A rectifier is a dispositive that ideally transforms the AC input voltage into a DC voltage
(voltage is always positive or zero). These diodes have the largest ratings and sometime
can be quite big in volume. As a rule of thumb, the bigger the diode (more pn surface
junction available for heat dissipation), the higher the ratings.
Half-wave rectifier
A half-wave rectifier is composed of a single diode that connects an AC source to a load. In
figure 3 the load is represented by a resistor. The diode conducts on AC voltage only when
its anode is positive with respect to the cathode (i.e. greater than 0.7 V for a silicon diode).
The output has therefore only a positive component with an average value:
The output peak voltage is the AC source minus the voltage drop of the diode, that in most
cases can be neglected.
Full-wave rectifier
In half-wave rectifiers, half of the power provided by the source is not used. To solve this
problem, we have to use full-wave rectifiers. The minimum full-wave rectifier is composed
of two diodes, but it requires a center tapped transformer. Figure shows a bridge rectifier,
composed of four diodes, that can use a “normal” transformer.
The AC current, according to its direction, flows either in the top or in the bottom part
of the bridge in each half-cycle. In the output voltage we will have a component for both
negative and positive parts of the input voltage. In both cases the current passes through
two forward-biased diodes in series, what produces a voltage drop of 1.4 V.
The average voltage of a full-wave rectifier is:
Full wave rectifier. In this case the voltage drop, not shown in the graphic, will be
1.4 V because two diodes are cros.sed.
