Computer Power Supply Units, or PSUs, are excellent power supplies that can be recycled once the computer is obsolete. In fact, even when the computer is long out of date, or its components have failed, it is more than likely that the PSU is still in good working order. Sadly, many of these perfectly working units will end up as landfill. In this instructable I will show you how to rebuild a PSU as a lab power supply.
SAFETY WARNING! I this instructable I will be opening the case, exposing potentally dangerous voltages. Do not attempt this if you do not understand the dangers of electricity.
LEGALEZE: I do not accept any responsibility for any injuries caused as a result of attempting this instructable.
Ok, several people have done projects like this. Just search Instructables for PSU and you'll see how many there are. So I thought I'd put my two cents worth in with this instructable, to demonstrate my "remote switch" idea, and just to show how I went about my version of this popular rebuild.
Most PSUs follow the same output configuration: +3.3V +5V, +12V, -5V, -12V and probably a +5V standby. This gives you a number of different voltages up to 24V; across the +12V and -12V there is 24V, +5V to -5V gives you 10V and so on.
The standby is also useful. When used in a PC, the standby is always on even when the PSU is otherwise powered down. This means you can still have a device running while the PSU is on minimal power.
This particular PSU has a master switch on the back, in addition to the "soft switch" that all PSUs employ. With that in mind, I decided to implement a "remote switch" idea that uses a switched 3.5mm jack. This works such that the PSU is powered up when nothing is plugged into the 3.5mm jack, but when you plug a switch into this jack, you can control the PSU from it. This switch can be on a lead so that you can control the PSU from a distance. It also means that you could have some device running off the 5V standby whilst the PSU is in standby, then that device could power up the PSU using the remote switch. At the end of this instructable there is a short video that demonstrates how the remote switch works.
Step 1: Disassembly
The first step was to remove the cover, and cut off all the connectors. I left some length of cable to work with.
During this step, I observed closely the cables and how they are connected. Most of the wiring will be standard across most PSUs:
Purple: +5V standby
Pink and brown: I was unable to determine what these are for. As you can see from the photos the pink wire connects to the red wire in the ATX connector, and the brown wire connects to the orange. At first, I thought these wires might serve as a detector to indicate that the connector is still connected and is good, but the unit still functioned as expected once I had cut the connector off, so I still don't know what these do. Elsewhere I read that the brown line is a "3.3V feedback line", but since it doesn't appear to function that leaves me none the wiser. I left these two disconnected.
Green: This is the "switch". Connecting it to common (a black wire) will cause the unit to power up. The normal arrangement is to have a switch on this line to turn the unit on and off. When it is off, the only power available is the +5V standby.
Grey: Apparently, this line is supposed to have a small load on it, like an LED and a resistor, but the PSU functions fine without it. I left this one disconnected also.