PC Form Factors and Power Supply Explained
PC Form Factors
XT Form Factor
Designed for the original IBM PC and its successors. These cases were primarily sold as desktops and were characterized by robust construction. Originally, they were quite large, as was their power supply, which had a power output of 63.5 W. Now obsolete.
AT Form Factor
It was a format similar to the XT, though there were key differences. For the first time, the power supply had the power button on the front. Cases began to be built in both desktop and tower formats, and this remained a standard for a long time. Now obsolete.
Baby-AT Form Factor
Very similar to the AT, except in size. It was a format that gained tremendous popularity until the rise of the ATX format. Baby-AT cases were available in a huge variety of styles and sizes.
LPX Form Factor
The key feature of the LPX format is the use of an additional card called a riser card that plugs into the motherboard. Expansion cards are connected to the riser card, parallel to the motherboard. This design allows the case to be shorter than the minimum height required by expansion cards.
NLX Form Factor
A design promoted by Intel to replace LPX. It is similar to LPX, but the key difference is that NLX is a standard, while LPX often referred to proprietary designs.
ATX Form Factor
This is undoubtedly the dominant format in the current market. It was developed by Intel in 1995 and has evolved to the present day, largely replacing Baby-AT. ATX cases introduced a number of significant features. First, the **I/O panel** on the back allows for different configurations of connectors for different motherboards. Second, the power supply is also specific, with new features and capabilities, as discussed below. Intel also specified some variants of this format:
Mini-ATX
A smaller variant.
Extended ATX (EATX)
A larger variant with dimensions slightly exceeding ATX.
PC Power Supply Basics
The main function of a power supply is to take input power from the electrical outlet (e.g., 220 V) in the form of **alternating current (AC)** and transform it into low-voltage **direct current (DC)** (typically 3.3 V, 5 V, and 12 V) so that the computer’s components can use it. Mains electricity is supplied as AC, and it is necessary to transform it to DC power to operate the PC. Thus, the first step is to convert AC to DC. Once converted, the power supply must provide a continuous supply of different voltages to power the various components of the computer.
Many external devices (printers, scanners, etc.) use power adapters that perform a similar function to the internal power supply. They transform the 220 V from the mains supply to 12 V or 9 V. The problem with simple adapters is that they waste a lot of energy, as the excess energy from the conversion is dissipated as heat. This is unacceptable in a PC, so **switching power supplies** are used due to their more efficient design. Less heat is generated, and this is dissipated by a fan within the power supply casing itself. These power supplies can radiate a strong electromagnetic field that can potentially interfere with other computer components. This is why PC power supplies are enclosed in a metal housing for shielding.
Common Output Voltages
- -12 V: Used in some serial port circuits, though very rarely used now.
- -5 V: No longer used. It was previously used by floppy controllers and some ISA cards in early PCs.
- 0 V (Ground): Considered the ground voltage. It is the reference voltage from which other voltages are measured.
- +3.3 V: A voltage introduced with ATX power supplies, primarily for components like microprocessors, memory chips, and the AGP graphics bus as their voltage requirements decreased.
- +5 V: Used extensively in older systems (Baby-AT and earlier) for the motherboard, older chips, and other components. It is still used today by many motherboards and components.
- +12 V: Commonly used to power motors in storage devices such as hard disk drives and CD-ROM drives.