Power-good signal

In short: The power-good signal (PWR_OK / PG) is sent by the power supply to the motherboard as soon as all voltages are stable and within the allowed range. Only then does the CPU start.

In more detail: At power-on the voltages need a moment to rise cleanly. If the CPU started too early, it could compute incorrectly or be damaged. The power supply therefore checks its outputs first and then pulls the “power good” line to about +5 V.

In Depth

• Sequence: Power button → motherboard switches the supply on via the “PS_ON#” signal → voltages rise → after 100 to 500 ms the supply sends “power good” → the motherboard releases the CPU reset → CPU initialization begins.
• Wire: Gray wire (pin 8) on the 24-pin ATX connector.
• If the signal disappears (voltage dip), the CPU is reset immediately. The computer restarts or stays off.
• Symptom: Computer starts briefly and shuts off again, fans twitch, no beeping, no picture. Cause: defective or too weak power supply, short circuit, bad contact.
• Testing: With a power supply tester or multimeter (paperclip test only with care and without connected components).
• Related: The standby voltage +5VSB keeps the motherboard and power-on logic ready even when the computer is “off”.
• Importance for startup: Without a valid power-good there is neither POST nor picture.

See also: Power supply and redundancy in the PC, Power supply (PSU) and 80 Plus, POST (power-on self-test)