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quick circuit board to power on an ATX computer power supply. wiring from top down:
Power Good; gray wire
+5V supply; red wire
+5V standby; purple wire
Power On; green wire
ground; black wire
when the PSU doesn't have mains power, all lights are off.
when mains is connected, the +5V standby light is on. standby power is good for an amp or two, more than enough to run an arduino and possibly enough to run a raspberry pi.
bring the Power On line to ground through a resistor, I use 10k. I put a switch here. when connected to ground, the power supply will turn on, the fan should start spinning and the +5V supply light will turn on.
the power good light should turn on about 0.2ms after the +5V supply light. this isn't very useful by itself; the motherboard is supposed to read it and work out what to do if it comes on too early or too late.
however Power Good can be useful when you do something stupid eg. short circuit the outputs. if the fault logic detects a fault, this light will go out and the power supply will need resetting.
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