Thursday, 17 November 2011

Universal-ish power supply unit

Hobbyists and home experimenters will know the feeling. You're working on a little project and you know you're going to need power. Often this results in a mad search for a battery box and some cells that still have a bit of life in them, or perhaps you raid your collection of "Wall Warts" (which usually means you can't charge your phone or use your router until playtime's over). Wouldn't it be nice to have a dedicated power supply for these occasions? What a luxury.

Contemplating this problem earlier today, my gaze fell upon a heap (literally a heap) of old computer PSUs gathering dust in the corner of the Secret Laboratory. I always salvage stuff from old PCs - better gathering dust than going to landfill.. Some of the PSUs were of no further use anyway, being of the old "20-pin" format rather than the "24-pin" that modern motherboards require. Some were rated as low as 200W, which doesn't really cut the mustard with modern PCs. For years I had been tripping over the very solution to my problems. It was a classic "DUH" moment...

So, the next Secret Laboratory project is going to be the conversion of one of these old computer PSUs to a useful universal-ish general purpose power supply. If it's good enough for motherboards, it's good enough for the shit I build.

The plan is to source a compact yet safe housing for the PSU, to prevent busy little fingers from poking stuff (accidentally or deliberately) through any dangerous holes. Terminals will be presented for +3.3V, +5V and +12V outputs (either separately else via a voltage selector switch - I haven't decided yet). I may include -12V for those of you who like to muck about with devices that require it. I never did fully understand why op amps need the dual polarity thing...

If you're following this project, the bit you're going to be working with is the 20-pin connector (usually marked P1), seen here to right of centre. You can ignore the other connectors (far left) - they just route the same power to other computer peripherals (hard drives, etc). You can get everything you need from the 20-pin connector alone.

Of great importance! Mark well the following cable colours:

  • Orange: +3.3V
  • Black: Ground
  • Red: +5V
  • Grey: Power Good (optional signal)
  • Purple: +5V Standby (optional power)
  • Yellow: +12V
  • Blue: -12V (optional power)
  • Green: Power On

There's some redundancy in there, so you'll find several cables of the same colour. Trust the colours but use a meter to check the voltage lines. I haven't given pin numbers because they differ between the 20 and 24-pin standards.

To turn the PSU on, you must ground the Power On pin. I plan to use a simple on-off switch for that.

+5V Standby is always on, even without Power On grounded. I plan to have a little LED across this just to show that the unit is plugged in and ready to go.

Power Good goes high when the PSU is delivering the right voltages to all outputs. Again, I plan to have a little LED across this one. You can't have enough lights on your unit, you just can't. The more lights, the better it looks (and it's a real babe magnet - well, it's always worked for me).

Regrettably, I must inform everyone that electricity can be hazardous and you really mustn't go mucking about with stuff you know not what of. In particular, do not disassemble the PSU itself and, if you do and you see something that looks like a little can of baked beans for a pixie's din-dins, don't touch it else you might end up all dead (even if you're not plugged in). That's my legal disclaimer. Can we see if we can get through this project like grown-ups?

More news soon, as soon as I've found a suitable housing for the PSU...

1 comment:

  1. Just thought - if I can find a suitable panel mount USB socket, I could include a handy little charger for USB devices also (it's just 5v - nothing complicated - just leave the data lines unconnected). Maplin do one but it's a bit overkill. Built by Neutrik, it's the kind of hardware that could survive a Theater (sic) of War.