What is Power Good (PWR_OK) Signal?
The Power Good (PWR_OK) signal is a control signal sent by an ATX power supply (PSU) to the motherboard after all output voltages have stabilized. It tells the system that power is safe and reliable, allowing the CPU to begin the boot process. Without a valid PWR_OK signal, a PC typically will not start correctly.
Key Takeaways
- PWR_OK is a status signal, not a power rail.
- It is generated by the PSU after voltage outputs become stable.
- The motherboard waits for PWR_OK before starting the CPU.
- It improves system reliability by preventing startup with unstable power.
- PWR_OK is a standard feature of modern ATX power supplies.
Why Does the Power Good (PWR_OK) Signal Exist?
Electronic components require stable voltage before they can operate safely. During startup, PSU output voltages briefly fluctuate while internal circuits stabilize.
The PWR_OK signal exists to:
- Prevent the motherboard from booting too early.
- Protect the CPU, RAM, and chipset from unstable power.
- Improve boot reliability.
- Ensure all major voltage rails are operating within ATX specifications.
Without this delay mechanism, a computer could crash, fail POST, or experience hardware instability during power-on.
How Does the Power Good (PWR_OK) Signal Work?
The startup sequence follows a defined order:
- The user presses the power button.
- The motherboard sends the PS_ON# signal to the PSU.
- The PSU enables its internal power conversion circuits.
- The +12V, +5V, and +3.3V rails stabilize.
- The PSU verifies that all output voltages are within acceptable tolerance.
- The PSU asserts the PWR_OK signal (typically a +5V logic signal).
- The motherboard releases the CPU from reset and begins the POST (Power-On Self-Test).
If voltage later falls outside specification, the PSU deasserts PWR_OK, allowing the system to shut down safely.
Key Characteristics
- Digital status signal from PSU to motherboard
- Used only during startup and fault conditions
- Indicates stable output voltage
- Part of the ATX power supply standard
- Works automatically without user configuration
Important Specifications
| Specification | Typical Value |
|---|---|
| Signal Name | PWR_OK (Power Good) |
| Source | ATX Power Supply |
| Destination | Motherboard |
| Logic Level | Approximately +5V |
| Assertion Time | Typically 100–500 ms after power rails stabilize |
| Standard | Intel ATX Power Supply Specification |
Compatibility
PWR_OK is compatible with:
- ATX motherboards
- ATX power supplies
- Desktop PCs
- Gaming PCs
- Workstations
- Servers using ATX-compatible power systems
Nearly every modern desktop computer relies on this signal during startup.
Advantages
- Prevents unstable system startup
- Improves POST success rate
- Protects sensitive electronic components
- Supports reliable power sequencing
- Reduces boot failures caused by voltage instability
Limitations
- Cannot fix poor-quality PSU voltage regulation.
- Does not protect against every electrical fault.
- Only indicates that initial output voltages are within specification.
- Hardware can still fail after startup due to other power issues.
Power Good (PWR_OK) Signal vs Power-On Signal
| Feature | PWR_OK | PS_ON# |
|---|---|---|
| Generated By | PSU | Motherboard |
| Purpose | Confirms stable power | Requests PSU startup |
| Direction | PSU → Motherboard | Motherboard → PSU |
| Timing | After voltage stabilizes | Before power rails turn on |
| Function | Allows system boot | Turns PSU on |
Common Misconceptions
Is PWR_OK the same as the power button?
No. The power button initiates startup, while PWR_OK confirms that power has stabilized.
Does PWR_OK supply electrical power?
No. It is a communication signal only and carries virtually no operating power.
Can a PC boot without PWR_OK?
Generally, no. Most ATX motherboards wait for the PWR_OK signal before beginning the POST process.
Real-World Example
When you press the power button on a desktop PC, the cooling fans may begin spinning immediately. However, the processor does not actually start executing code until the PSU sends the PWR_OK signal. This brief delay, usually a fraction of a second, ensures stable power before the motherboard begins loading the BIOS or UEFI firmware.
Related Technology Terms
- PS_ON# Signal – Motherboard control signal that tells the PSU to turn on.
- ATX Power Supply – Standard desktop PSU design that defines PWR_OK behavior.
- Power-On Self-Test (POST) – Hardware diagnostic process performed during startup.
- Voltage Regulation – Process of maintaining stable output voltage under varying loads.
- Hold-Up Time – Duration a PSU maintains output after AC input is interrupted.