Introduction: ATX to Lab Bench Power Supply Conversion

By www.mbeckler.org/
About: If you need to get in touch, please email me instead of sending an instructables message. matthew dot beckler at gmail dot com

In my sophomore year of college at the University of Minnesota, I started into my main electronics classes, and needed a good power supply for working on lab projects at home. My roommate Adam told me about somebody online who had converted an ATX computer power supply into a lab bench power supply, so I decided to do the same thing. You can also check out this link for a very similar guide by their user Abizarl. I have also documented this project on my website at http://www.mbeckler.org/powersupply/ if you are interested.

Warning! There are several large capacitors in ATX power supplies, that will store a dangerous charge for a long time. Please let your power supply discharge, completely unplugged from the wall outlet, for a few days before opening it up. You can probably be seriously hurt, so please be very careful. I am not a lawyer, but I hereby release myself from as much liability as I can, for any sort of injury you sustain, or any trouble you get into.

Step 1: Background

First, a bit of background on a typical ATX power supply:

Computer power supplies are Switch Mode Power Supplies (SMPS), which use high-frequency switching circuit elements to provide a high-quality output voltage, with good energy efficiency. One side effect of this technology is the minimum load requirement that each power supply has. In order to function properly, the power supply needs at least a very small electrical load connected to it. In other words, ATX power supplies will only work if you have something connected to it. We will be using a power resistor to provide this minimum load.

Also, modern power supplies do not simply have an OFF/ON switch, they have what is known as a "soft" power switch. This normally makes no difference to the user, as the computer behaves the same, but when you shutdown your computer, the motherboard can turn off the power supply when it has finished shutting down. This requires us to add our own power switch to the power supply chassis.

To protect our circuit from accidental (and careless!) short circuits, we will install some fuse-holders and fuses, which will disconnect the circuit supply lines if too much current flows. The size of the fuses are up to you, but a 1 amp fuse will work just fine for most circuits. You really should put fuses on all supply lines.

Update: While the diagrams show fuses on all voltage rails and no fuse on the ground line, when I actually built my power supply, I was young and foolish and only put a fuse on the ground wire. It's much safer and a better idea to put fuses on all signal lines and not the ground line. Thanks to many emails and messages on Instructables about this oversight.

Step 2: Planning

Planning is the most important step of any successful project. To plan this project, I created a few images. I am going to be using four binding posts, a power switch, a fuse holder, a power resistor, and two light emitting diodes (LED's) with current-limiting resistors. The first image details the circuit connections inside the power supply, where everything will be connected

When the power supply is connected to the wall socket, but not yet turned on, it provides a +5v standby signal, that can be used by the motherboard for things like wake-on-LAN functionality. We use this signal line to indicate when the power supply is plugged in with a red LED and a 330 ohm resistor. On my power supply, this signal line has a purple wire, and is labeled "+5VSB" on the circuit board.

When the power supply is first turned on, it must go through a start-up sequence, to ensure that everything is working, and that it is able to provide stable power to the computer. When the start-up sequence has completed, it signals the motherboard by providing +5v on the "Power Good/Steady" signal line. We will use another red LED and 330 ohm resistor to indicate when the power supply is running. On my power supply, this signal line has a gray wire, and is labeled "PGS" on the circuit board.

The power resistor is a 10 ohm, 10 Watt resistor, commonly called a "sandbar", because they are usually coated with a material that feels like sand. Most power supplies need a minimum load to keep them running, so this sandbar resistor provides a constant minimum load between the +5V rail and Ground. I've heard that newer power supplies also need a load on the 3.3v rail, your mileage may vary.

In the second image, you can see the diagram for the front of the power supply. Here I have marked where the components will go, including the LED's, the binding posts, the fuse holder, and the switch.

The third image is what the power supply looks like without any modifications. You can see the various voltages I am going to use along the front edge.

Step 3: Drilling Holes

Here, I planned out and drilled the holes in the case. My power supply was a smaller form factor, (It was from a mini-tower case), so there wasn't a lot of space to work with.

Step 4: Connecting Front-panel Items

Here, I am connecting the appropriate wires to the binding posts, power switch, fuse holder, and LED's.

In an ATX power supply, there should be a wire that is used to turn on the power supply. You can see this wire (It's green) in the second picture; it is the green wire in the middle, where it says "ON/OFF" on the PCB. I connected this to the switch, and the other pole of the switch went to ground. The +5, +12, and -12 are connected right to their wires on the PCB. The ground wire is connected through the fuse holder before the binding post.

Initially, I was going to use green LED's, but I realized I had many more red LED's than green LED's, so I switched them over to reds. In the first picture, you can see the holders I installed into the front. I connected the LED's through a common resistor to ground. The LED on the left (from the front view) is a standby LED. It is lit whenever I have the power supply plugged into the wall. It is connected to the +5V standby wire on the PCB. In my PS, it's purple. The other LED is the "Power On" LED, and it is lit when I have the power supply turned on. It's connected to the "Power OK" signal wire, which goes to +5V when the power supply detects that it has stabilized the voltages. In my PS, it's the gray wire.

Step 5: Power Resistor

Most modern ATX power supplies require a small load to stay in the ON mode. I added a 10 ohm, 10 watt resistor between +5V and ground to provide this small load. It is strapped to the back wall of the power supply, where it should get plenty of air flow. It doesn't actually even get warm during normal operation so it's not a big deal.

Step 6: Finished Project

Here you can see the finished project, both with and without the cover. If you have any questions, please leave a comment and I will try to check back often to answer them. Thanks for looking, and good luck!

Keep in mind that while I built my power supply many years ago with only the ground line fused, you should put fuses on all your signal lines and leave the ground line directly connected.