MiniMake: Benchtop PSU

Hello Sunday!

So as I’m working on Project Eta right now, I need a source of power while I’m prototyping. Historically, I’ve been using 9v batteries and AA battery holders to provide power while prototyping, but obviously, that is less than perfect as a solution. So I had a couple old computer ATX power supplies laying around, and I decided to turn one of them into a benchtop power supply, as these handy little supplies provide 12v, 5v and 3.3v in neatly regulated cables. This project takes less than a weekend to finish,  and presumes you have basic skills with soldering connections and a grounded understanding of how electricity works.

WARNING! POWER SUPPLY UNITS CONVERT WALL OUTLET ELECTRICITY INTO LOW VOLTAGE HIGH CURRENT. ELECTROCUTION HAZARD AND DAMAGE TO YOURSELF/YOUR HOUSE’S WIRING/YOUR PSU AND YOUR PET GERBIL IS POSSIBLE. UNDERTAKE THIS PROJECT ONLY IF YOU KNOW WHAT YOU ARE DOING. KIDS, DON’T TRY THIS AT HOME.

Components

  • Computer PSU, 2nd hand works well, ATX preferred.
  • LED/bulb for indicator
  • Switches
  • 2.1mm plugs, screw fit connectors, crocodile clips, etc.
  • Heat shrink tubing & solder

Tools

  • Multimeter
  • Drill
  • Sheet metal snips and tools
  • Soldering Iron
  • Wire Stripper

On Choosing ATX PSUs-

Most PSUs should work ATX works well because they usually have a bit more headroom for circulation, and they’re the most common and cheap. It’s advisable to use a second hand PSU for this project, as long as it works. Some old PSUs require a minimum load on the 5v line in order to properly regulate the 12v line. This can be fixed by adding a resistor onto the 5v line, but at low current applications it shouldn’t be an issue.

Benchtop PSU (2)

Here’s my PSU’s specs, and I doubt I’d be reaching anywhere near the limits

Procedure

Start by unplugging your PSU and leave it alone for around 15min, just in case the capacitors have any remaining charge in them. Remember to exercise caution and possibly even common sense while working.

Unscrew the 4 screws on the top of the box, and remove the upper shell. The PSU has two U shaped shells, one upper and one lower, which forms the body. The lower one has the electronics, fan, power port, and cables attached. If you want to paint your PSU afterwards, peel off the labels now, and give the upper shell a thorough scuffing with sandpaper, 120 should do it.

Now plan your layout for what you plan add to the box. I’m using 3 sets of crocodile clips, a 2.1mm jack for arduinos or the like, a 2.1mm port, a potentiometer, an indicator bulb and a power switch. You may want to adjust this depending on your anticipated usage, maybe using header ports instead of barrel plugs, your choice.

Use a marker or scriber to mark your layout, then use a drill to make the holes. Keeping a scrap of wood under the sheet metal as you’re drilling will prevent it getting dented or damaged as you drill. After drilling,  I used a pair of snips to cut a square hole for the main switch, and clean up the edges of the circular holes.

Benchtop PSU (3).JPG

Upper shell of the PSU, drilled and ready to receive the components.

After drilling the holes, give your box a gentle scuff of sandpaper to prepare it for painting. For my texture, I decided to go with hazard stripes, a lightning bolt and the word HAZARD on the top shell, and leave the lower shell as plain metal. I painted the entire box black with gesso primer, then I used 1in masking tape to create a diagonal masking pattern on the box. Using a paper cutter,  I cut out the details out of the masking tape strip, and then painted the entire shell lemon yellow.

Benchtop PSU (4).JPG

After the paint dries, peel off the tape, and behold your work. You can screw in your accessories at this point, I went with an old fairy light bulb for a mildly retro look, a potentiometer and a 2.1mm port.

connector_atx_pinout

Standard Pin layout for the thick wire bundle from your PSU, older PSUs use version 1, mine was v2

Next, we start the fun part, wiring. Observe the tangle of wires on your PSU, you’ll notice one thick bundle of around 20ish wires, and several thinner bundles. Use a wire cutter to cut the plugs off the thinner bundles. All the wires should be colour coded, although the colours vary depending on the manufacturer, but typically the yellow wires should be 12v, red is 5v and black is common. Use a multimeter to check just in case. Leave the PS_on, -12vdc, 5vsb and PWR_OK wires connected to the big plug for now, and cut all the 12v, 5v, 3.3v, and ground cables from the plugs as long as you can, pull them into the case and group them.

Benchtop PSU (5)

12v yellow, 5v red, ground black, 3.3v orange

Next, carefully observe which wires are PWR_OK, -12vdc, and 5vsb, and cut them from the plug. You can either snip these cables as close to the board as you can, or tie them up inside the case. The PS_ON wire is useful because when it is connected to ground, the PSU turns on. This cable is what we will connect to the power switch on the case. The 5vsb provides 5v dc at approx. 400ma even when the PSU is plugged in but off, so it can be used if you want to control the outputs with a micro-controller.

Next, we start soldering. I connected a ground cable and the PS_ON cable to the red main switch, and  I connected one of the 3.3v cables to my bulb, so that when the PS_ON switch is flipped the light comes on to indicate the PSU is live. Two 12v lines went to a pair of crocodile clips and a 2.1mm barrel plug. I used the potentiometer as voltage divider to control the power to one set of croc clips. I like to twist the wires around each other to keep them together, with a little ring of heat shrink at each end to prevent them unraveling. Remember to insert heat shrink before soldering!

Benchtop PSU (6).JPG

Once you’ve soldered all the wires you’re going to use, curl up the additional wires, zip tie them and push them to the side of the case. Remember, airflow is important inside the case to prevent overheating, so ensure that you haven’t blocked off the fan or its airflow path. Do a final test with a multimeter to check that all switches, plugs and wires function properly. Pull the crocodile clip wires out through the hole in the PSU case, and close up the case. Check that the tabs on either side of the upper shell properly align with the ones on the lower shell, and that its facing the right direction, and tighten the screws.

Thats it! Your PSU should be fully functional now, plug it in and give it a trial run. As a last minute feature, I glued a small strip of PVC foamboard (sunboard) to the rear of the case, and made some impressions in it so that I could clip my crocodile clips to it. This prevented the clips from accidentally touching and shorting out.

I hope you enjoyed this short MiniMake weekend project tutorial log, and I hope it was of use to you. If you’ve got any queries, please do leave a comment and I’ll reply as soon as I can and do consider sharing and subscribing for more weekly content.

Until next time,

Signing off,

~Adithyaa

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