March 9, 2014

Laboratory ATX Power Supply Unit

Now this is a more recent project that I did back in 2012. As I started getting to work on electronics along came the need to have a decent power supply for my experiences, instead of always using different, cheap and weak power adapters. Since I had a few power supply units from old computers laying around, I thought I should modify one to fit my needs. They have a few different voltages available and can be quite powerful.

This was the final result. I wanted it to have a cool high-tech look so I painted the case all black and put some green LEDs inside.

So the first thing you need to know about the computer power supplies or sometimes called ATX PSU, is the wiring color scheme. These are the most common colors in use and their respective voltages. However there are some PSU with different colors and voltages, so the best way to be sure is to test them with a multimeter.

To start a PSU just plug it in to the AC mains and connect the green wire (PWR ON) and one of the black wires (GND), the fan should start working.

The GND is the ground common for every one of the output voltages. As the image shows, the yellow, red, orange, white and blue wires correspond to 12V, 5V, 3.3V, -5V and -12V respectively.

The other colors have different functionalities:
  • The green wire or PWR ON is responsible for the start up, that's why you connect it with a GND wire to start the power supply. 
  • The purple wire is the Standby terminal, it always has 5V available, even if the PSU is turned off, as long as it is plugged in to the AC mains. 
  • The brown wire (sometimes also orange but thinner) is the Sense terminal, it's usually connected with the 3.3V or orange wires, and it is responsible for making sure that these outputs always have 3.3V. If the voltage is different than that, it'll probably shut down the PSU.  
  • The grey wire or PWR OK is a confirmation that all outputs are working with the proper voltages. When the PSU starts up, it takes a moment until all the outputs have the right values, during this time the grey wire is fixed at 0V. When all the outputs reach their voltages, the grey wire changes its output to 5V, indicating that the PSU is ready to use.
With this in mind, it's time to start designing the circuit. So all I'm gonna need is the output voltages, therefore I'm gonna drill 6 holes for 6 connectors: -12V, -5V, GND, 3.3V, 5V and 12V.

For the ON/OFF switch, the PSU I'm using already has one on the AC mains side, and since I don't need the 5V Standby terminal, I can use the mains switch as my ON/OFF switch. So I will connect the green wire directly to a black wire. Alternatively you could also put a switch between this connection.

Now the 3.3V Sense terminal. For this you're gonna need to test your PSU to see if it works without it or not. I removed the brown wire and my PSU still worked fine, but this is not true for all PSUs. If you want to avoid this doubt, you can just connect the brown wire (Sense) with the 3.3V wires, this way you'll be sure it will work as it should.

I used the PWR OK terminal or grey wire to plug in my Power LED. This way my ON indicator will also tell me if all the voltages are correct. Since this terminal gets up to 5V when everything is ready, I can just put an LED in series with a 150 Ohm resistor. Here are the connections to the terminals:

I left more than one wire in some terminals to allow them to use more current. The power specifications for each output is different and you can find it on the PSU casing, but in order to use it properly you should leave more wires in parallel. Otherwise, depending on the application of course, you could reach the current limit of the wire, and the voltage drop would also increase.

Finally and for the looks, I put 2 LEDs on the inside of the PSU connected to the 5V terminal (red wires), in series with 150 Ohm resistors. I used the 5V terminal instead of the PWR OK terminal because the PWR OK is just an indicator terminal and for that reason its current output is very limited. Even though it could probably drive 2 more LEDs.

I also left one connector for HDDs or disk drives in case I need to test one. And here is the final aspect of my PSU:

This is how it looks inside:

And this is how I transformed an old computer PSU into my new laboratory power supply. An upgrade that could be done to this would be adding a variable output with an LM317 for example.

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