A PC power supply is the cheapest way to make a powerful power supply for your electronic assemblies. With a classical power supply (200W), you can get 20A from +5V and 8A from +12V.
There are two kinds of PC power supply, AT (the oldest), and ATX (the newest).
Standard color codes are :
The PG wire switches to +5V when the power supply is ready (half a second after power on). Be careful, you cannot take more than 1mA from this wire (then it is difficult to directly light on a LED). Moreover, you cannot take more than approximatively 1A from the power supply while the PG wire is not ok. It means that you may not be able to start the power supply with the assembly already connected.
With an ATX power supply, you must connect one wire to +5V in order to have it working. TODO