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ATX PSU mod

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Introduction

I found an old ATX power supply unit in my storage room. I thought I could make this unit useful again by turning it into a bench power supply. I did some research and actually managed to find the schematics for the unit.

Schematics

Schematics for the mod.

3D model

I did a 3D model of the PSU unit with the planned modifications. The 20 pin Molex connector can give me outputs at many different voltages. I ended up with 5 outputs:

  1. +3.3V
  2. +5V
  3. +12V
  4. -5V
  5. -12V

I'm feeling lucky since the -5V pin was removed on newer ATX revisions. A fuse will be connected to each output as overcurrent protection. The finished 3D model can be seen in the following picture.



Hardware

After finishing the 3D model I started drilling the holes for the outputs. Banana jacks will be used for all the outputs.





I can get rid of a lot of the cables since I only need the 20 pin Molex connector.





The result can be seen in the following picture.





I chose not to unsolder the unused cables. I just cut them and put some shrink tube on them in case I want to expand the number of outputs some day.



I gave the PSU a brushed metal finish and added a couple of layers of clear coat. This gave the unit a nice matte look.





In order for the PSU to have a stable voltage output, a constant current must be drawn from it. I fixed this by adding a 10 ohm/10W resistor to one of the 5V outputs. This resistor will get hot from all the current flowing through it (500mA @ 5V). To fix this problem I mounted the resistor against the metal chassis and added a thin layer of thermal compound in between.





After mounting the resistor I connected the PS_ON cable to the switch. The PSU will start when PS_ON is grounded. I also connected the PWR_OK cable to an LED.





I got rid of some unnecessary cables and ended up with a ground cable and one cable for each output voltage. All that remains now is to connect the output cables to the top of the PSU.





Here's the top of the PSU with the attached banana jacks and fuse holders





I soldered each output to its corresponding banana jack and fuse. Since I only have one ground cable, I connected all black banana jacks in series. I like this solution since it reduces the number of ground cables.





Here's the unit, fully assembled and ready for testing. Since each output voltage has a different maximum current rating, I had to be careful when choosing the fuses for the different outputs.





Testing



I tested all the outputs with a multimeter. The measured voltage for each output can be seen in the following table.

Output Measured Voltage (V)
3.3V 3.309
5V 5.061
12V 12.10
-5V -4.705
-12V -11.18

Multimeter: Uni-T UT61D



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last edited: 2015.11.04

Components used

Innovation + Robotics = Innovabotics