Semiconductor Diode as a Rectifier

A semiconductor diode works as a switch because it conducts current only in forward bias. It does not conduct current in reverse bias. That means a semiconductor is on in forward bias and off in reverse bias. Thus P-N junction diode acts as a unidirectional switch in an electronic circuit.

Rectification and rectifier: The process of convention AC into DC is known as rectification and the device used in rectification is called rectifier.

P-N Junction as a half wave rectifier:

Figure showing AC input, Half wave rectifier and DC output

Half wave rectifier converts only half cycle of AC into DC. A half wave rectifier consists of a transformer, diode and a load resistance as shown in figure. The input AC is applied across the primary P of the transformer. The secondary S is connected in series with load resistor RL. When AC input is applied to the primary, a voltage is induced in the secondary end. During the first half cycle of AC voltage, when upper end of secondary is at positive potential and lower end is at negative potential, the P-N junction is forward bias, due to which the current flows through the diode.

During the next half cycle of the AC voltage when upper end of secondary is at negative potential and lower end is at positive potential. The P-N Junction is reverse bias, due to which no current flows through Diode. In this way, only uni-directional current flows through the  load resistor and acts as a half wave rectifier.

Diode as a full wave rectifier:

Figure showing AC input, Half wave rectifier and DC output

A full wave rectifier consists of a transformer, with a centre tapped secondary coil with two diode D1 and Dconnected at the upper and lower end of the secondary coil of the transformer as shown in figure.
During the first half cycle of AC voltage, when the upper end of secondary is at positive potential, the lower end is at negative potential and that of central tapping is at zero potential. The diode D1 is forward bias, D2 is reverse bias. During the next half cycle of AC voltage, when upper end is at negative potential, the lower end is at positive potential and that of central tapping is at zero potential. The diode D2 is forward bias and D1 is reverse bias. The current flows through the diode D2 and no current flows through D2. Thus in both half cycle of AC voltage, the current passes through load RL in the same direction. Hence, combination of D1 and D2 works as a full wave rectifier due to which output rectified DC voltage is obtained across the load.

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