How to test a PC ATX power supply with a paperclip

How the Power-On trigger signal works in ATX power supplies

When we press the power button on a computer case, the motherboard does not send heavy operating power directly to the system. Instead, it sends a low-voltage control signal to the power supply unit. This signal is known as the Power-On pin (or PS_ON#, where the hash sign indicates active-low logic, meaning it shuts down with high voltage and turns on when grounded). For the ATX power supply to wake up, it must receive this specific command through the main 24-pin power connector.

In practice, this means your computer's power supply has two main electrical states. The first is the standby state, where only the 5 Volt standby rail (or 5VSB) remains active to power basic motherboard circuits, such as the chip responsible for monitoring the power button and selected USB ports. The second state is full operation, which occurs when the PS_ON# pin is connected to ground (GND). This intentional short circuit tells the power supply's internal circuitry that it can release all its main currents and voltages, such as the 12V, 5V, and 3.3V rails.

Understanding this logic frees us from myths and demonstrates why the power supply never turns on by itself just by plugging it into the wall outlet. Before performing any practical bench procedure, it is also worth understanding the electronics involved in the motherboard power-on circuit, since the actual issue might be in the case button or the motherboard itself, rather than the power supply.

The paperclip trick: how to simulate the motherboard

The famous paperclip test is a classic bench procedure used by hardware technicians for decades to isolate power failures. If your computer shows no signs of life when pressing the power button, the first logical step is to find out whether the fault lies in the power supply or the motherboard. Since the ATX power supply is an independent block that only needs to receive a ground signal on the green pin to turn on, we can use a piece of conductive metal to trick the power supply and take the motherboard's place.

In practice, the main 24-pin power connector features a universal color coding standard created by the ATX consortium. The PS_ON# pin is invariably identified by a green wire, while any ground pin uses a black wire (GND). By inserting an unfolded U-shaped paperclip joining the green wire hole to any neighboring black wire hole, we close the exact electrical circuit that the PC button would actuate. If the power supply is healthy, its internal circuits will recognize the immediate grounding and spin up the cooling fan within milliseconds.

However, it is worth noting that this quick test only indicates whether the primary circuit and basic switching stage are responding. If you suspect deeper board problems that may have damaged circuits, it is wise to learn how to diagnose motherboard shorts before reconnecting anything, preventing you from discarding a working power supply or burning a brand-new part.

Step-by-step guide to safely test the power supply on the workbench

Before getting your hands dirty, it is essential to ensure that the power supply is completely disconnected from the electrical outlet and any computer components, such as graphics cards, SSDs, and the motherboard itself. Working with electricity requires caution, and internal power supply capacitors can retain dangerous charges even after disconnection from the mains. Have a clean paperclip ready and, if possible, a digital multimeter to measure output voltages during activation. To explore these architectural concepts further in production, see our related technical analysis on Why CORS blocks requests and how to configure in Node.

1. Turn off the rear power switch of the power supply (if equipped) and unplug the power cord from the wall outlet to eliminate any shock hazard while handling connectors.
2. Disconnect absolutely every cable plugged into storage drives, expansion cards, and the motherboard, leaving the power supply 100% isolated.
3. Take the main 24-pin connector and locate the green wire (PS_ON#) and an adjacent black wire (GND), keeping the plastic latch facing upwards.
4. Insert the two ends of a U-shaped paperclip directly into the metal terminals corresponding to the chosen green and black wires.
5. Plug the power supply cable into the wall outlet and turn on the rear switch; immediately observe whether the internal fan starts spinning continuously.

What to do if the power supply turns on or refuses to start

When you close the contact between the green and black wires and the fan instantly fires up, you have strong evidence that the startup circuit and the 5VSB auxiliary supply are healthy. However, remember that fan rotation only proves the basic stage has turned on; it does not guarantee that the power supply is delivering clean, stable energy required to power a modern CPU and demanding graphics card. For absolute certainty, use a digital multimeter on the direct current (DC) voltage scale to measure the yellow pins (12V), red pins (5V), and orange pins (3.3V).

On the other hand, if you placed the paperclip correctly, plugged the power supply into the wall, and absolutely nothing happened — not even a twitch from the fan —, there are two likely scenarios. The first is a catastrophic failure of the internal fuse or active PFC circuit due to a power surge or prior short circuit. The second scenario, common in modern high-efficiency units, is the triggering of strict protections against zero load conditions (OPP/OCP). Some reputable power supplies feature sensors that refuse to boot under total empty load, requiring at least a tiny current draw (like an old 3-pin cooling fan) to authorize startup.

In summary, the paperclip test is a fast and extremely efficient triage tool on any tech enthusiast's workbench. It separates primary electrical issues from the rest of the hardware in seconds. Combining this knowledge with visual inspection best practices and precision instrument measurements saves time, prevents unnecessary spending on new parts, and provides clear insight into the internal behavior of your computer's energetic heart.