue powering it from the 3.3v pin on the Z probe connector?
    BLTouch manual says current draw is 300mA peak

  • administrators

    3.3V probably isn't enough voltage to power the solenoid on the BLTouch.

    You can get 5V from the PanelDue connector if your system doesn't include a PanelDue.

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