The Power Supply Unit (PSU)

The Power Supply Unit converts 240AC to 12, 5 and 3.3 volts DC. There are two types of PSU which are measured in Watts; modern computers have a 350watt supply approximately. There are two types of computer cases and Motherboards, AT and ATX. ATX is the only one used in modern computers.

The easy way to tell the difference between an AT PSU and an ATX is the power connection to the Motherboard. The old AT PSU has two motherboard connections labelled P8 and P9 ( see pics below) whilst the new ATX has one large connection called P1. The difference doesn’t stop there however as the newer ATX PSU has a soft power supply button that can be set to do different things when activated. The ATX system always has a 5v power supply to the motherboard as long as it’s plugged into the wall.

Connecting the power supply is fairly straightforward. First screw it into the computer case usually with four screws. If it’s an ATX style you just plug the PI into the only place on the Motherboard it’ll fit, there’s a guiding pin that ensures connectivity. On the older power supply AT PSU there are two connectors P8 and P9. Again these will only go in one way ensuring that all the black wires are next to each other and in line.

P1 Connection.

As you can see in this picture the P1 connection can only be connected one way with it's alternate square and D shape pins.

P8 and P9. Black to black as it would be when connectd to the Motherboard.