What is PS_ON?

What Is PS_ON?

PS_ON (also written PS_ON# or PS-ON) is a logic-level control signal found on the ATX main power connector that links a computer's power supply unit (PSU) to its motherboard. It is the mechanism by which a modern computer performs a "soft" power-on and power-off, rather than relying on a mechanical switch that physically cuts mains power. The signal is defined in the ATX power supply design guide and is present as pin 16 (green wire) on the standard 24-pin ATX connector (formerly pin 14 on the original 20-pin connector).

Understanding PS_ON is important for IT technicians because it explains how a PC can be turned on with a momentary push-button, how it can be woken from sleep states, and how a PSU can be bench-tested outside a case.

How PS_ON Works

The PS_ON signal is an active-low logic line. This means:

• When PS_ON is high (pulled up internally to about +5V), the power supply's main outputs (+3.3V, +5V, +12V rails) remain off. Only the +5V standby rail (+5VSB, the purple wire) is active.
• When PS_ON is pulled low (connected to ground, roughly 0V), the power supply turns its main rails on.

The motherboard controls this signal. When the system is plugged into mains and switched at the wall, the PSU immediately provides the +5VSB standby voltage. This standby power keeps a small portion of the motherboard alive, including the circuitry that monitors the power button. When you press the front-panel power button, the motherboard's power management logic drives the PS_ON line to ground, and the PSU energizes all rails.

Because PS_ON simply needs to be connected to any ground (COM) wire to activate the supply, technicians can test a PSU without a motherboard by using the "paperclip test" or a dedicated PSU tester.

The Paperclip Test

To verify that a PSU powers on independently of a motherboard, a technician can:

1. Unplug the PSU and disconnect it from all components.
2. Insert a bent paperclip or jumper between the green PS_ON wire (pin 16) and any black ground wire (pin 15 or 17).
3. Switch on the PSU. If the fan spins and voltages appear, the supply can at least turn on.

This does not fully load-test the PSU but confirms the PS_ON activation circuit functions.

Related Signals

PS_ON works alongside two other important ATX control signals:

• +5VSB (Standby) — The purple wire supplies a small amount of always-on power used to keep management circuits and features such as Wake-on-LAN alive while the PC appears off.
• PWR_OK / Power Good (PG) — The gray wire is asserted high by the PSU once all rails are stable and within tolerance after PS_ON is engaged. The motherboard waits for Power Good before releasing the CPU from reset. This handshake prevents the system from booting on unstable voltages.

The typical sequence is: mains applied → +5VSB active → motherboard drives PS_ON low → main rails ramp up → PSU asserts Power Good → system boots.

Why PS_ON Matters

Soft Power Control

Before ATX, AT-style systems used a physical switch that literally broke the AC mains circuit. PS_ON enabled the modern behavior where the operating system can shut the machine down in software (for example, Windows' Shut Down command), because the OS instructs the motherboard to de-assert PS_ON.

Power Management and ACPI

PS_ON is fundamental to ACPI (Advanced Configuration and Power Interface) power states. Sleep and hibernate states rely on the interplay between standby power and PS_ON to reduce power consumption while retaining the ability to wake quickly.

Wake Events

Features such as Wake-on-LAN, wake-on-USB, and RTC (scheduled) wake work because +5VSB keeps monitoring circuits powered while PS_ON is high. When a wake event occurs, the motherboard asserts PS_ON to bring the system fully online.

Common Use Cases and Considerations

• PSU troubleshooting: A dead system that shows no signs of life may have a failed PS_ON circuit, a faulty power button, or a PSU that never receives standby power.
• Server/lab automation: Some remote power controllers manipulate PS_ON to enable remote power cycling.
• Custom builds: When testing components on an open bench, jumping PS_ON is a quick way to power a build without the case power button.

Best Practices

• Always unplug and discharge a PSU before manually jumping PS_ON, and never bridge it to a live voltage rail — only to ground.
• Use a proper PSU tester when possible, as it applies a minimal load and reads all voltages, giving safer and more meaningful results than a bare paperclip.
• Remember that a PSU passing the PS_ON test may still fail under real load; it only confirms the supply can start.

Real-World Example

Suppose a customer's PC appears completely dead. A technician confirms the wall outlet works, then removes the PSU and performs the paperclip test. The fan spins, indicating PS_ON activation and basic rail output are functional. The problem is therefore likely elsewhere — perhaps a faulty front-panel power button, a shorted motherboard preventing PS_ON from being asserted, or a component drawing excessive current. This isolates the fault quickly and demonstrates why understanding the PS_ON signal is a practical, exam-relevant skill.