Case–I: Half Wave Rectifier
From the V–I characterstic of a junction diode we known that is allows current to pass only when it is forward biased. So if an alternating voltage is applied across a diode the current flows only in that part of the cycle when the diode is forward biased. This property is used to rectify alternating voltages and the circuit used for this purpose is called a rectifier.
If an alternating voltage is applied across a diode in series with a load, a pulsating voltage will appear across the load only during the half cycles of the ac input during which the diode is forward biased. Such rectifier circuit, as shown in Fig(a) is called a half–wave rectifier. The secondary of a transformer supplies the desired ac voltage across terminals A and B. When the voltage at A is positive, the diode is forward biased and it conducts. When a is negative, the diode is reverse–biased and it does not conduct. The reverse saturation current of a diode is negligible and can be considered equal to zero for practical purposes. (The reverse breakdown voltage of the diode must be sufficiently higher than the peak ac voltage at the secondary of the transformer to protect the diode from reverse breakdown).
Therefore, in the positive half–cycle of ac there is a current through the load resistor RL and we get an output voltage, as shown in Fig(b) whereas there is no current in the negative half–cycle. In the next positive half–cycle, again we get the output voltage. Thus, the output voltage, though still varying, is restricted to only one direction and is said to the rectified. Since the rectified output of this circuit is only for half of the input ac wave it is called as half–wave rectifier.
Case–II: Full–Wave Rectifier
The circuit using two diodes, shown in Fig(a) gives output rectified voltage corresponding to both the positive as well as negative half of the ac cycle. Hence, it is known as full–wave rectifier. 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(b) the voltage rectified by each diode is only half the total secondary voltage. Each diode rectifies only for half the cycle, but the two diodes 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.
Input wave forms given to the diode D1 at A and to the diode D, at B (c) Output waveform across the load RL connected in the full-wave rectifier circuit
