Introduction: Use a Computer Power Supply for Desktop Electronic Projects

Building electronics with Arduinos, motors and other gadgets, I came up against a common problem: powering the circuits with a consistent voltage and adequate current. Like many people, I realized that a computer power supply can be a perfect solution, albeit with some modifications. It provides regulated 3.3V, 5V and 12V power at a current level high enough to satisfy almost any project requirement.

Most of the examples I found online did not fit my requirements, so I decided to modify my own. The requirements I set out were:

- ability to switch 24 pin ATX connector for other projects
- variety of connectors
- fused
- ample power

Bill of Materials
1x 300W power supply with a rear fan - $30 - Newegg
1x Cytron ATX Power supply breakout board - $9 - Robotshop
1x Power distribution block - $2 - Adafruit
5x 2.1mm DC barrel jack connectors - $5 - Various
5x 5mmx20mm fuse holders - $5 - Various
4x M3 brass standoffs
5x 5mmx20mm fuses (assorted of 5A, 3A, 1A) - $10 - Various
5x 5mm screw terminal pairs - $2 - Various

Tools
Assorted drill bits
Step drill with 3/16", 3/8" and 1/2" sizes
Power drill
Assorted screwdrivers
Soldering iron
solder
wire stripper
diagonal cutters

The core component that makes this project easy is the ATX breakout board. It takes a 24 pin ATX power cable and splits the components out to their different voltages, while also providing indicator lights, an ON/OFF switch, and room for screw terminals.

Be sure to properly research the power supply: you'll need to select one with a full 24 pin ATX power cable, rear fan instead of the top and enough connectors for your project. If you decide to copy my barrel jack configuration, you'll be using 1 4 pin Molex and 1 SATA cable.

Please remember that a computer PSU provides a dangerous amount of power and has a warranty sticker on it. Since this is a hacking contest and website, I'm sure this won't deter many people. Just be smart.

Step 1: Planning

Since I'm an engineer, I did a lot of planning before actually drilling any holes. This included loading as many parts as possible into a 3D CAD application. Not only would this let me lay out the parts, but I could print a template for drilling each hole in the shell. This step becomes more important as as each project becomes more complicated.

By the end of my design stint, I confirmed that my particular power supply unit would fit 5x barrel jacks, a power distribution block, the ATX board and 5 fuses. The barrel jacks would have 2 with 12V, 2 with 5V and 1 with 3.3V.

If you plan on using similar components in your project, here are some notable dimensions:

3/8" - 2.1 mm DC panel jack
1/2" - 5mm fuse holder chassis jack
1/8" - M3 standoff holes
3/16" - M4 holes for power distribution block

Step 2: Open and Cut the Power Supply

Use the screwdriver to open the shell of the power supply and make sure your planned connector layout will work with the internal setup of the PSU. This means confirming the fuses on the side don't reach the heatsinks and the top connectors don't meet the capacitors and bottom shell.

Once you're satisfied with the layout, crack out the drill and have at it. I suggest printing the template and using a center punch to dent the sheet metal enough for the drill to bite. My step drill started at 3/16" so I was able to use it for the fuses and barrel jacks. Despite the shifting of the paper I used, the holes ended up being spaced properly.

As a result of the thin sheet metal, you'll likely have to deburr the underside of the holes with a file or Dremel tool. Once that's done, you can install the jacks and fuses in the holes you've drilled. Do the same for the ATX breakout board holes and install the standoffs at the same time.

Step 3: Make the Wired Connections

Start the electrical connections by cutting the Molex and SATA connectors off the cables you'll need. For my setup here, you'll need to cut 1x peripheral cable and 1x SATA cable. Slide the cables through the shell so you can access them directly from the PCB at the bottom.

The wires exiting the PCB are as follows:
orange - 3.3V
red - 5V
yellow - 12V
black - common ground

To make the connections work, the positive wires from the PCB go to the fuse block, then to the barrel jack. The associated ground wire can go directly from the PCB to the barrel jack. By cutting the wires to the correct length, you should be able to tuck them all in the available space.

After soldering the connections in place, assemble the power supply back together for testing.

Step 4: Test It Out

After reassembling the shell, add the desired fuses and break out the voltmeter to test the connections. I'm testing the ATX breakout board power in the photos, but be sure to do the same for the barrel jacks.

It's worth noting that in its current configuration, the power distribution block has no power of its own. That way it can distribute power at whatever voltage is required.

With the breakout board's screw terminals, it's easy to hook a breadboard up to all of the different voltages and power whatever project you have in mind.

Comments

author
MohitS17 (author)2015-09-08

I built my power supply with the similar instructions as yours using only the 12Volt output to power my Car Airpump (12V 14Amp max).I joined all 5 12V wires (Yellow) together and same for the ground connections to get the max current on the 12V rail which is rated for 17A on this supply.

With no load connected, the SMPS turns on and works fine but as soon as i connect the Car Airpump it trips (shuts itself down) not sure why ?

Now if i connect the same Power supply with a Digital Ampere meter connected in series, the Air Pump works perfectly. Could you help me understand the possible cause for this behaviour and how could i go about fixing or diagnosing it.

author
russ_hensel (author)2015-01-09

Just a note to let you know I have added this instructable to the collection:
Encyclopedia of ATX to Bench Power Supply Conversion
>> https://www.instructables.com/id/Encyclopedia-of-ATX-to-Bench-Power-Supply-Conversi/
Take a look at about 70 different approaches to this project.

author
Philiprocks (author)2013-10-23

Hi, I found this instructable very useful, congratulations. I have a question what's the name of the 3d cad software?

author
rimar2000 (author)2012-11-04

Useful info.

Do not need to do a short between two pins in a connector?

author
Gelfling6 (author)rimar20002013-04-05

Some of the other instructables, which do a full conversion, they also seem to want to short PG (Power Good) and the +5V-Stby (the constant +5 used for momentary switch on/off control).. Sometimes, yes, Sometimes, see 2nd reply to wesg... caused a smoke-show. the PG signal is supposed to be a signal back to the switching chip in the supply, from the computer, that either everything is A-O-K, and if not, shut-down A-S-A-P!

author
wesg (author)rimar20002012-11-04

The ATX breakout board has a switch that does the switching for you. It shorts between the green cable and ground.

author
Gelfling6 (author)2013-04-04

I've run into a few of the older ATX supplies, which required a load across the +5V (Not stby) to keep them going, does this breakout board add any negligible load for this purpose? Or, is it relying on the supply already having the load resistor built in?

author
wesg (author)Gelfling62013-04-04

That's a good point, and one I didn't mention. So far in my testing I've found that to be the case; the PSU shut off after a few minutes without anything connected to it. The breakout board doesn't add any load, as far as I can tell, so I think I'm going to try adding a small power resistor to keep the system going.

author
Gelfling6 (author)wesg2013-04-05

One of the converted supplies I have, (a 350W ANTEC), had a 33-Ohm, 5W metal film resistor hard-soldered to the board between the +5V & GND.. Some of the instructables for converted supplies, suggest a 10-ohm, 10W, which honest truth, seems a wee low for resistance.. (unless they want it warm?) when I converted a 200W Dell supply (fan on top), I clamped a 33-Ohm, 5W sandblock to the opening on the side, and sacrificed 1 +5 wire and one GND wire to it. Though, mine, I completely scrapped the ATX connector, also because I've heard some of the Dell supplies, possibly even some of the Compaq supplies before they standardized under HP, have an incorrect wiring scheme (swapping a fan signal with -5V, PG & Pwr-On, etc.. I smoked one supply that way..(Nice loud POP, followed by a wisp out through the fan.))

author
Gelfling6 (author)2013-04-04

In the first photo (on this step), note the two canisters at the bottom-left, as "The Micro Killers!" which people need to be wary of.. these are the capacitors which hold the converted AC to DC before it is fed into the switching circuitry. Specifically, these range from 200-400uF, but with a 200-250V working voltage. equate these to a miniature version of a defibrillator paramedics & doctors use to stop/restart the heart, with the exact same potential! Most supplies have a bleeder resistor (I see one, to the left, above the "ZD" print, not sure if that one to the upper-right of the right one is.. looks too small.) across both, which should bleed-down the high voltage within about a minute or so.. but, if those resistors are corrupted (seeing these as metal film type resistors, they can still burn through like fuse), those capacitors can knock you on your keester, or worse!

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Bio: I'm a mechanical engineer combines software with hardware to make something new. Usually that involves an Arduino.
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