Computer Cases
The type of case is the first decision when building a computer. The case is made up of the metal chassis (or frame), and a cover, usually constructed of metal or hard plastic. The case is the housing unit for the internal components and protects against dust and damage. The case usually comes with the power supply needed to power the computer and the installed components. Computer cases are either desktop or tower models. The following is a description of each type:

• The desktop model sits on a desk horizontally. The monitor can be set on top. This choice can be a space saver.
• The tower model stands upright in a vertical position that allows easy placement on the floor. Mini-tower, mid-tower, and full tower cases are available.

The choice of a desktop case or tower is a matter of personal preference. However, it is important to consider the workspace before choosing a case.

Hardware components are installed in the bays of the case. The bays are placeholders for the drives so that they are neatly organized. Devices may be easily interchanged from bay to bay if necessary. Some bays are normally left unfilled in a new computer, so the machine can be upgraded with a ZIP drive, tape back up, or a CD-ROM burner.

Tables and summarize information about the different parts of a typical computer case and the factors to consider when selecting a case.

Power Supplies
A power supply provides the needed voltage to power the various electronic circuits that make up the PC. It receives external power as AC electricity. It also produces power for the operating parts installed in the case. The power supply is contained in a metal box. Within this box, a transformer converts the current that is generated from standard outlets into voltages and current flows that the computer parts need to operate.

Power supplies are rated in watts, and a typical computer has a 250-300 watt power supply. A fan installed in the power supply prevents the computer and its components from overheating. This is accomplished by maintaining cool air within the case. It is critical that these components be kept at a consistent operating temperature to ensure their best performance. The fan is built into the power supply with openings on the backside of the case. The fan inlet port should never be blocked or covered.

The power supply converts AC to DC. There are several different types of power supplies, varying in size and design. The more common types are known as the AT and ATX power supply form factors. AT type supplies are no longer used in new systems, and are generally found on computers built prior to the mid-1990s. ATX is the more common. The easiest way to distinguish the AT and ATX is from the nature of their connectors. Common PC power supplies are switched power supplies, as opposed to linear power supplies. More about the physical identification of the various power supplies is discussed in the lab exercises.

The devices that attach to the power supply use +5v, +12v, and +3.3v DC power. Older devices (PC/XT and early AT) also use -5v and -12v DC power. The power supply must support the type of processor to be used. Each power supply comes with all these specifications. Tables and describe the components of a typical ATX power supply and the factors required when selecting a power supply.

Interactive PhotoZoom Activity  (Flash, 487 KB)   PC

Interactive PhotoZoom Activity  (Flash, 337 KB)   Power Supply

Worksheet  (PDF, 10 KB)   PC Power Supply