Diodes allow current to pass easily in one direction only. If an alternating current is supplied to a voltage, the output from the diode will be as in the top image below. Only half of the input produces a voltage across the output terminals. This is called half wave rectification.
The second image below shows the effect of connecting a capacitor across the diode. The output from the diode is smoothed out.
Full wave rectification can be achieved using the arrangement of diodes below.
Current flows from the input terminal P through AD, through the resistor R, back through BC and to the other input terminal Q for one half of the cycle, and from Q to CD, through R and back through BA to terminal P for the other half of the cycle. Again, the output can be smoothed using a capacitor acroiss the output terminals.
