A rectifier is an electrical device that converts alternating current (AC) into direct current (DC) by allowing a current to flow through the device in one direction only. The process of converting AC current to DC current is known as rectification. A rectifier can take several physical forms, such as solid-state diodes, vacuum tube diodes, mercury-arc valves, silicon-controlled rectifiers, and various other silicon-based semiconductor switches.

Rectifiers have many uses, but are most often used as the primary components of DC power supplies and high-voltage direct current transmission systems. In power supplies, the rectifier is normally placed in series following the transformer, a smoothing filter, and possibly a voltage regulator. Below are some of the applications and uses of rectifiers:

• Deriving DC power from AC power: The primary application of the rectifier is to derive DC power from AC power. Rectifiers are used inside the power supplies of almost all electronic equipment.
• Powering appliances: All electrical appliances use a DC power supply to function. Using a rectifier in the power supply helps in converting AC to DC power supply. Bridge rectifiers are widely used for large appliances, which can convert high AC voltage to low DC voltage.
• Electric welding: Rectifiers supply polarized voltage for welding. In such circuits, control of the output current is required, which is sometimes achieved by replacing some of the diodes in a bridge rectifier with thyristors, effectively diodes whose voltage output can be regulated by switching on and off with phase-fired controllers.
• Flame rectification: In gas heating systems, flame rectification is used to detect the presence of a flame.

Rectifiers are classified into two types: uncontrolled rectifiers and controlled rectifiers. Uncontrolled rectifiers are further divided into half-wave rectifiers and full-wave rectifiers. Controlled rectifiers are classified into thyristor rectifiers and other types.