Introduction: Benchtop DC Power Supply
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. Electricity is no joke. Most technicians I know, myself included, have been "bitten" before. ALWAYS verify that power has been removed before working on electrical equipment (and 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. Other sources include Mouser, Digikey, and Amazon. I spent about $50 on parts because I wanted multiple outputs. A variable output is possible, but fixed voltages are ideal for my applications.
Shrink tubing (or electrical tape)
Drill and selection of bits
Paint marker, stamping set, label maker, or Sharpie
Step 1: Parts List
For this project, I wanted +12V and 5V. The ATX supply also provides 3.3V, so I added a jack for that. When I originally built this, I had in my mind that I would be using it a lot to test car stereo equipment and other automotive parts. Since then, I've done much more work with TTL, CMOS, and microcontrollers. Consider your needs and plan accordingly.
I used the following components:
2 black banana jacks for ground and -12V
4 red banana jacks for positive voltages
1 on/off toggle switch
1 red LED to indicate that power is applied
2 banana plugs
1 set of test leads with alligator clips (36") (cut in half to create two leads)
*Note: you can buy test leads that are already terminated with gator clips
**Additional note: If I were to rebuild this today, I would use color-coded jacks, with red for 5V, yellow for 12 volts, and maybe green or blue for 3.3V. This isn't necessary, but I think it improves safety by making it very clear which voltage level you are accessing.
Step 2: Open the Case!
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 always 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 internal components of the case. Once you work out your positioning, drill your holes to the appropriate size. The packaging often indicates what size mounting hole is required, but you could also measure with calipers if this information isn't provided.
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, and 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 LED, which serves to indicate that the power supply is working. Be sure to include a current-limiting resistor (220 ohm is often ideal).
Step 3: Put It All Together!
4: After you drill your mounting holes and mount your components, you can reattach the case cover. 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 you should label the output jacks. Mine are as follows:
The leftmost black jack provides -12V while the right is ground. The red jacks, from left to right, are 3.3 (x1), 5 (x1) and 12v (x2). As mentioned above, if I were to redo this project today, I would add more 5V jacks. I would be tempted to omit the 3.3V, but it may be useful if I start working with low voltage controllers in the future.
The +12V is great if you do a lot of tinkering with operational amplifiers. A bipolar power supply simplifies the design process for AC signal gain considerably. In addition, most circuits only realize the difference between two sources. As such, -12V and 12V will provide +24V, -12V and +5V provide +17V, and -12V and +3.3V provide +15.3V.
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 Would I Do This?
The uses for a cheap, stable power supply are limitless. It can provide power for breadboard projects for engineering or technology students, be used to test automotive or computer components, or power arduino and/or Raspberry Pi projects and peripherals without depending on your computer's USB ports (a risky proposition).