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Building an ATX to bench power supply converter

posted 3 Jul 2010 13:37 by James Gibbard   [ updated 24 Jul 2010 09:25 ]
I was looking to get hold of a bench PSU to allow me to test small electronics projects at home. Unfortunately even the basic ones are quite expensive, for example this one from rapid is nearly £100. After seeing an instructable on converting a computer PSU for use as a bench supply I decided that this was a cheaper and more interesting route to follow.

I decided to go about the project slightly differently to the way described in the instructable. Instead of sacrificing a working computer PSU that I use when repairing computers I decided to make a box that would allow me to plug in the PSU and output the different voltages through terminal posts. This box would also contain a two status LEDs and a switch to turn the power supply on and off. In addition to the standard outputs of a computer PSU (+12v, -12v, +5v, -5v) I also decided to add a variable voltage regulator to allow any voltage from 1.2v to 24v.

The parts required were:
  • ABS Box 130 x 68 x 44 mm  (It was a tight squeeze!)
  • 6.4 mm Clear heat shrink
  • Small piece of veroboard
  • 100 nF Capacitor
  • 1 mircoF Capacitor
  • 120 ohm Resistor
  • 2 x 1N4002 1A 100v Rectifier diode
  • 10 ohm 10 watt Resistor
  • LM317  3-Terminal adjustable voltage regulator
  • Heat sink for voltage regulator
  • Latching push switch
  • 1x Red LED and 1x orange LED (and suitable resistors for 5v operation)
  • 5k Linear potentiometer
  • Control knob for potentiometer
  • 20 pin atx to 24 pin atx converter 
  • Molex to sata power cable
  • 5x Red terminal posts
  • 1x Black terminal posts

The first step was to drill holes in the box for the 2 LEDs, switch, potentiometer and the 6 terminal posts. Once these were made the square holes for the input from the computer power supply had to be made by drilling, cutting and filing to a neat(ish) finish. 

The actual wiring was very similar to the method described in the instructable. The main difference being that instead of cutting the wires on the power supply I used a 20 pin to 24 pin atx cable and cut the 24 pin end off. This left me with the male socket for a motherboard power connector with all the wires ready to connect up. Since each wire is rated at around 1.5A, and a modern power supply can provide up to around 20A, I used multiple wires to connect to each terminal block. As one +12v wire was being used for the variable voltage regulator only one other +12v wire was available  to connect to the +12v terminal. In order to allow more current to flow (This is only necessary if you intend to you the power supply for high current applications) I also cut the sata ends off a molex to sata power adapter and wired the +12v wires off these to the 12v terminal. The picture below shows the connectors once they were attached to the box.

A 10 ohm resistor is connected between any +5v and ground line. This is because some power supplies require minimum load to function correctly.

The standard wire colours of a computer PSU are as follows:
Yellow +12v
Blue - 12v
Red +5v
White - 5v
Green - Turn the PSU on when connected to ground
Purple +5v when the PSU is connected to mains  (standby LED)
Grey +5v when the PSU is running (ON LED)
Black - Ground

The +12v, -12v, +5v, -5v and ground wires were connected to the correct terminal posts and then heat shrink wrapped to insure that no short circuits occur. Next the power switch and 2 LEDs were connected up. 

The final step was to to add the LM317 voltage regulator.  A circuit similar to this one was used. The output of the voltage regulator was then connected to the final terminal post. The picture below shows the completed insides of the box.

The final result: