Half-Wave Rectifier Circuit:

Whenever a diode is connected to a source of  alternating voltage, it will be instead forward-biased, and then reverse-biased, in during every cycle of the AC sine-wave. Current will flow through the circuit only during one-half of the input voltage cycle Figure when a single diode is used in a rectifier circuit.   For this reason, this rectifier circuit is called a half-wave rectifier.  The output of a half-wave rectifier circuit is pulsating DC.

Figure : Half-Wave Rectifier