ATX Power Switch Substitute

An additional push-button switch is normally required for the ATX
Power Switch/Soft Power Switch signal, but you can do without it if you
use this simple circuit. It is an artful design, but it has been
repeatedly tested. The zener diode is intended to provide protection
against excessive voltages and reverse-polarity connection. In the
latter case, the resulting short-circuit current (approximately 1A) will
exceed the allowable limit and cause the ATX
power supply to shut down after around five seconds. It might be
possible to use a smaller capacitor; this must be tested experimentally
in actual use.

If the motherboard documentation is poor, you should verify the
earth pin using a continuity tester. The resistor is only needed if you
want to be able to switch on the PC within ten seconds after switching
it off. It discharges the capacitor quickly enough to make this
possible. With a 1-kΩ resistor, the time constant is around 0.5 s. Since
the capacitor also tends to stabilize the voltage, this circuit could
also help in situations in which the ATX power supply switches off unintentionally due to voltage fluctuations on the PWR Supply On line.

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