A few weeks ago I did what I had wanted to do for a long time: take the old computer power supply I had laying around and convert it to a simple bench power supply. There are plenty of instructions on the web on how to do this. The idea behind all of them is quite simple: Since a standard computer power supply provides several different voltage levels to the motherboard and the built-in peripherals (hard drives, graphics card, etc.), it would be useful to make these outputs available via standard connectors (often banana plugs). You can then use these voltages during prototyping of any of your electronics projects. This is especially useful when you need multiple voltages. The two main voltage levels provided are +12 Volts and +5 Volts, but often you also have a +3.3 Volt output and a few negative supplies (-5, -12). For my conversion I used the three positive outputs only, though I may connect the negative outputs in the future as well. In my supply, the red socket provides the 5V output, blue the 3.3V and yellow the +12V. The two black sockets are ground.
The switch came from an old hot air gun that had stopped working. Even though the PSU has a power switch on the mains input side, I wanted a switch on the ‘user’ side as well. I also added a red LED, which I connected to the “power good” signal (via an appropriate resistor).
When I first opened the PSU, I was shocked to see the poor quality of the device. It came from a fairly cheap old PC, so I didn’t expect great quality, but I was surprised by all the exposed mains connections and the giant solder blobs on the bottom (I should have taken a picture!). I also had to replace two of the capacitors – the ones that where in there had blown. That at least wasn’t a surprise. The one positive surprise was that the fan still works flawlessly, and is probably one of the quietest fans of that size I had ever heard. When I first turned it on after my conversion, I thought the fan wasn’t working any more because I could hear nothing. But a quick look showed that it was spinning happily.
So, all in all the conversion seems to have gone well, and I can now stop using wall-wart supplies or batteries when I want to try out a new circuit. If you are considering doing such a conversion yourself, be sure to follow the appropriate safety precautions. Remember that you are dealing with mains voltages in parts of the circuit, so there are all kinds of real dangers! More so if you have a cheaply constructed design such as I had!