Friday, 3 August 2012

FREE Desk Power Supply

I found a dead computer at the top of our driveway (we share it with a motel apartment thing), and someone had obviously abandoned it. Not sure if it got left out in the rain, probably.... does care. Soooooo like any right-minded engineering student, I immediately grabbed it and scuttled back home to see what I could scavenge.

As well as 2 disc drives and a floppy drive (which will be dismantled later, A] for shits and gigs and B] for potential motors, linear actuators and other goodies), the PSU was still inside. While it is only rated for 300W which is not enough for a modern computer, that's more than enough for a desk power supply.

The unit provides supplies for GND, 3.3V, 5V and +-12V, as well as a standby 5V which is always on when its plugged in. There is also a wire allowing a switch to be added for turning it on and off without unplugging it. I decided to add an LED to indicate that it was turned on as well, just in case.

I have never been able to do electronics at home (apart from what runs on USB) due to the lack of a scope and a power supply, so this solves half the problem at least.

I found a tutorial online which made it seem pretty easy, at .


First I cut the plugs off, leaving long leads just in case I needed long ones.

I scrounged (from the techs at uni) some banana plug binding posts of various colours, and a switch. In my pile of potentially useful things, I also found an LED and some crocodile clips.

Once I knew what connectors and things were going on it, I checked there was room inside the case and then drilled holes for them all to be mounted on the outside using the drill press in the uni workshop.

The switch and binding posts I just screwed onto the case, as the wires can then be attached to the base of these. The LED I left to be wired up later.


Inside the case, there were heaps of wires so in order to sort it out a bit I "thinned" them (e.g. probably don't need 10x 5V so I took out like 5 of them). There are more wires for some voltages than others, and this corresponds to the PSU's current rating for each.

I left a long 5V and GND to be attached to the clips, because having clips is always useful for easy connection to things e.g. onto the contacts in a battery holder or component leads.

Otherwise I tried to shorten them as much as possible while still allowing the unit to be dissembled easily, because there is not too much room inside but the wires must be long enough to allow the cover to be removed while they are attached.


The binding posts come with a wire attachment loop which is soldered to the leads, and then loops over the bolt and is attached with a second nut.
Each group of wires had a piece of heatshrink stuck over it, then the ends were soldered together and soldered to the binding loop. The heatshrink was then pulled back up and shrunk on to cover the live ends. This was repeated for all the voltage levels, before the leads were attached to the binding posts.

The green wire is the ON control for the PSU, and the unit runs when this is shorted to ground. the GND and green wires were attached to the switch, so that up is off and down is on.

For the LED, I attached it with a 560 ohm resistor to the 12V line (because I had an extra 12V spare), and then attached the other end directly to the ground binding post. The LED is just wedged in the hole, but the sizing is pretty tight so it is reasonably secure.

The brown "voltage sense" wire was tied to the 3.3V line as this is used to regulate that level, and the grey "power ok" wire was removed because it does nothing.

I attached the crocodile clips to the long GND and 5V lines, and these went out the hole left by the original wire mess.


With everything attached, the case was put back on.

Just to make sure I did it properly and it wasn't going to short out my flat (wouldn't be the first time) or explode and catch fire (as happened with the toaster yesterday), I got it properly electrically certified checked by Tim the technician at uni.

With a pass mark (the caveat being "don't put your fingers in the hole"), my new desk supply was plugged into the wall and the voltage levels tested with a digital multi-meter. Everything worked, and all in all the project was a great success.

While I now have a home desk power supply with several voltages including a dual-rail line, I also have something that looks almost professionally hacked and can live on my shelf in pride.

Hackage desk supply, something every engineer needs.


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