How to turn on ATX PSU without connecting a motherboard ?
It is sometimes necessary to power on devices and use an ATX power supply without actually connecting and booting up a motherboard.
You can turn on ATX power supplies by connecting a short wire/paper clip between the green (/PS_ON) wire on the large 20/24 PIN ATX connector and ground (black wire). The green /PS_ON wire is pin 14 on 20-pin connetors, and pin 16 on 24-pin connectors.
The power button on ATX cases goes through the motherboard, but still shorts that /PS_ON wire to ground when pressed.