Convert ATX Computer PSU to Bench Power Supply

 

ATX Bench Supply

This is a great way to get a cheap lab power supply, and serves as an introductory project in electronics. It was actually my very first successful electronics project. Computer PSU's supply a good range of low voltages, which is useful for all sorts of smaller electronics projects. Your garden variety ATX supply has a +5, +12, +3.3, -5 and -12 volt rails. The positive 5 and 12V rails can supply enough amps to power projects consuming hundreds of watts, while the other rails are rated for just one amp of so. The negative rails have actually been regulated by 7905 and 7912 ICs in many of the supplies I've disassembled. However these PSUs won't function outside of a computer without some hacking first. Most of what's written here applies for AT supplies too.

Modified ATX PSU with Indicator LEDs

First you have to know which voltage is on which line. Typical wiring configurations for AT and ATX PSU's are shown below.

AT Power Supply Pin out

Pin	Name		Color	Description
1	PG		Orange	Power Good, +5 VDC when all voltages has stabilized.
2	+5V		Red	+5 VDC (or n/c)
3	+12V		Yellow	+12 VDC
4	-12V		Blue	-12 VDC
5	GND		Black	Ground
6	GND		Black	Ground

ATX Power Supply Pin out

Pin	Name		Color	Description
1	3.3V		Orange	+3.3 VDC
2	3.3V		Orange	+3.3 VDC
3	COM		Black	Ground
4	5V		Red	+5 VDC
5	COM		Black	Ground
6	5V		Red	+5 VDC
7	COM		Black	Ground
8	PWR_OK		Gray	Power Ok (+5V & +3.3V is ok)
9	5VSB		Purple	+5 VDC Standby Voltage (max 10mA)
10	12V		Yellow	+12 VDC
11	3.3V		Orange	+3.3 VDC
12	-12V		Blue	-12 VDC
13	COM		Black	Ground
14	/PS_ON		Green	Power Supply On (active low)
15	COM		Black	Ground
16	COM		Black	Ground
17	COM		Black	Ground
18	-5V		White	-5 VDC
19	5V		Red	+5 VDC
20	5V		Red	+5 VDC

In the best of cases shorting the green "Power Supply On" wire to ground will be enough to turn on the PSU. To run outside of a computer most ATX PSU's require a minimum load. A power resistor between +5v and ground or +12V and ground is used. One 2.7Ω 10W for the 5V line or 10Ω 15W for the 12V line will work with most PSU's. In some cases Power OK must be shorted to ground to make the PSU run. Putting a LED on Power Ok will solve this and indicate when the PSU is on. A reader has informed me that the new 24-pin ATX supplies have a sense wire which must be connected to the +3.3V rail. This sense wire is usually brown. These are usually the only steps it takes to start a PSU.
If all is good you should be getting 12V on the yellow wires and 5V on the red wires, give or take 0,5V. If the voltages are a little low the minimum loads might need to increased, which is to say the load needs lower resistance.