Power-Supply Sizes
Power supplies are available in a few standard sizes and shapes. However, the names for power supplies are anything but standard. They are based on the types of case they will be used in and the types of motherboard connections they will support. This is because different styles of cases place items such as plug fittings, mounting screws, and fans in different places.
A few years ago, a new type of motherboard cable power fitting began to appear on the market. The older models are known as AT-style, and the newer ones are known as ATX. We cover both in this tutorial, because you will need to be able to work with either one. The ATX design simplifies the placement of connections, so there is little to worry about with case compatibility. The main issues to be aware of are how much wattage the PC needs to power its parts and how many connectors for peripherals are required. Generally speaking, older Pentium-based computers and all 486-based and earlier PCs used AT supplies; almost all Pentium II and later-based systems use ATX supplies. The ATX design is preferred for two reasons:
- The on/off power control circuit (not the button) on ATX boards is built into the motherboard. On AT PCs, it comes from the power supply.
- AT power supplies connect to the motherboard through a pair of six-wire connectors. ATX power supplies connect through a single 20-pin connector.
A few motherboards and power supplies provide both AT and ATX fittings and switch support. They are rare, but will open up more options if you have to repair such a system. Generally, you should use ATX for all replacements, if possible.
It's a good idea to compare the existing power supply to the new one. Make sure that they are physically the same size, have the same connectors, and that the new one has at least the same power rating. Some high-quality power supplies offer "silencer" fans, that are much quieter than most models.