The ATX power supply units (PC PSU's) provide regulated and stabilized DC current that can be used on a breadboard for prototyping purposes with 3.3V, 5V and 12V.

This units are turned on/of externally, anything that shunts the black and green wires will do.

Some uinits need some load to turn on, if its the case an HDD will do.

After sucessfull power-up we should check the voltage values with a multimeter.