Full Wave Rectifier:
In a full wave rectifier circuit the output voltage current is obtained for both the half cycles of input alternating voltage. Here two junction diodes are so used that one diode represents the positive half cycles of input ac and the other represents the negative half cycles. Figures shows a circuit of full wave rectifier.
Here, the P-side of the diodes are connected to the ends of the secondary of the transformer. The N-side of the diodes are connected together and the output is taken between this common point of diodes and the midpoint of the secondary of the transformer. So for a full wave rectifier the secondary of the transformer is provided with a center tapping end so it is called center tap transformer. Suppose, the input voltage to A with respect to the centre tap at any instant is positive. It is clear, at that instant, voltage at B being out of phase will be negative. So diode D1 gets forward biased and conducts (while D2 being reverse biased is not conducting). Hence, during the positive half cycle we get an output current (and a output voltage across the load resistor RL). In the course of the ac cycle when the voltage at A becomes negative with respect to centre tap, the voltage at B would be positive. In this part of the cycle diode D1 would not conduct but diode D2 would giving an output current and output voltage (across R) during the negative half cycle of the input ac. Thus, we get output voltage during both the positive as well as negative half of the cycle. Obviously, this is a more efficient circuit for getting rectified voltage or current than the half wave rectifier.
For a full wave rectifier a centre tap transformer is necessary. Here normal transformer cannot be used. If the current is to be obtained at high voltage then this transformer should be step up transformer and if the current is to be obtained at low voltage then the transformer should be step down transformer.