The Bridge Rectifier

This is another type of full wave rectifier that produces same output waveform as full wave rectifier. However the bridge rectifier has advantages over full wave rectifier that the transformer does not need to produce twice the voltage required and the secondary is in use all the time. The bridge rectifier circuit uses four rectifying diodes connected in a closed bridging loop configuration to produce same output as full wave rectifier.

As shown in the figure, the current will flow through diode D2 and return through diode D4 during the positive cycle. Then during the negative cycle the current will pass through diode D3 and return through diode D1. 

Image source: technologyuk.net

As the current flowing through the load is unidirectional, so the voltage developed across the load is also unidirectional the same as for the previous two diode full-wave rectifier, therefore the average DC voltage across the load is 0.637Vmax.