I wanted a power supply to use on my desk with a breadboard for electronics- based projects. Lab bench power supplies are very cost-prohibitive, but luckily power supplies for desktop computers can easily provide 30A for under $20.
Inside the power supply, you will find a lot of different colors of wires. These all have different meanings:
- Black: GND
- Orange: +3.3V
- Red: +5V
- White: -5V (older PSUs)
- Yellow: +12V
- Blue: -12V
- Green: Power On
- Purple: +5V Standby
- Gray: Power Good
- Brown: +3.3V Sense
The brown sense wire is hooked up to one of the orange +3.3V wires. When the power supply sees “Power On” (green) get grounded it checks to make sure the 3.3V sense is within acceptable range and then turns on the rest of the outputs while sending +5V over the “Power Good” wire to tell the motherboard it can begin executing instructions on the CPU.
Some power supplies also check to see if there is a load present before activating, so we will need to add a 10W resistor or 2x 5W resistors in parallel. Most power supplies should be happy with about 10 Ohms of load on the +5V rail, but this may vary across different PSUs.
We can either add a switch to the “Power On” wire or just ground it permanently and use the AC switch on the back of the PSU. We can also optionally add an LED to the “Power Good” wire so we know when the PSU is supplying DC power. Both of these options are shown in the diagram below:
Here are photos from my build:
I like to start off by separating the wires by color so it’s easier to find the ones I need to work with. You can snip some of the wires off at the board, and epoxy over them to make it easier to connect your outputs to banana plugs.
Here is the LED that I wired into the “Power Good” wire, and simply connected to ground:
Below is the whole thing assembled according to the schematic. You can see the 10W, 10 Ohm resistor on the bottom right, which was heat shrinked between a +5V and GND wire. The LED was soldered to breadboard between Power Good and GND and then electrical taped to prevent shorts.
All DC wires were isolated from the AC wires in the foreground, and soldered onto banana plugs with heat shrink to prevent shorts. Finally, a push button switch was added between Power On and GND.
And here is the final result! It can achieve 24VDC by using +12V and grounding to -12V.
It’s important to note that these power supplies are capable of providing many amps, so you must be very careful when using ICs or other delicate components.
It would be wise to use a fuse between the PSU and your circuit. This could be built in before the banana plugs, but I would rather use one externally.