I always find that I am looking for a 12V, or 5V power supply to test some piece of electronic nonsense that I am building. For a while now I have used an old power supply from a PC with a couple of plugs cut off. If you haven’t done this before keep reading, they are the cheapest and best power supplies you will find. You can get 24V, 12V, 5V and 3.3V out of these things, which covers most of what you need, but what is better is the 12V rails can run around 30-40Amp depending on the power supply (the -12V will only be around 0.5-1amp though, so don’t expect much from your 24V supply). You can get them for $50, if not for free out of an old PC you have lying around at home. For this project I wanted to make something that allowed for easy connection and disconnection of different devices, something that had a USB port, and something that was a bit neater than a PC power supply sitting on my desk. Note that this project will expose harmful voltages so don’t run the power supply without the circuit board being encased, and don’t do this if you don’t have some idea what you are doing.
Making Your PC Power Supply Start Up
So the first trick to getting your PC power supply to run as a test power supply, is to get it starting up without a motherboard. This is actually quite simple, all you need to do is connect the green wire on the motherboard plug to any of the black wires. I used to use a piece of solder just poked into each pin and it worked fine, but as mentioned about I wanted to make this power supply a bit nicer, so I soldered them together inside the case.
Getting The Power Supply Ready For The New Case
To get the power supply ready for the new case I had to do a few things. First of all I stripped everything out of the old case, this involved cutting a few things out, like the power switch and power connector. To cut them out I was able to unscrew the connector then used a pair of tin snips to cut through the case enough to pull the cables out. I also cut off all the excess cables that I didn’t need. I was only going to use 2x12V, 2x5V, 1×3.3V, 1x-12V and 5x0V connections, so I cut everything off close to the power supply circuit board and taped what was left together to keep it out of the way. The power supply I was using also had a voltage selector switch, I worked out if this needed to be open or closed and then either cut it off or soldered the two wires together (can’t remember which).
Designing The New Case
My day job gives me access to solidworks, so I was able to draw up a case design that fitted the circuit board from my donor supply. I have uploaded this to Thingiverse here. I put in some cutouts to add in some screw down terminals for all the different voltages, as well as cutouts for fan, switch and power cable connection. The screw down terminals I used were these ones, and the switch I just had lying around. The USB connector I de-soldered off an old printer I had lying around. I made the case in two parts on my 3D printer, one part being the top and the back, the other being the rest. Doing both the top and back in one part significantly reduced the amount of supports that needed to be used when printing.
I decided that I need some indication that the power supply was on. I wired up an LED onto a 5V supply with a 220 ohm resistor. This was then hot glued into the small hole I had left at the top of the case.
You may notice in the photos above that there is big line of hot glue down the inside of the case. This is because I had an issue with the case printing at about eleven of twelve hours. I didn’t want to print the whole thing again so I cut off the section that failed (using a dremel), then printed the last hour again. I then carefully glued the two sections together. The result is not bad, and given it is the back of the case I’m not too concerned about it. This has been a really useful project to do, and I am currently using it to run my CNC Dividing Head. Having both the USB and 12V power available from the one unit is awesome, and means I don’t have to run two power supplies.