P-N Junction as Rectifier

 

Half wave rectifier

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The A-C to be rectified is connected to the primary P₁P₂ of a step down transformer. S₁S₂ are the secondary coil of the transformer, which are connected to diode and load resistance R as shown. The output is taken across the resistance R. During the positive half cycle on account of induction, S₁ is +ve and S₂ is -ve. It forward biases the junction diode and hence a current flows in direction shown. We therefore get output across load resistance. During the negative half cycle S₁ is negative and S₂ is positive. The pn junction is reverse biased. It offers high resistance and hence there is no flow of current due to majority charge carriers and thus there is no output across load. In the output only one half of the wave is present and the other half is missing.

The amount of DC voltage across the output of half wave rectifier is given by

It is also called as average voltage, it is less than V_m always since out put of half wave is not a pure DC it is pulsating DC. Therefore the component of ripple is calculated by ripple factors it is denoted by gamma.

i.e., 1.21 percent of ripple is present at the out put when compared at input that means unwanted signal is still increased by 21 percent. Therefore half wave rectifier is poor rectifier.

P-N junction diode as full wave rectifier

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For full wave rectification, we use two PN junctions. During the positive half of the input A-C (fig), the upper PN junction diode i.e., D₁ is forward biased and D₂ is reverse biased. The current flows in circuit due to majority charge carriers of D₁ in the direction shown. During the negative half cycle, the diode D₁ is reverse biased and D₂ is forward biased. The current flows in the circuit due to majority charge carriers of D₂ in the direction shown. The output signal voltage is unidirectional and current flows through the load resistance R during both the halves.