Section 1.3: Working with Power Supplies

In personal computers, the power supply is a metal device, placed at a corner of the case to provide power to the computer, as shown in Figure 31. The Power supplies are often called "switching power supplies", because they use switcher technology to convert the AC input to lower DC voltages. The typical voltages supplied are: 3.3 volts, 5 volts, and 12 volts. The 3.3 and 5 volts are typically used by digital circuits and the 12 volt is used to run motors in disk drives and fans.

The PC power supply converts the110 volts or 220 volts Alternating Current (AC) available in your home to the Direct Current (DC) needed by your personal computer. For example, an AT motherboard needs +5 VDC, -5VDC, +12VDC, and -12VDC voltages to operate. On ATX motherboard, an additional voltage of 3.3 VDC is provided.

Thus from a 110 VAC input, the PC power supplies such as +3.3VDC, +5VDC, -5VDC, +12VDC, and -12VDC are produced.

Figure 31

The AC in, DC out voltage pairings accurately represents the input and output respectively in Power supplies and AC adapters. They use standard wall outlets for an input of AC voltage and they convert the AC voltage to the DC voltages required by the devices that use DC Voltage.

Power supplies are rated in watts. A watt is the product of the voltage in volts and the current in amperes or amps. While replacing the power supply in your PC, you must be careful about the wattage it provides. You need to ensure that all the connected devices and the devices inside the computer do not require more wattage than the chosen power supply can offer.

A 20 pin main connector from the power supply to the motherboard is standard for all ATX power supplies In addition an auxiliary power connector of either 4 or 6 pins is used to provide additional power. In year 2004 the standard ATX 12V2.0 was passed which changes the main connector from 20 pins to 24 pins. This connector did not require additional 4 or 6 pins auxiliary power connector.

Advanced Configuration and Power Interface (ACPI) is an industry specification for the efficient handling of power consumption computers. ACPI must be supported by the computer motherboard, basic input/output system (BIOS), and the operating system.

In addition, the motherboard and processor must support the standard.