The Case and Power Supply Unit (PSU)

Selecting the case unit is mostly a matter of deciding for a style.

In broad terms there are two types of cases:

• Desktop Case

• Tower Case

whereby tower cases are usually divided into:

• Full Size Tower

• Midi Tower

• Mini Tower

Both types of cases vary by size and profile. Whereas the desktop case sits (as the name implies) on the desktop, the tower case is designed to stand upright and can be much larger.

Before choosing the case you should think about your requirements for your computer. If you need a simple office machine you might decide for a desktop style. If, however, you want to keep your options open for upgrading your computer at a later stage, maybe by adding more hard drives or other devices, then you might consider a tower with more room for add-ons. Also make sure that there are enough expansion slots for your purposes and that they match the expansion slots on the motherboard (compatibility). 

If you decide to build an ATX system you also need an ATX case, if you decide to build an AT system you need an AT case. These days most systems are ATX, especially if you decide for an Intel processor. You can distinguish the cases best by looking at the back. AT cases have usually eight slots for expansion cards, two little slots for the serial port connectors and one for the parallel port connector, one round hole (approx. 1.5cm diag.) and maybe a smaller round hole next to it for PS/2 mouse support. As ATX motherboards come with all sockets already onboard, ATX cases have a large square hole next to the expansion slots, which accommodates the I/O shield (a panel that closes around the connectors of the motherboard. All ATX cases come with a variety of I/O shields in order to cater for all types of motherboards.

Make sure you decide for a case that provides good access to the components. Don’t get one where you have to cram everything into. Think about what happens if a component has to be replaced, you don’t want to disassemble the entire system to replace it.

Whichever case you buy, it will most likely include a power supply to produce a regulated source of electricity. The electrical connectors supply the motherboard and all other devices with +5V, -5V and +12V, -12V. A power supply is rated by the number of watts of power it can deliver to all connected devices. Although today’s computer components are more energy efficient than a few years ago it is always a good idea to look for a power supply that is a little bit stronger than actually needed to ensure a safety margin in the event of having to cater for additional devices. Standard power supplies for AT systems are normally rated for 200W to 250W, which is enough for any normal computer. But if you intend to build a machine with multiple hard drives and lots of other internal devices, try get a 300W supply. Better too much than too little. ATX systems don't come with a power supply unit of that strength any more as the most power consuming component of the PC now plugs directly into the power socket: the monitor. Today's monitors don't like to be switched off any more but rather switch themselves off when the computer sends the signal to do so.

Another important factor is the cooling fan, which normally comes with the power supply. Because today’s computers produce much more heat than in earlier years (especially the processors), it is important to ensure that the case has no unnecessary open slots, as those will affect the efficiency in which the heat is removed from within the case. Always make sure that you get a working cooling fan with the power supply as excessive heat can cause premature failure of the components. This is another good reason for buying a larger case, a bigger case means less heat. If you are going for a larger system you might even think about a second fan. If you do decide for a second fan, make sure it supports the air flow within the case and blows in the same direction as the power supply fan.

Make sure that the case you select contains all the necessary mounting and accessory hardware such as screws, washers and brackets. These are usually attached to the case in a small box or plastic bag. Also decide which other connectors you need, such as LED-indicators or a build in speaker. A lot of cases offer a wide range of accessibility features, such as a slideable bottom tray.

As you search for the right case you will come across many special offers called ‘barebone systems’. Such a system usually contains just the bare case with power supply and fan and the motherboard, sometimes even the processor or other components. When considering such an offer it is important to make sure that all components are equally up to date, as some manufacturers combine their products in order to clear (sometimes out of date, or second user) stock. As with all components of a computer it is best not to try to safe money at the wrong places. Don’t go for a cheap case if you can avoid it, one day you might regret it.