Powering the High Current Source while Vin is Un-powered?

  • I am working on some tweaks to my Duet 3 6HC Mainboard setup. One issue I have had is that the switching power supply powering the system has enough internal capacitance that it powers the mainboard and the Raspberry Pi for almost a minute after I sure off mains power.

    Maybe I am just being paranoid, but I really don't like the slow dropoff of power for the mainboard and SBC.

    I am considering leaving the power supply on but having a switch that shuts power off to the Duet 3 Vin and the SBC power. Effectively shutting off the system.

    The issue comes from finding a switch that can also handle the current I am using for the bed heater. Without that item in the mix, I have some great illuminated switched that would work great. But none of them will also handle the bed current that I am supplying separately.

    My question is if leaving the High Current Input powered while shutting off the Vin will damage the board in any way. Will it end up powering the board somehow? Is it going to damage any of the high current control circuits? I am hoping to just be able to leave the high current Input unmanaged but before I go this route I want to be sure I won't damage anything.

    If needed I can set up a relay that turns on when the board power comes on. This would allow me to shut down the High Current supply at the same time as Vin, but it is more parts and effort.

    Any suggestions would be welcome.

  • Its relatively common for high power power supplies to have enough capacitance to take a while to shut down with little to no load. Higher end ones will turn off the output as it falls below the specified regulation however, and even provide a power good output signal.

    instead of a high power switch or a relay, if you have a spare normally closed contact on your switch, you could add a discharge resistor to do a controlled discharge of the power supply when off (but the value and ratings would have to be determined according to the amount of energy to deal with).

    another aspect is you could run separate wires to supply the bed input, and use a small-ish switch to cut vin for the rest of the board.

    however unless the supply has transients i doubt the sbc will care about the delay as long as it guaranteed a cold start before power comes back on potential brownout issues shouldn't be a problem.

    bottom line is a quality supply shouldn't be a problem, and a better supply will solve the problem.

  • administrators

    @drobertson123 said in Powering the High Current Source while Vin is Un-powered?:

    My question is if leaving the High Current Input powered while shutting off the Vin will damage the board in any way.

    You can safely leave the high current input powered when the rest of the board is unpowered.

  • @dc42 Thanks, this is exactly what I needed.

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