211 SRI RAMAKRISHNA MISSION VIDYALYA POLYTECHNIC COLLEGE.

  VI Characteristics of Zener Diode The graph given underneath shows the V-I characteristics of the Zener diode.    V-I characteristics of a Zener Diode can be studied under the following two headings, Forward Characteristics of Zener Diode Forward characteristics of the Zener Diode are similar to the forward characteristics of any normal diode. It is clearly evident from the above diagram in the first quadrant that the VI forward characteristics are similar to other P-N junction diodes. Reverse Characteristics of Zener Diode In reverse voltage conditions a small amount of current flows through the Zener diode. This current is because of the electrons which are thermally generated in the Zener diode. As we keep increasing the reverse voltage at any particular value of reverse voltage the reverse current increases suddenly at the breakdown point this voltage is called Zener Voltage and is represented as V z .

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  DC Excitation System The DC excitation system has two exciters – the main exciter and a pilot exciter.  The exciter output is adjusted by an automatic voltage regulator (AVR) for controlling the output terminal voltage of the alternator.  The current transformer input to the AVR ensures limiting of the alternator current during a fault. When the field breaker is open, the field discharge resistor is connected across the field winding so as to dissipate the stored energy in the field winding which is highly inductive.