A rectifier is a device that creates a DC source from an AC source. All electronic devices that get their power from the standard 60Hz power grid employ them.
The figure illustrates a simple half-wave rectifier.
Current can flow through the diode only in the direction of its arrow, so current flows in only one direction through the resistor: the voltage drop VS is never negative. The capacitor serves to smooth out the output voltage, by supplying current during the times that the input voltage is small or negative. The figure below shows the resulting output voltage VS with or without the capacitor.
The term half-wave refers to the fact that in the absence of the capacitor, a voltage exists only during the time when the primary source is positive. One can make a steadier source with a full-wave rectifier as shown below.
In the full-wave rectifier the current flows from left to right through the resistor for all parts of the AC cycle. This results in a positive voltage at all times as shown in the figure below. The addition of a capacitor would easily smooth out the resulting DC current.
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