What Is the Power Good Signal?
The Power Good signal (often abbreviated PWR_OK or PG) is a control signal produced by a computer's power supply unit (PSU) to inform the motherboard that the DC voltages it supplies (+3.3V, +5V, and +12V) are stable, regulated, and within their specified tolerance ranges. Only after this signal is asserted will the motherboard's chipset allow the CPU to leave its reset state and begin executing the boot firmware. This mechanism is a foundational part of how modern PCs achieve a safe, orderly startup.
The Power Good signal matters because computer components are sensitive to voltage fluctuations. During the brief moment when a PSU is first switched on, the voltages ramp up and can be erratic. If the CPU and other components attempted to operate during this unstable period, the result could be data corruption, crashes, or hardware damage. The Power Good signal acts as a safety gate that prevents the system from running until conditions are safe.
How It Works
When a computer is powered on, the PSU begins producing its output voltages. However, these rails do not stabilize instantly. The Power Good circuitry within the PSU monitors the output voltages and, once they have all reached and held their nominal levels within tolerance, it raises the Power Good line to a logic high state (typically +5V) after a defined delay.
- Timing: The ATX specification requires the Power Good signal to be asserted between 100 and 500 milliseconds after the output voltages are within regulation.
- Signal level: When power is good, the signal is held at approximately +3V to +6V (a logic high). When power is inadequate or failing, it drops to 0V (logic low).
- Connection: In the ATX standard, the Power Good signal travels on the gray wire (pin 8) of the 20/24-pin main power connector.
Interaction With the CPU Reset
The motherboard ties the Power Good signal to the processor's reset logic. As long as Power Good is low, the CPU is held in reset and does nothing. When Power Good goes high, the reset is released and the CPU begins fetching instructions from the BIOS/UEFI firmware. If the voltages ever drop out of tolerance during operation, the PSU de-asserts Power Good, immediately triggering a reset to protect the system.
Key Components and Concepts
- Voltage regulation: The PSU must keep each rail within roughly ±5% of its nominal value.
- Power-on delay: The intentional delay ensures rails have settled before the system starts.
- Brownout protection: If input AC voltage sags, the PSU can drop Power Good to force a shutdown rather than run on bad power.
- PS_ON signal: A separate signal from the motherboard tells the PSU to turn on; Power Good is the PSU's response confirming readiness.
Common Use Cases and Applications
The Power Good signal is present in virtually every ATX-compliant desktop computer and many servers. It is especially relevant in the following scenarios:
- System boot sequencing: Ensuring the CPU only starts after stable power.
- Troubleshooting no-boot conditions: A failing or missing Power Good signal is a classic cause of a system that has power (fans spin) but will not POST.
- PSU testing: Technicians use PSU testers and multimeters to verify the Power Good signal timing and level.
- Fault recovery: Automatic reset when voltages fall out of tolerance.
Best Practices and Important Considerations
- Verify timing when diagnosing intermittent boots: A Power Good signal that arrives too early can cause a system to try to boot before voltages stabilize, leading to random crashes.
- Use quality power supplies: Cheap or aging PSUs may generate a premature or unreliable Power Good signal, which is a common hidden cause of instability.
- Understand the paperclip test limitation: Jumping PS_ON to ground will spin up a PSU's fan but does not verify that the Power Good signal is being correctly generated under load.
- Check the gray wire: When probing, a healthy Power Good reads around +5V; a reading near 0V while the system is otherwise powered indicates a fault.
Note: A PSU whose fan spins and rails read correct voltages can still fail to produce a valid Power Good signal. This is why a motherboard may appear "dead" (no POST) even though the power supply seems to work. Always check Power Good behavior before condemning the motherboard.
Real-World Example
Imagine a technician facing a desktop that powers on (fans and LEDs light up) but never displays anything or completes POST. After ruling out RAM and GPU, they connect a PSU tester and observe that the Power Good indicator never lights, or a multimeter on the gray wire reads 0V instead of ~5V. This confirms the PSU is failing to assert Power Good, so the CPU stays in reset and the machine cannot boot. Replacing the power supply resolves the issue. This example illustrates why understanding the Power Good signal is essential for hardware troubleshooting and is a common topic on the CompTIA A+ exam.