Building a bench top power supply with a computer PSU
Electronic tinkering and building have become a bigger hobby for me as I finished my university degree. The Arduino development platform is quite a remarkable system, and the online community of users is growing every day. The problem with many of the projects that I do is that many systems require more power than the 50mA or so the Arduino can provide. You could use different AC-DC wall adapters, but that becomes unwieldy.
The DIY solution, as many Arduino users have discovered, is to repurpose a used computer power supply unit for cheap, regulated 3.3V, 5V and 12V DC power. I decided to build my own based on some other examples, while adding things that I need for my projects. I documented the procedure in an Instructable but thought I would expand on the design and build process here a little more.
I did lots of planning before cutting any sheet metal because I wanted to follow the engineering procedure. The major requirement for this is to decide what features the final system needs to be successful. I chose the following:
- Ability to remove the 24 pin ATX cable to power other projects
- Access to all available PSU voltages (3.3V, 5V, 12V)
- Different connectors to make powering different components easy
- Fuses to prevent powered systems from breaking
- Keep some external cables for future expansion
- Connectors on the top of the PSU
Bill of Materials
With the requirements in hand, I started browsing for and collecting components. One of the things I noticed right away is that any power supply with a top mounted fan would not useable. This means that my system is limited to about 300W or less, which for desktop projects is completely fine. Older, cheaper and lower power units were the first place I looked, but I quickly discovered another requirement: a 24 pin ATX connector instead of the older 20 pin standard. This limitation meant that I had to build a new one from Newegg instead of on the used market. The cost worked out to be the same.
After waiting a few weeks for delivery, the following parts were on my table:
- Computer Power Supply unit
- Cytron ATX Power Supply Breakout board
- Power distribution Block
- 2.1mm DC barrel jack connectors
- 5mm x 20mm fuse holders
- 5mm x 20mm fuses (various amperage)
- M3 brass standoffs
The ATX breakout board handles the operation of the PSU by properly shorting the on/off wire and providing indicator LEDs. It also breaks out each of the voltages on the cable to a useful screw terminal pad. If you choose to use a different method, turning the power supply on is as easy as shorting the green wire with a common ground. The other parts I found on eBay or various electronics retailers on the internet (try Adafruit, RobotShop, Sparkfun or OddWires).
My first step was to measure and replicate each part as accurately as possible using a 3D CAD package. If you’re looking to do something similar, take a look at Solidworks, Autodesk Inventor, SketchUp or Pro/Engineer and decide which to learn. They all perform the same functions, although SketchUp is less about dimensions and more about sculpting 3D objects.
With the components in my system, I was able to lay out all of the holes on the PSU shell to confirm space. I was able to fit 5 2.1mm DC barrel jack connectors, the power distribution block and ATX board on the top surface, with the fuses moved to the side. The software made it very easy to print templates for the hole locations on each surface.
One of the things I needed to look out for was preventing interference between the barrel jacks on top and the internal heat sinks. That meant modelling additional components inside the shell, but doing that extra step prevented numerous hangups further down the process.
As a quick note, please keep in mind that this is a record of my procedure and by no means a complete step-by-step instruction manual for hacking power supplies. These units produce an evil amount of electricity and can be dangerous when misused. Remember that before breaking open the tool box and realize that it’s entirely your choice to void the warranty.
Drilling and installing the components
Cutting into the sheet metal container can’t be done by hand, of course, so here’s a list of tools I used:
- Power drill
- Assorted drill bits
- Step drill (with 3/16″, 3/8″ and 1/2″ sizes)
- Round file
- Dremel deburring tool
- Assorted screw drivers
- Wire stripper
- Diagonal cutters
If or when I build another unit, I’ll add a center punch to that list in order to accurately dent the material to start drilling. Without it, I needed a finishing nail and hammer, which reduced accuracy somewhat.
Start by ignoring the warranty labels and cracking open the PSU shell.
Inside you’ll find a semi-organized collection of wires, circuit boards, capacitors, MOSFETs and heat sinks.
Now is a good time to tape the hole template paper to the shell, if you’ve prepared one. For the most part the holes aren’t for accuracy relative to the shell, but more for proper spacing between the individual holes.
Drill the 1/2″ holes for the fuses with the step drill. The thin sheet metal will produce burrs on the underside that you’ll need to clean up for the fuse holder to sit flat.
Do the same for the 3/8″ holes for the DC barrel jacks. Both of those components can then be loaded into their respective locations because they’ll need to be installed in order to solder the wires.
Wiring and soldering the connections
Routing the cables from the bottom PCB to the fuses and on to the barrel jacks was somewhat of a challenge because of the arrangement of the heat sinks and bottom components. I cut the external connectors and passed the wires inside to test for length. The overall path for the positive cable of each voltage would be PCB – Fuse – barrel jack and the associated ground wire went directly to the barrel jack. Be careful not to pull too hard on the PCB because tracing the wire back to the soldering pad can be a real pain.
Generally the fuse to barrel jack wire was soldered first, because it was the easiest to trim and arrange. The fuse holder only has two pads, but the barrel jack has three, which means before soldering anything you’ll have to learn the pinout configuration. The most common barrel pinout is center positive, so that’s what I used. Finding which pad was which meant connecting a wire to the jack and checking continuity with a multimeter. Not difficult by any means, but it can be time consuming.
After reinstalling the shell, I soldered on the screw terminals to the ATX board and connected the 24 ATX cable. The brass standoffs are used on the ATX board to raise it from the metal shell, so those were installed next.
Testing the entire unit was a simple matter of connecting the multimeter to the screw terminals and barrel jacks.
After testing, how you use the power supply is completely up to you. The easiest way to incorporate the power is to feed lines from the screw terminals to a branch on a solderless breadboard. Saving the cut off internal wires is a great way to wire them together.
This was a great project for me to test design principles and following through with a project from start to finish. It now opens the doors to building systems with motors, thermoelectric coolers or any number of other sensors.