This project is a great way to bring life back to a retired ATX power supply. These make a great, and relatively high current, benchtop switch mode power supply. This build provides four, non-isolated, fixed output rails at 3.3V, 5V, 12V, & -12V. With a small effort we could later add a variable output, but for now I’m happy with the fixed outputs.
Here is a quick outline of the specs on my 300W supply.
3.3V @ 28A = 92.4W
5.0V @ 30A = 150W
12V @ 15A = 180W
-12V @ -800mA = 9.6W
These are the maximum outputs for each individual supply, but when used in combination there are more limitations. The 3.3V & 5V rails combined can source no more than 180W, and the 3.3V, 5V, & 12V rails combined can source no more than 280W.
All you need to pull this off is an ATX power supply, 8 banana jacks, a screwdriver, a drill, a soldering iron, and a bit of solder. I’ve got all my stuff ready, so lets unplug all the cables and get started.
Step 1) Open the Case
This one is tough. Locate the 4 screws on top of the enclosure, remove them, and then remove the top half of the case. This is the piece with which we will mount our banana jacks.
Step 2) Install Banana Jacks
Mark a grid with a Sharpie marker so that each square is 0.75 inch by 0.75 inch. Spacing the jacks 3/4 of an inch apart makes it compatible with the dual banana plugs that are available. There are 1 inch spaced dual plugs, but they are not as common. Next, use a center punch or other sharp object to make a small circular dent in each intersection of the grid. This will help keep the drill bit centered when starting the hole. It also might help to drill a 1/8 inch starter hole before drilling the final holes with a 5/16 inch drill bit. After the 8 holes are complete, slide in the banana jacks and cinch up the retaining nuts on the back side.
Step 3) Identify Power Wires
Now its time to connect the output power wires to our banana jacks. ATX supplies follow a color code where black wires are ground, orange wires are 3.3V, red wires are 5V, yellow wires are 12V, & blue wires are -12V. Sometimes the voltages are also printed on the PCB itself.
Step 4) Solder Power Wires
Now that you’ve identified the wires you need, its time to solder them to the jacks. I soldered a bundle of 3 wires to each jack, except for the -12V since there is only one blue wire. With the exception of the green wire, which we will use later, I chopped off all of the remaining wires.
Step 5) Identify Power On Wire
Now its time to revisit that green wire mentioned earlier. This is the PS_ON wire and needs to be tied to ground to enable all the other power wires.
Step 6) Solder Power On Wire
Any ground will do, but I tied this wire to chassis ground as shown in the image above. Alternatively you could have connected this to any of the black wires.
Step 7) Close the Case
Everything should be working at this point. So just slap the top back on, and put back the four screws.