Introduction: Yet Another ATX Lab Bench Power Supply Conversion

By workshop.electronsmith.com

This project builds upon the ideas of a previous instructables project: https://www.instructables.com/ex/i/D5FC00DAB9B110289B50001143E7E506/?ALLSTEPS
The big difference is that I decided I didn't want to destroy my ATX power supply in the conversion. Another advantage is that you can just plug in a new power supply if the old one goes caput.

Tools:
Drill and bits
Screwdriver ( for taking project box apart)
Soldering iron
Desoldering iron (optional)
Label maker (optional)

Materials
Banana jacks
Project box
A lighted 12V switch
3/4" strain relief
ATX connector (purchased or scavanged)
Miscellaneous heat shrink tubing sizes
Some wire 14-20 Ga (colors matching standard ATX if possible)

Step 1: Obtain an ATX Motherboard Connector

For my power supply I desoldered an ATX motherboard connector from a fried motherboard. An easier, less time consuming, but more expensive solution would be to buy an ATX extension cable or an ATX 20 to 24 pin adapter for about $10.

Desoldering components can be tricky, so take your time. The easiest way I have found is to first flood all the pads with as much solder as they'll hold. This allows me to heat the entire pad and pin evenly with the desoldering iron. I use a simple Radio shack model with a squeeze blub.

Step 2: Connect the Neccessary Wires to the Connector.

Again if you bought an extension or converter cable you need just to cut off the opposite end.

Pinouts for the ATX connector can be found at:
http://xtronics.com/reference/atx_pinout.htm
or
http://pinouts.ru/Power/atxpower_pinout.shtml

The pinout is a little wierd, I think it was dreamed up by somebody who wasn't very familiar with electronics. Pin 1 and 11 are at the same end. Ditto for pins 10 and 20.
What I did to avoid confusion was to plug the connector into the power supply and note the color of the wires . Then I soldered wires to the following pins

Pin Standard Color My Color Use
1) Orange Orange +3.3V
7) Black Black GND
10) Yellow Yellow +12V ( I solderd two wires to this pin)
12) Blue Solid Grey -12V
13) Black Black GND
14) Green Green Switch on
18) White Grey/Red -5V
20) Red Red +5V

To finish off the connector I bundled the wires with some heat shrink tubing.

Step 3: Layout the Front Panel

I used a small project box that I had lying around. I wanted the spacing of the holes such that I could use standard dual banana plugs, so I laid out the hole patterns so I could have one plug in ground and the other in the desired voltage. You can see from all the pictures how this works.

I also had to drill an additional hole for the lighted switch. The beauty of using the lighted switch is that it can be both a power indicator and the load to keep the power supply on.

Step 4: Drill Cable Strain Relief Hole

Next I drilled out the back to accept a standard 3/4" strain relief.

Step 5: Assemble Hardware

I put all the hardware in the box. That should be pretty self explanitory from the pictures.

I then soldered the leads to the posts. Looking from the front:
The center post is ground.
The leftmost posts is -5V.
The rightmost post is +5V.
The bottom post is 3.3V.
The top left post is -12V.
The top right post is +12V.

The switch has 3 terminals: +12V, GND, and the switched contact. When the switch is off, the switched contact will be open and the light will be off. When the switch is on, the switched contact will be grounded and the light will be on.

The other yellow wire goes to the +12V of the switch.
The second black goes to the switch ground.
Finally the power supply switch wire goes to the switched contact.

Step 6: Apply the Finishing Touches

I used my cheap Casio label printer to make some labels to remind me what terminal is what.

One notable flaw with my design is that it relies on the overload circuitry of the power supply. It's hard to add a fuse without cracking open the power supply, and to fuse each output would have been more work. ATX power supplies are cheap. Some may have internal overload protection and some may not. I'll take my chances. That's the beauty of my design, I can just plug in another ATX power supply if the first one fails.

Another problem may be that the lighted switch won't pull enough current to keep the power supply running. If it didn't, you would have to add a load resistor to one of the outputs, or in series with the power to the lighted switch.