ATX Based Lab Power Supply

Computer power supplies cost around US$15,but lab power supplies can run you $100 or more! By converting the cheap (free) ATX power supplies that can be found in any discarded computer, you can get a phenomenal lab power supply with huge current outputs, short circuit protection, and very tight voltage regulation.

In this instructable I will show you how to quickly convert one of those many computer power supplies into something that you can use to power your electronics projects, for electroplating, for electroetching, for heating wires for foam cutting, etc.

The voltages that can be output by this unit are 24v (+12, -12), 17v (+5, -12), 12v (+12, 0), 10v (+5, -5), 7v (+12, +5), 5v (+5, 0) which should be sufficient for most electrical testing. Many ATX power supplies with a 24-pin connector for motherboards will not supply the -5V lead. Look for ATX power supplies with a 20-pin connector, a 20+4-pin connector, or an AT power supply if you need -5V.

PLEASE, YOU ARE WORKING WITH AC VOLTAGE!!! IF YOU ARE NOT SURE OF WHAT YOU ARE DOING - DO NOT ATTEMPT THIS.

These instructions were originally posted by me on http://www.wikihow.com/Convert-a-Computer-ATX-Power-Supply-to-a-Lab-Power-Supply

I finally had to package the PSU in a nicer box. The wooden book was from a local craft chain called Michaels which I lined with foil and then packaged the electronics in. I also added back the 3.3V terminal as this was useful and I missed not having that in my previous version.

Harvesting the ATX Power Supply

1) Unplug the power cord from the back of the computer. "Harvest" a power supply from a computer by opening up the case of the computer, locating the gray box that is the power supply unit, tracing the wires from the power supply to the boards and devices and disconnecting all the cables by unplugging them.
2) Remove the screws (typically 4) that attach the power supply to the computer case and remove the power supply.
3) Discharge the power supply by either letting it sit unconnected for a few days, or by attaching a 10 ohm resistor between a black and red wire (from the power cables on the output side). Using a resistor will only take a few seconds to fully discharge the power supply.
4) Gather the parts you need: binding posts (terminals), a LED with a current-limiting resistor, a switch (optional), a power resistor (10 ohm, 10W or greater wattage, see Tips), and heat shrink tubing.

Preparing the ATX Power Supply

5) Cut off the connectors (leave a few inches of wire on the connectors so that you can use them later on for other projects).
6) Open up the power supply unit by removing the screws connecting the top and the bottom of the PSU case.
7) Bundle wires of the same colors together. The color code for the wires is: Red = +5V, Black = Ground (0V), White = -5V, Yellow = +12V, Blue = -12V, Orange = +3.3V, Purple = +5V Standby, Gray = power is on (output), and Green = Turn DC on (input).

Drill Holes for Attaching Binding Posts

8) Drill holes in a free area of the power supply case by marking the center of the holes with a nail and a tap from the hammer. Use a Dremel to drill the starting holes followed by a hand reamer to enlarge the holes until they are the right size by test fitting the binding posts. Also, drill holes for the power ON LED and a Power switch.

Attach Binding Posts and Start Wiring

9) Screw the binding posts into their corresponding holes and attach the nut on the back.

Connecting the Wires

10) Connect one of the red wires to the power resistor.
11) All the remaining red wires to the red binding posts.
12) Connect one of the black wires to the other end of the power resistor.
13) One black wire to a resistor (330 ohm) attached to the anode of the LED(see the next image)
14) One black wire to the DC-On switch
15) All the remaining black wires to the black binding post.
16) Connect the white to the -5V binding post, yellow to the +12V binding post, the blue to the -12V binding post, the gray to the cathode of the LED.

Note that most power supplies have either a mauve or brown wire to represent "power good"/"power ok". Check the ATX plug (the plug with many connections) to see if there is a small mauve or bown wire plugged into the same hole as an orange wire (+3.3V) or a red wire (+5V). If the small wire is connected to the orange in the ATX plug then do the same, hook these two together. If it is connected to the red, then hook it to the red wire. This wire must be connected to either an orange wire (+3.3V) or a red wire (+5V) for the power supply to function. When in doubt, try the lower voltage first (+3.3V).

Wrapping Up the Connections

17) Connect the green wire to the other terminal on the switch.
18) Make sure that the soldered ends are insulated in heatshrink tubing.
19) Organize the wires with a electrical tape or zip-ties.
20) Check for loose connections by gently tugging on the wires.
21) Inspect for bare wire, and cover it to prevent a short circuit.
22) Put a drop of super-glue to stick the LED to its hole.
23) Put the power supply cover back on.

Testing the Power Supply

24) Plug the power cord into the back and into an AC socket.
25) Flip the main switch on the PSU (on the back). The fan will come on.
26) Check to see if the LED light comes on. If it has not, then power up by flipping the switch you placed on the front.
27) Plug in a 12V bulb into the different sockets to see if the PSU works, also check with a digital voltmeter. It should look good and work like a charm!

Updated the packaging a bit so it looks sexier and wife acceptance factor increases ... used a ready made unfinished wooden box from Michaels.

Repackaging the Power Supply in Nicer Enclosure

I still use that supply quite a lot but Wife Acceptance Factor was low as it looked - well -ugly. So I used one of the cool looking wooden boxes we had bought from Michaels as a new skin for this power supply (above). Essentially installed the circuit board into the box (after lining the insides of the box with foil and then with insulating tape), reconnected all the wires to the terminals, installed a smaller fan, and added an old car amp meter that I had bought from Harbor Freight and viola a new sexier looking power supply.

Stained the box, carved some cool lettering with a Dremel ... still have to add labels for the different terminals ... but it now looks good when placed in the bookshelf with no hint of what it is till you flip it around!

Troubleshooting

# If you are not sure of the power supply, test it in the computer before you harvest. Does the computer power on? Does the PSU fan come on? You can place your voltmeter leads into an extra plug (for disk drives). It should read close to 5V (between red and black wires). A supply that you have pulled may look dead because it does not have a load on its outputs and the enable output may not be grounded (green wire).
# If the LED light does not come on, check to see if the fan has come on. If the fan in the power supply is on, then the LED may have been wired wrong (the positive and negative leads of the LED may have been switched). Open the power supply case and flip the purple or gray wires on the LED around (make sure that you do not bypass the LED resistor).
# ATX power supplies are "switch-mode supplies"; they must always have some load to operate properly. The power resistor is there to "waste" energy, which will give off heat; therefore it should be mounted on the metal wall for proper cooling (you can also pick up a heatsink to mount on your resistor, just make sure the heatsink dosn't short anything out). If you will always have something connected to the supply when it is on, you may leave out the power resistor.
# You can add a 3.3 volt output to the supply by hooking the orange wires to a post (making sure the brown wire remains connected to an orange wire) but beware that they share the same power output as the 5 volt, and thus you must not exceed the total power output of these two outputs.
# If you don't feel like soldering nine wires together to a binding post (as is the case with the ground wires) you can snip them at the PCB. 1-3 wires should be fine. This includes cutting any wires that you don't ever plan on using.
# The +5VSB line is +5V standby (so the motherboard's power buttons, Wake on LAN, etc. work). This typically provides 500-1000 mA of current, even when the main DC outputs are "off". It might be useful to drive an LED from this as an indication that the mains are on.

Options: You don't need the switch in the front, just connect the green and a black wire together. The PSU will be controlled by the rear switch, if there is one. You also don't need the LED, just ignore the gray wire. Cut it short and insulate it from the rest.

Almost 10 Years Later ...!

Am still using the original Power Supply though now it is packaged in a more professional case. I also added an ammeter in the 0 V line so that I could measure the Amps being delivered. I also added a USB socket to charge USB devices. The last diagram shows a simplified wiring diagram of all the parts together.

The fact that this PSU is short circuit proof has turned out to be a big advantage. It has withstood a lot of abuse.