What is PWR_OK?

What Is PWR_OK?

PWR_OK, also known as Power Good or PG, is a control signal produced by an ATX power supply unit (PSU) that tells the motherboard the DC output voltages (+3.3V, +5V, and +12V) have stabilized after power-on. Until this signal is asserted, the system's processor and chipset are held in a reset state, preventing the computer from attempting to boot while voltages are still ramping up and potentially unstable.

The PWR_OK signal is critical because it protects the system from erratic behavior or data corruption that could occur if the CPU began executing instructions before power rails reached their proper levels. It is one of the fundamental handshake signals in the ATX power specification.

Why It Matters

Modern power supplies do not deliver stable output the instant they are switched on. There is a brief ramp-up period during which voltages rise toward their target values. If the CPU tried to start executing during this window, unpredictable results could occur. PWR_OK acts as a gatekeeper, ensuring the logic circuitry only wakes up once the electrical environment is safe.

A missing or improperly timed PWR_OK signal is a common cause of "no boot" or "dead system" symptoms even when the power supply fans spin up.

How It Works

The PWR_OK signal is carried on pin 8 of the 24-pin ATX main power connector and uses the gray wire in standard color-coded PSU cabling. Its operation follows a defined sequence:

1. The user or system asserts the PS_ON# signal (green wire), instructing the PSU to turn on its main outputs.
2. The power supply begins generating its DC rails and their voltages start to rise.
3. Internal monitoring circuitry within the PSU checks that +3.3V, +5V, and +12V are within tolerance.
4. Once all rails are stable, the PSU drives PWR_OK from a logic LOW (0V) to a logic HIGH (typically +5V) after a delay of 100 to 500 milliseconds.
5. The motherboard releases the reset line, and the CPU begins its startup and POST (Power-On Self-Test) sequence.

If any voltage rail drops out of tolerance during operation, the PSU pulls PWR_OK back LOW, which forces the system into reset and effectively shuts it down to prevent damage.

Key Electrical Characteristics

• Logic LOW: less than 0.4V (power not good / not ready)
• Logic HIGH: +2.4V to +5V (power good / stable)
• Turn-on delay: 100 ms to 500 ms after voltages stabilize
• Connector location: Pin 8 of the 24-pin ATX connector
• Wire color: Gray

Relationship to Other Power Signals

PWR_OK works alongside several other important power management signals:

• PS_ON#: The signal the motherboard uses to command the PSU to turn on the main outputs.
• +5VSB (Standby): The always-on standby rail that powers wake circuitry even when the system is "off."
• +3.3V Sense: A feedback line allowing the PSU to compensate for voltage drop.

Common Use Cases and Diagnostics

Understanding PWR_OK is essential for hardware technicians troubleshooting boot failures:

• Diagnosing dead systems: If fans spin but the system does not POST, a faulty PWR_OK signal may be the culprit.
• PSU testing: Power supply testers and multimeters can measure the gray PWR_OK wire to verify it reaches +5V after startup.
• Paperclip test caveat: Jumping PS_ON# to ground (the classic paperclip test) will spin the PSU fan, but it does not confirm that PWR_OK is functioning correctly under load.
• Aging power supplies: As PSUs degrade, the PWR_OK timing may drift, causing intermittent boot failures that are difficult to diagnose.

Best Practices and Considerations

• When replacing a power supply, ensure it meets the correct ATX specification so PWR_OK timing matches the motherboard's expectations.
• Use a quality PSU tester that reports the Power Good delay in milliseconds; values outside the 100–500 ms range indicate a marginal supply.
• Do not rely solely on the fan spinning as proof of a working power supply, since PWR_OK is a separate signal.
• Cheap or counterfeit power supplies sometimes hardwire PWR_OK to always report "good," bypassing safety protection and risking hardware damage.

Real-World Example

A technician receives a desktop that appears completely dead. Pressing the power button causes the CPU fan to spin briefly and then stop. Using a multimeter on the gray wire of the 24-pin connector, the technician measures a voltage that never rises above 0.3V after power-on. This indicates the PWR_OK signal is never asserted, meaning one of the voltage rails is out of tolerance. Replacing the failing power supply restores the correct +5V PWR_OK signal, and the system boots normally.