Converting an ATX PSU to a bench supply i found out how to start it by shorting the green wire to ground with a Push-button, and that I have to place a minimum load (preferably on the rail with highest amp rating) to keep it running.

What I want is another Push-button to shut it down again. (I don´t want to use a on/off switch between green and ground, I also dont want to use the on/off switch on the back of the PSU)

How does a mainboard turn off the PSU? does it simply disconnect all loads? if so, how can I achieve this for all rails (3.3, 5, -5, 12, -12) with a single Push-button?

  • \$\begingroup\$ Don't they just use latching toggle push buttons? \$\endgroup\$
    – vicatcu
    Sep 15, 2018 at 13:38

1 Answer 1


An ATX motherboard connects PWR_ON# (green wire) to ground to turn on power and disconnects it from ground to turn power off. All power (except stand by 5V) is controlled by that.

If you want separate push buttons for on and off, you’ll need to make a latching circuit, probably powered by the SB5V, to pull the green wire to ground. Your ON button triggers the circuit. The OFF button could be a normally closed button to cut power to the circuit or a NO button to trigger a reset to the circuit. This circuit could be as simple as a DPST 5V relay where the second poll hold power to the relay, or an SR flip flop (Wikipedia) made from a couple of BP transistors.

  • \$\begingroup\$ Thank you for your answer, I think my misconception was that PWR_ON# and ground just have to be connected momentarily to turn the PSU on. So in a PC the power button makes the initial connection (momentarily) and the MB keeps it that way (permanently, till shutdown) \$\endgroup\$
    – downforme
    Sep 15, 2018 at 14:01
  • \$\begingroup\$ Additionally, the dummy load is there for voltage regulation only and not for "keep alive" \$\endgroup\$
    – downforme
    Sep 15, 2018 at 14:07
  • \$\begingroup\$ Actually, the power button is connected to an input on the MB. The MB logic itself holds and releases the PWR_ON# line, similar to the SR FF I mentioned. Otherwise, your comments are correct. The process of powering on and off an ATX system is actually much more complex, including the MB monitoring the PWR_GOOD line from the power supply to assume power is stable before starting the CPU. \$\endgroup\$
    – DoxyLover
    Sep 15, 2018 at 14:12

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