Here the p-side of the two 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 centre tapping and so it is called centre-tap transformer . As can be seen from Fig.
. (c) the voltage rectified by each diode is only half the total secondary voltage. Each diode rectifies only for half the cycle, but the two do so for alternate cycles. Thus, the output between their common terminals and the centre- tap of the transformer becomes a full-wave rectifier output.
(Note that there is another circuit of full wave rectifier which does not need a centre- tap transformer but needs four diodes.) Suppose the input voltage to A FIGURE . (a) Half-wave rectifier circuit, (b) Input ac voltage and output voltage waveforms from the rectifier circuit. with respect to the centre tap at any instant is positive. It is clear that, at that instant, voltage at B being out of phase will be negative as shown in Fig.
. (b). So, diode D gets forward biased and conducts (while D being reverse biased is not conducting). Hence, during this positive half cycle we get an output current (and a output voltage across the load resistor R L ) as shown in Fig.
. (c). 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 D would not conduct but diode D would, giving an output current and output voltage (across R L ) during the negative half cycle of the input ac.
Thus, we get output voltage during both the positive as well as the negative half of the cycle. Obviously, this is a more efficient circuit for getting rectified voltage or current than the half- wave rectifier The rectified voltage is