Repair your PSU by the numbers,
not by guesswork.
Pick a wattage class to see its rail budget, then tap a wire color to read the exact voltage it should carry. A simulated multimeter shows you what a healthy probe looks like.
1 · Choose a PSU by wattage
Set your estimated system load, then compare rail capabilities across the common ATX wattage classes. The unit marked PICK is the smallest one with a healthy ~30% headroom.
Recommended minimum: — · formula: load × 1.3 (peak headroom)
Repair tip: when a unit trips under load, measure the +12 V rail with a dummy load. A drop below 11.40 V points at dried capacitors or a failing rectifier.
2 · Pick a wire color → read its voltage
ATX pinout color codes are standard. Tap any color to see its nominal voltage, tolerance window and what the wire actually does. Then press Take measurement to simulate a healthy probe reading.
All tolerances per ATX specification (±5% on main rails). Legacy −5 V (white) is absent on modern ATX 2.2+ units — treat a missing white wire as normal, not as a fault.
3 · Bench diagnostic workflow
A repeatable order of operations that keeps you and the hardware alive.
Discharge & inspect
Unplug, wait 5 minutes, then discharge the primary caps through a 10 kΩ / 5 W resistor. Look for bulged caps, brown glue, and cracked solder joints.
Check the fuse & bridge
An open fuse is a symptom, not a fault. Test the bridge rectifier and the main switching transistors before replacing anything.
Standby rail first
Power up and measure +5 VSB (purple). No standby means the fault is on the primary side — stop there.
Force it on
Short PS_ON# (green) to ground through a 1 kΩ resistor. The fan should spin and all rails should appear within ~500 ms.
Load test every rail
Apply a dummy load and verify each rail stays inside tolerance. Ripple above 120 mV on +12 V means the output caps are done.
Verify PWR_OK
The gray wire must rise to ~5 V between 100–500 ms after the rails settle. A late or missing signal causes random shutdowns.
4 · Common faults & likely causes
Start from the symptom, then work backwards through the circuit.
| Symptom | Suspect | Quick test | Severity |
|---|---|---|---|
| Completely dead, no standby | Blown fuse, shorted bridge rectifier, main MOSFET | diode mode across the bridge |
High |
| Standby OK, won't start | PS_ON# driver, standby controller IC | Short green to GND through 1 kΩ |
Medium |
| Starts then shuts down instantly | Over-voltage / over-current protection, shorted output diode | Measure each rail into a dummy load | Medium |
| Random reboots under GPU load | Aged +12 V output capacitors, high ESR | Scope ripple — look for > 120 mV | High |
| Loud squealing / ticking | Transformer, snubber network, failing fan bearing | Isolate fan by unplugging it briefly | Low |
| Rails sag under load | Undersized +12 V rectifier, bad PWM feedback loop | Compare no-load vs full-load readings | Medium |
⚠ Safety first
- The primary-side capacitors hold up to 400 V DC and can keep a lethal charge for minutes after unplugging. Always discharge them.
- Never work on a PSU that is plugged into mains. Use an isolation transformer for live measurements.
- Do not bridge PS_ON# with a bare wire while the unit is connected to a motherboard — use a proper jumper or resistor.
- Replace capacitors with the same or higher voltage rating and a low-ESR, 105 °C part.
- If the PCB is scorched, the transformer is burnt, or the case is deformed — recycle the unit. Some repairs are not worth the risk.