Turn an ATX power supply into a bench supply (safely)
ATX 24-pin wire colours and voltages, how to switch the supply on with PS_ON#, when a load resistor is needed, and how to add fuses and binding posts.
A spare PC power supply gives you solid 3.3 V, 5 V and 12 V rails for next to nothing. It isn't a lab supply — there's no adjustable voltage and no current limit — but with a few precautions it's a handy fixed-voltage source for projects and tests.
Wire colours
| Colour | Signal | Notes |
|---|---|---|
| Yellow | +12 V | Highest current on modern supplies |
| Red | +5 V | |
| Orange | +3.3 V | One orange wire is often the 3.3 V sense line (sometimes brown) — keep it connected |
| Black | Ground (COM) | |
| Purple | +5 V standby (5VSB) | Live whenever mains is connected, even when “off” |
| Green | PS_ON# | Pull to ground to switch the supply on |
| Grey | PWR_OK | Goes high when the outputs are stable — a signal, not a power output |
| Blue | −12 V | Low current only |
| White | −5 V | Only on old supplies; dropped from the ATX12V spec long ago |
Switching it on
Connect the green wire to any black wire through a small toggle switch: closed = on. An LED with a 330 Ω–1 kΩ resistor across a +5 V output makes a useful “on” indicator.
Does it need a load?
Older, group-regulated supplies regulate +5 V and +12 V together and may drift or shut down with no load. Modern supplies with DC-DC converters usually run fine unloaded. If yours cycles or shows odd voltages, add a load resistor on +5 V: 10 Ω rated 10 W draws 0.5 A and dissipates 2.5 W — mount it on metal, it gets warm.
Make it safe
- Fuse every output you bring to a binding post (for example 2–5 A fast-blow blade fuses). The 12 V rail can deliver 20–40 A — enough to burn a track, a thin lead or your fingers.
- Use proper binding posts and heavy wire; bundle and insulate the unused wires rather than cutting them loose.
- Don't use it to hunt shorts on a board. Without current limiting it will simply burn the shorted part — or the tracks around it. For repair diagnostics use a current-limited bench supply; see the no-power troubleshooting plan.
- Never open the supply itself while it's connected, and remember the primary capacitors can hold a dangerous charge after unplugging.
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Torry's Delphi Editor & curator
Hi, I'm Torry's Delphi. For more than ten years I've been writing software that collects and organises data — scrapers, catalogues, automation pipelines. Repair documentation turned out to be the perfect mess to clean up.
The files listed here were collected by the community in the Telegram channel @schematicslaptop, plus a Google Drive archive. I wrote a parser that reads every file name, recognises the brand, the ODM manufacturer (Compal, Quanta, Wistron…) and the board number, and turns it into pages you can search and browse.
Next to the catalogue I keep a small set of practical notes on electronics and PCB work — KiCad libraries, moving designs from Eagle, making prototype boards at home, LCDs and EEPROMs. If something is sorted wrong or a note is unclear, write to me and I'll fix it.