Winter Break Projects 1: PC to Bench Power Supply

Final Result

Every electronics tinkerer needs to do this. A bench power supply is an absolute necessity, and can be quite expensive. Building your own supply from scratch is possible, but time consuming and prone to errors for even one with experience. Even a small PC supply has more wattage than you'll need. If you're building something larger and more complex than a complete computer, you're probably sitting in a clean lab with a white coat, anti static booties, and a million $$ power supply. This project is not for you.


There are plenty of excellent full how-tos on the web for this project (check instructibles), so I won't go too crazy with steps and photos here.. I will simply point out some key issues you'll probably face, and mention a few specifics of my build. If you're savvy, you really don't need a how-to, It's a very simple project that is relatively universal, regardless of your actual PSU model.


OBLIGATORY CAUTION: It goes without saying that this project involves 120V AC, and has some danger factor. Additionally, THESE SUPPLIES HAVE LARGE CAPACITORS THAT RETAIN 120 VOLTS EVEN AFTER IT IS UNPLUGGED!! You have to let them drain before you poke around in there. How long to wait will vary from one supply to the next. If you don't know what you are doing, don't do it. You've been warned.



  • PC supply - Pull one out of an old tower, like that win98 machine in your corner that you said you were going to rebuild as a home automation controller three years ago. City sidewalks and local dumps are other good sources. Sidewalk computers are usually picked clean of anything valuable, but the supply is most likely still there. Where a commoner sees junk, a hacker sees opportunity.
  • Power cable - There are two kinds of people in the world. Those who have more of these than they'll ever use, and those who never seem to have enough. I'm the latter. This is a very standard cable for all things computer, so they are easy to track down. Specifically, it's a type C13, IEC 60320-1 cord.
  • Connection terminals - I went with standard banana jack binding posts, like you'd find on a breadboard. This allows either bare wire or banana style connections.
  • Switch - a simple SPST switch would do (either a toggle or latching pushbutton). I used a DPDT as explained later. LED's (optional) - These are nice addition, both aesthetically and for troubleshooting.
  • Power resistor - YMMV. Most units need this. Mine didn't. You want some thing low in the ohms scale (I've seen as low as 10 ohms). It should be at least a few watts. 5-10 watts should be fine. Note: These are not the typical resistors you use commonly. Those are more in the ¼ watt range. You want a POWER resistor.
  • PC screw terminal block - Some people call these Wago or Keystone PC terminal blocks. This was specific to my build, you may not need this.


The hardest part is trying to get your new electronics to fit around the existing bits. It's was a tight squeeze for me. Also, the "front" of my unit (the side opposite the power connector) was mesh for ventilation. This meant I had rotate mine and have the power cable coming out of the left side. You can get 90 degree IEC cables, but in my case the supply sits on the left edge of my bench, so it was no big deal.


1. Open the case and tidy the cords. You can clip the connectors off the wires. We won't use those. In fact you can clip all but about 6 inches off. Leave yourself enough to work with. I usually leave a few inches more than I think I'll need because my first hunch is usually wrong. When deciding how much to clip, imagine putting the finished product back together. You need enough slack to slide the case on and off. This is another thing I usually remember too late.



2. Inspection and Bench test. Inspect the board and wires. The board will be labeled, and the wires will be in groups of colors. Black is usually a shoe in for ground, but other colors may vary. For me, it was like this; Yellow = 12V, Red = 5V, Blue = -12V, Orange = 3.3V. Look for the wires that seem all alone, and trace them to the board. You're looking for the standby line, and the power on line. The labels should be fairly obvious, like SB and PS-ON, etc… My STANDBY was purple, and my ON wire was green. Make sure you write all of these discoveries down. o see this thing in action, we need to power it up. Put the board on a non-conductive surface and make sure it is not shorting on the chassis or screws or tools that may have rolled under it. Clip a test lead to a ground wire, and to the STANDBY line. Plug it in, and you should read 5V. Next, try shorting the ON wire to ground. You might see the fan spin up. Mine did. If not, you'll need the power resistor. Connect it between a 5V line and Ground. That should set things right. Once it's in action, you can test voltage on the other lines.


3. Clip unneeded wires. You won't need all of the grounds or all of the voltage lines, so trim back any that you don't need. I left them a few inches long so I could tape them off with electrical tape to prevent shorting (tape them in separate groups, ground, 5V, et….). I kept one 5V line, one 12V line, and one 3.3V line. I also kept one ground line for each, as well as one gowned line for STANDBY, and one for ON (5 ground lines total).


Soldered up4. Solder and assemble. You're on your own here. This will depend on your chassis and chosen connectors. This is where my terminal block came in. I didn't have enough room for a front mounted 3.3V line, so I dangled it out of the side with the terminal block connection. I decided I wanted an ON LED, and that I wanted my STANDBY LED to go out when the unit was on. You can skip all of this and use an SPST to short ON the ground. I've provided the schematic of my switch circuit below in case you want the same.



That wraps it up. It's very functional and industrial looking, if you're into that sort of thing. It's not as pretty as the supply I built in 2010, but it's way more reliable. You could opt to ditch the chassis and build a slick enclosure. That would give you freedom with the panel layout. I think keeping the original chassis is the way to go. This type of supply has become a sort of hacker status symbol, or right of passage, if you will. The original case has that "I void warranties" vibe that I like. Until next time, keep making stuff.


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