Power Supply

Power supply plays a vital role in the operation of PC and their peripherals. It converts AC into various level of DC that can be used by electronic devices like mouse, keyboard, HDD, CD drive.
In a personal computer, power supply is the metal box usually found in a corner of case. It is visible from back of many systems because it contains the power-cord receptacle and a cooling fan. Capacity of a power supply is measured in watts.
Types of Power Supply-
There are two types of power supplies- 1. Linear supply , 2 Switch Mode Power Supply
Linear supply- Term linear means line or straight. It essentially operates in a straight line from AC input to DC output. All linear supplies contain same basic sub-sections: transformer, rectifier, filter and regulator
Switch Mode Power Supply[SMPS]- Power supplies use switcher technology to convert AC input to lower DC voltages. Typical voltages supplied are-
3.3 volts
5 volts
12 volts
3.3 and 5 volts are typically used by digital circuits, while 12-volt is used to run motors in disk drives and fans. Main specification of a power supply is in watts. A watt is product of voltage and the current in amperes or amps.
How SMPS works-
For example a 5V regulated supply:

Each of the blocks is described in more detail below:
Transformer - steps down high voltage AC mains to low voltage AC.
Transformer only

The low voltage AC output is suitable for lamps, heaters and special AC motors. It is not suitable for electronic circuits unless they include a rectifier and a smoothing capacitor
Rectifier - converts AC to DC, but the DC output is varying.
Transformer + Rectifier

The varying DC output is suitable for lamps, heaters and standard motors. It is not suitable for electronic circuits unless they include a smoothing capacitor.

Smoothing - smooths the DC from varying greatly to a small ripple.
Transformer + Rectifier + Smoothing

The smooth DC output has a small ripple. It is suitable for most electronic circuits.
Regulator - eliminates ripple by setting DC output to a fixed voltage
Transformer + Rectifier + Smoothing + Regulator

The regulated DC output is very smooth with no ripple. It is suitable for all electronic circuits.