Saturday, 12 July 2008

Regulated Electronics Power Supply - Old Computer's PSU

I came across this post on Instructables a while ago. I am so glad I made my own; It has been an invaluable resource for many projects I've done recently. It is really quite simple to build, and a good project for a few of those scavanged parts many of us have lying around.

The parts I managed to scavange are:
  • An old (ish) SPST 240V switch for mains-powered devices
  • An LED from the camera I took apart recently (for the high speed photography)
  • A potentiometer (variable resistor) that used to be the volume control for a pair of speakers
  • Wire for the leads from a piece of mains cable that was lying around
Sadly, there were some parts i needed to buy:
  • 8 - 4mm test lead sockets (as there known by Maplin)
  • A variable volatage regulator (I got a negative one by accident - that didn't really matter though)
  • 4 - 4mm test lead jacks
  • 4 - small insulated crocodile clips
Buying these parts individually, as I did from Maplin, was a little costly: about £10 for the lot.

The other common components that were needed, but I already had, were:
  • capacitors (for the voltage regulator - smooth the output)
  • resistors (for the LED - protection from a current that would kill the diode)
There's not much point posting schematics for this one, you just solder and insulate the wires that would normally exit the PSU in a big bunch, after finding out what voltage they carry, to the test lead sockets. The switch connections are simple: connect one side of the switch to the 'PS-ON' wire (which, in my case, was green) and the other side to ground (black wire). The LED connections were also easy: connect the anode of the LED to the resistor, with the other end of the resistor connected to a 3v3 line (dependant on LED), and the cathode of the LED to ground.

The voltage regulator circuit scematic will be available in its datasheet. This will be able to give information much more specific than I can. One thing I would suggest, if you have a single-rotation linear potentiometer like I do, is that you don't use +12V and -12V as the input and ground reference connections to give you a greater output range. With one rotation, it can be dificult to set the voltage accurately since the slightest movement will cause quite a large jump in the output voltage. What I did (effectively - mine was a -ve V.R. though) was to input +12V, with 0V as the ground connection. This gave me an output of about +1V - +11V relative to ground (0V). So, for a voltage between +1V and +11V, the output would be taken relative to ground (0V). For a voltage in the range +13V - +23V, you simply take the output voltage relative to the -12V connection. This will become clearer when you look at the regulator's datasheet. For those of you who cannot wait, here is a quick diagram:

All this leaves me to do now is post some pictures:

I am glad I made this power supply for all my electronics projects, using batteries was getting a little tyring when what needs to be done should be quite quick; especially when you're one battery short and it's raining outside!

Thanks for reading,