ATX PSU MOD.

Intro

Regulated PSUs are expensive.
Why not just use a PC power supply. They can be expensive too, but they can also be free from old computers being thrown away.
PC Power supplies work in switched mode, and need to be loaded before they will work. They are also not terribly reliable or accurate in the voltages.
For my purposes, All I needed was a 12V supply to power my nicad peak detect charger. A computer PSU would be adequate. A search on google newsgroups gave me the information I needed.

Piccies

The modified PSU - The insides

Parts list:

1 x 230W PC PSU (or greater).
6 x 10 ohm 10W ceramic power resistors. (Maplin code H10R)
1 x 10A SPST toggle switch. (Maplin code JK25)
3 x banana jack pairs. (Maplin codes HF02 - black, HF07 - Red)
1 x LED clip (Maplin code UK14)
1 x 5V LED (Maplin code CK47 - Green)
Some plastic tie wraps
Some 2 mm heatshrink wrap
A piece of stiff yetflexible plastic.

Disclaimer

Caution ... the insides of a PC PSU can be dangerous. I left mine unplugged for 6 months before opening it up. (Not deliberately, it just hadn't been touched)
The Caps can retain power for some time, and can give you a nasty kick.
It can be fatal.
Disclaimer... You open up a PC Power supply at your own risk.

Build

Lay the resistors out in two groups of three and sloder the legs together as shown below.

Cut the piece of stiff flexible plasic to a size big enough to mount the resistors on with some space to spare.
Drill holes in the plastic and tie wrap the resistors one atop the other to it.
Solder the tags of the resistors together at the bottom such that all six are now connected together in parallel.
Drill more holes in the plastic, and mount it just behind the fan inside the PSU case. I managed to find or drill holes in the large vertical heatsinks in which to put more tiewraps. You may have enough room to use standoffs and screws into the heatsink itself..
Visiually check that there is no part of the 5V subsystem which will earth out on any part of the PSU. Insulate everything in sight with heatshrink and/or insulating tape.
Attach a grounding strap. If you don't know what one of these is, you shouldn't be doing this procedure. Open up the case.
Drain the capacitors by connecting bulbs across the various +12V, +5V, -5V, -12V output wires.
Use bulbs rather than resistors so you can see them dim for a visual check that there is no power left.
Use a multimeter across the same wires to show there is no power left in the system.
Drill holes in the case
- 3 pairs of holes into the case, next to the hole the wire bundle comes through for the terminal posts to fit into.
  Label them +5V and +12V.
- One for the toggle switch
- One for the 5V LED
Fit the toggle switch and the terminal posts into the case and tighten them well.
Cut all wires except Red, Yellow Black, (and Green on an ATX PSU), back to an inch or so. "Safe" the ends with a bit of the heatshrink.
Yellow is 12V, Red is 5V and black is earth.
Cut all the plastic plugs off the wires and separate into matched bundles of wires.
You will need ...
- a 5V bundle of half the red wires with an equivalent amount of black wires to go to the 5V terminal posts.
- a 5V bundle of the rest of the red wires with an equivalent amount of black wires to go to the resistors.
- a 12V bundle of half the yellow wires with an equivalent amount of black wires to go to one of the 12V terminal posts.
- a 12V bundle of the rest of the yellow wires with an equivalent amount of black wires to go to the other 12V terminal posts.
Cut the wires to the correct lengths and solder them to their destinations. (See Picture above of the inside of my modified PSU)
On an ATX power supply I connected the green wire to one side of the switch and the other side of the switch to earth. This does have the unfortunate side effect of drawing current through the resistors even when the switch is "off".
I should get around to switching the 240V AC Current at source rather than using the green ATX Signal wire.
On an AT power supply, the 240V power goes to the front of the case via 4 wires. You need to work out which two to connect together, and which two to connect to the switch.
Take a wire from the 5V feed somewhere, (solder a wire the the 5V termial post or just use a spare red wire), and solder it to the positive side of the LED. (Normally marked with a small flat on the LED casing)
Solder the negative side of the LED to earth.
Fit LED clip to the Case and insert LED.
11. Close the case, you're done.

Q & A

Q1. Why use expensive resistors, why not use bulbs ?

A1. Well, they're not that expensive at circa 25 pence each, and they wont break like a glass bulb might.

Q2. How much current can I pull out of this thing ?

A2. I'm not an expert but I can give you some empirical numbers.
The 230 Watt PSU which is pictured above has had 10A drawn from the 12V rail by using two nicad chargers at the same time.
I have used the 5V rail to power an Orion Orbital 12x3 electric motor to run the brushes in. The draw into a resistive load was measured at 15.4 Amps.

Last updated 29th December 2002