The system unit draws power from the AC mains through a power protection device. This power is not directly supplied to the internal components. Instead, one of the components, called the internal power supply, converts the AC input into DC output of 5 and 12 volts. Normally, the internal power supply is referred to as Switched Mode Power Supply (SMPS). The SMPS provides cable connectors to supply the required voltage to the other internal components like the floppy drives, the hard disk drive, the motherboard and external device such as the keyboard. The ON/OFF switch of the system unit is actually a part of the SMPS. Thus, when you switch the system unit on or off, you actually switch the SMPS on or off. Figure shows the various components of the system unit.
Components of System Unit