. A PPLICATION OF J UNCTION D IODE AS A R ECTIFIER From the V-I characteristic of a junction diode we see that it 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. . , 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 R L 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 be rectified . Since the rectified output of this circuit is only for half of the input ac wave it is called as half-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 .