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The power supply for one of my computers needs a new fan. How do I discharge the power-supply capacitors to make it safe to open up?

With an ATX power supply, it's just a matter of unplugging it, waiting for the standby power LED on the mainboard to go off, and punching the power switch a few times to discharge any remaining power. But AT doesn't have standby power, and the power switch is connected between mains power and the PSU. Is it simply a matter of turning it off and waiting a few seconds, or is there more to it?

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Normally you would just let it sit for a few minutes. While the output capacitors may have some residual charge, the most any one would have is 12 Volts, which isn't really dangerous to your fingers. The input filter capacitor after the full wave rectifier, however, is a more likely threat, but killing the AC input while the power supply is under load will almost immediately drain the input DC filter capacitor.

The power supply sources the current required for it to operate from the AC input, which is then converted to DC and filtered by the input capacitor. When the AC input is no longer present, the power supply will continue to try to operate from the residual voltage in the input capacitor. If the power supply is no longer capable of powering the system, that means that the input filter capacitor has been mostly drained.

It never hurts to check first with a meter, and/or discharge through a resistor.

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    Likely accurate IF THE PSU IS INTACT. Remember that the input filter capacitors could remain charged for a long time if you open up a dead PSU!
    – rackandboneman
    Feb 5, 2020 at 17:14

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