The half-wave rectifier

The simplest rectifier circuit uses just one diode (or a series or parallel combination) to “chop off” half of the ac input cycle. 
In a half-wave circuit, the average output voltage is approximately 45 percent of the rms ac input voltage. But the PIV across the diode can be as much as 2.8 times the rms ac input voltage. It’s a good idea to use diodes whose PIV ratings are at least 1.5 times the maximum expected PIV; therefore, with a half-wave supply, the diodes should be rated for at least 4.2 times the rms ac input voltage.
Half-wave rectification has some shortcomings. First, the output is hard to smooth out, because the waveform is so irregular. Second, the voltage output tends to drop when the supply is connected to a load. (This can be overcome to some extent by means of a good voltage regulator.)
Third, half-wave rectification puts a disproportionate strain on the power transformer and the diodes.
Half-wave rectification is useful in supplies that don’t have to deliver much current, or that don’t need to be especially well regulated. The main advantage of using a half-wave circuit in these situations is that it costs a little less than full-wave or bridge circuits.