This has been done probably hundreds of times here on Instructables, but I think that this is a great starter project for anyone interested in getting into electronics as a hobby. I am a U.S. Navy Electronics Technician, and even with costly test equipment at my disposal, I still consider this cheap mod among my favorite and most versatile pieces of equipment.
CAUTION: this Instructable requires the use of power tools. Always use eye protection when operating power tools. Also, electricity is no joke. Most technicians I know, myself included, have been "bit" before, and many of them have lost friends or coworkers to electrocution. ALWAYS verify that power has been removed before working on electrical equipment (or properly protect yourself).
The great thing about this project is that it is cheap and nearly anyone can do it. The basic piece is just a regular ATX-style power supply from a junk computer. Check craigslist, someone near you is probably giving one away!
The piece-parts, however, you probably will have to purchase. I bought mine at radio shack because it is across the street. You can try mouser.com, amazon, there are a ton of great places to get parts. Mine cost about $50 because I specifically wanted multiple outputs. A variable output would be cheaper (but I smoked my only potentiometer early on screwing around) and there are countless instructables on how to (properly) wire that in if you prefer.
Step 1: Parts List
This is really on you, The more outputs or features, the more you need.
Minimum, you will need 2 jacks, 2 plugs, and 2 alligator clips.
Here is what I used:
2 black banana jacks
4 red banana jacks
1 on/off toggle switch
1 red LED
2 banana plugs
1 set of test leads with alligator clips (36") (cut in half to create two leads)
Step 2: Open the Case!
A smarter person might have taken more pictures...that person isn't me, so you're going to have to use your imagination a little bit.
1. Disconnect power
2. Open your case: There is a bundle of color coded wires inside. Use a meter (or read the board) to determine the voltage transmitted through each. In my case, 12V was yellow, red was 5V, and orange was 3.3V. Black is (nearly) always ground, but feel free to verify.
3. Decide where you want to mount your controls: I had to play around with my case a little bit to figure out where I could mount the banana jacks without interfering with the "innards" of the case. Once you figure this out, just drill your holes to the appropriate size (the package often tells you what size mounting hole a component requires) and mount your jacks/switch/LED.
3a:. I cut most of the wires out, keeping a few of each voltage level for redundancy. Cut those remaining wires to length, strip the ends, and solder them to the appropriate terminals.
3b: Most computer power supplies require a signal to turn on. Mine was no different. You can see in the picture that the green and white wires go to the switch. When the switch in closed (ON), this "wakes up" the power supply. The 5V is also tapped for the L.E.D., which serves to indicate that the power supply is working. Do not tap 12V to the LED, you will ruin it.
Step 3: Put It All Together!
4: I probably could have broken this down into more steps, but you are smart, I trust that you can follow this pretty well. After you drill your mounting holes and mount your components, you can reattach the case cover. Again, this may require some finesse to get everything to fit. Liberal use of heat shrink tubing, electrical tape, or even scotch kote (it's a paint-on rubber sealer) will prevent any potential short-circuits from occurring.
5: I wire brushed the case to give it a clean look (and also erase all of my pencil marks). At this point a smart person would label which jacks are which. I didn't, because I like surprises!
The 2 black jacks, the left is the -12V wire, the right is ground. The red jacks are 3.3 (x1), 5 (x1) and 12v (x2). The reason for the -12V is that -12 and +12 will create +24V, -12 and +5 create +17, and -12 and +3.3 create +15.3(ish), thus allowing more voltage options for the user (that's you!)
6: At this point, you can plug in your new power supply and verify the voltage levels using a multimeter. For the leads, I used a set of alligator clip test leads, cut in half and soldered the cut ends to banana plugs. Banana plugs are a great choice because they can also be used in a meter, limiting the number of different tools and attachments required for your tool kit.
Step 4: Why Do All of This?
Why go through the trouble? Well, if you got this far, I was kind of hoping that you had your own reasons...
Really, though, the uses are limitless. If you create breadboard projects, this is a great way to power your circuits. If you work on cars, you will know that 12V is everywhere: this is a great way to bench test stereos, switches, and sensors before installation. I used mine most recently for another project that I should probably add an instructable for, which was hacking the factory stereo in my truck to add a bluetooth feature. I was able to use a junk speaker and a few short lengths of wire, to prove that everything would work the way I wanted it to before reinstalling everything.
If you have any suggestions for other builders, or if you tried it out and like it, post below! If you have any questions, feel free to post below, I'll try to get back to you quickly, or maybe another user will drop by and help you out.
Thanks for reading!