So for a long while I've wanted to make a bench power supply from an old ATX supply from an old computer. As I don't have a lot of equipment on hand, I've had to resort to scavenging wall transformers or power bricks to do my projects. I'm still using a 1A wall supply a teacher had us modify(just cut off the barrel plug and added banana plugs) in our introductory electronics course in college. But while I can get a decent variety of voltages by hacking up a unused supply I have to keep a box of these around and most can only offer up to 1A or so. And negative voltages? Not without some work.
These limitations can be frustrating. It's a hassle to rummage through the box of supplies to find the right voltage with enough juice. Once I was trying to diagnose the cabin blower fan for a friend's car in the middle of a cold winter. I removed the fan from under the dash and wanted to test it. But when it's -10F and dark outside, you'd prefer not to stand outside and try wiring it up to the vehicle battery. However, testing indoors is a little tough when you need a +12V supply that can provide, say 10 or 20 amps. It would have been nice to have that ATX supply handy... In the end I had to confirm the operation of the fan out in the cold. Turned out the fan worked, but the contacts in the fuse panel had roasted themselves to the point of melting some of the plastic and were essentially open circuits. A little dis-assembly and sanding of the contacts got things working again... so far.
Well back to the project at hand: the ATX bench supply. Initially I did a search of the internet and found that many people have done just what I planned. So I plodded on without reading their write-ups cause I wanted to do my own thing. I grabbed a 250W unit out of an old Dell and got to it.
The first task: turn the ATX supply on. This I had already accomplished while troubleshooting some motherboards for various people. All it really took was a web search for the ATX power spec which provided this little diagram:
This pretty much tells us everything we need to know. The main Molex plug that normally goes to the motherboard includes all the signals and voltages. All the other wires going to plugs for various drives, fans, and whatnot aren't really necessary and can be removed altogether, which I did. So turning this thing on. You'll notice as signal called 'PS_ON#'. When this signal is connected to ground our supply fires right up. Design wise, the project is kinda done. The rest is physically rigging the supply up to be practical.
You can see that I've added a couple LED's for indication. There is always a low current standby 5V source when AC is plugged in (via 5VSB), which will be designated by the yellow LED. When the outputs settle, the PWR_OK signal goes high, which I use to drive the red LED on. I didn't find how much current the PWR_OK signal could source, so to play it safe I just drove a transistor with it. Not much else to really say.
 |
| Unmodified power supply. |
 |
| Wire bundle from power supply. Most redundant wires were removed. |
 |
| Terminal strip mounted, dry erase marks for switch and LED drilling. |
 |
| Internal modified wiring with switch and LED board mounted. Used a spare piece of proto board cut from another project, held in place using hot glue. |
 |
| Power supply reassembled. Future modification would be to drill a new hole to feed the wires up in order to clean things up. Other than that, works just as intended. |
