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Semiconductor Electronics : Materials Devices and Simple Circuits — Physics STD 12 Science — Question

Bihar BoardEnglish MediumSTD 12 SciencePhysicsSemiconductor Electronics : Materials Devices and Simple Circuits5 Marks
Question
How is $p-n$ junction diode is used as an half wave rectifier? Explain its action with proper circuit diagram. Show the input and output waveform also.
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Answer

$p-n$ junction diode as a Half Wave Rectifier : "A device that converts an alternating current or voltage into direct i.e., unidirectional current or voltage is called rectifier." The rectifier which converts only half cycle of alternating current into direct current is called half wave rectifier.
A $p-n$ junction diode has asymmetric conducting property i.e., it is highly conducting when forward biased and non-conducting when reverse biased. In this way a $p-n$ junction diode allows the electric current to pass through it only in one direction, hence it on be used as a rectifier.
The circuit for a $p-n$ junction diode used as a half wave rectifier is shown in the figure (a).
Image
Working : A.C. from the main supply is fed to the primary P of suitable step-down transformer. The desired low voltage A.C. is obtained across the terminals S1 and S2 of secondary S. The load resistance R1. through which direct current (d.c.) is required is connected to terminals S1 and S2 through $p-n$ junction diode D.
Image
The potential of terminal S1 with respect to S2 varies as sine function of the time as shown in figure (b). It is input waveform. It is obvious that the polarity of S1 and S2 alternately becomes positive and negative with time.
During the first half-cycle of the a.c. input, when the terminal S1 of the secondary is supposed to be positive and S2 is negative, the junction diode is forward biased. Hence it conducts and current flows through the load RL in the direction shown by arrows. The current produces across the load an output voltage of the same type as the half-cycle of the input voltage. During the second half-cycle of the a.c. input, the terminal S1 is negative and S2 is positive. The diode is now reverse-biased. Hence there is almost zero current and zero output voltage across RL. The process is repeated during the next cycles.
In this way the output current is a continuous series of unidirection pulses [Fig. (c)]. It is output waveform. Thus the output current is D.C. current (i.e., flowing in one direction) which is not steady but fluctuating between 0 and maximum value Im.
Since the output current corresponds to one half of the input voltage wave, the other half being missing, the process is called half-wave rectification and the $p-n$ junction diode is called half-wave rectifier.

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