Power conversionBench electronicsTest and identify
ATX computer supply
Also called PC PSU, desktop power supply.
The highest-current multi-rail supply in ordinary trash. Also a mains device with a primary capacitor that stays charged. Use it as a supply, or harvest the fan and the hardware, but do not treat the open board as a toy.
If you are new to this category
These parts change wall power into the lower voltage a device uses. A sealed supply with a label is the safer kind. An open board from a lamp can still be connected to the wall on the side that looks like low voltage. Isolated means the output is not electrically connected to the wall. If you are not sure, treat the output as live.
Read the label out loud: volts, amps, and the polarity symbol. Then measure. A number you remember from a similar brick is not a measurement.
Yellow is 12 volts, red is 5 volts, orange is 3.3 volts, black is ground. The supply stays off until the green wire is connected to a black one. Purple is a standby 5 volts that is present whenever the cord is plugged in, even if the fan is still. Unplug and discharge before you open the metal box.
Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.
Stored high voltage. Capacitors hold a charge with the cord in your other hand. Microwave HV capacitors, CRT anodes, xenon flash caps, and primary bulk caps are the usual ones.
| What to look for | Usual figure |
|---|---|
| Rails | +12 V yellow, +5 V red, +3.3 V orange, −12 V blue (small), +5 V standby purple. Black is ground. |
| Start | Green PS_ON to black, active low. Grey is power-good. |
| Sense | Brown is 3.3 V sense. If you move the 3.3 V wires far from it, regulation suffers. |
| Honesty | No-name wattage labels are marketing. Derate. The 12 V rail is the one that matters for motors. |
Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.
What people actually use it for
- Bench 12 V and 5 V
- A project that already lives in the PC case
- The fan, the IEC inlet, the switch, and the harness wire if the supply itself is dead
Leave this one when
Bulged caps, a burnt smell, or a fan that does not start. Part it. Do not 'just try it' on a nice board.
Buying a new one is the better idea when
A new supply if this one will sit under a desk unattended running a 3D printer. Fire insurance is a spec.
How to tell if this one still works
Inspect, discharge, jumper PS_ON, measure each rail under a small load.
Step-by-step on the bench: Start an ATX supply on the benchTools you will want: Meter, Discharge resistor, A load.
Words on this page
Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.
- mains
- Ordinary household wall power. It can kill. Unplug before you open anything that used it, and do not experiment on live wiring.
- capacitor
- A part that stores a small amount of electrical energy. Large ones in power supplies can stay charged after you unplug. Discharge them before you touch the leads.
Where people actually find it
1 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.
| From | Call | How many | How often | Where it sits |
|---|---|---|---|---|
| Desktop PC | Prize | 1 | always | Top or bottom of the case, IEC inlet. |
- In this particular machine
- A multi-rail supply. Treat the label wattage as a claim, not a measurement.
- Where to look
- Top or bottom of the case, IEC inlet.
- How to get it out
- Four screws, unplug the harness. Keep the case closed if you will use it as a bench supply.
- The condition to expect
- Often fine when the PC was scrapped for speed. Reject it if the fan is dead or the caps are bulged.