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Zener Diode as Voltage Stabiliser


Zener Diode as Voltage Stabiliser

Zener Diode as Voltage Stabiliser

 






A zener diode can be used as a voltage regulator to provide a constant voltage from a
source whose voltage may vary over sufficient range. The circuit arrangement is shown
in Fig.1.





Fig-1




The zener diode of zener voltage Vz is reverse connected across which
constant output is desired. The series resistance R absorbs the output voltage
fluctuations so as to maintain constant voltage across the load. It may be noted that the
zener will maintain a constant voltage V z (=E 0 ) across the load so long as the input
voltage does not fall below Vz.
When the circuit is properly designed, the load voltage E 0 remains essentially
constant (equal to V z ) even though the input voltage E 1 and load resistance R L may vary
over a wide range.





Fig-2





(i) Suppose the input voltage increases. Since the zener is in the breakdown
region, the zener diode is equivalent to a battery Vz as shown in Fig.2 .





(ii). It is clear that output voltage remains constant at V z (=E 0 ). The excess
voltage is f=drop across the series resistance R. This will cause an
load current remains constant. Hence, output voltage E 0 remains constant
irrespective of the changes in the input voltage E 1.





(ii) Now suppose that input voltages constant but the load resistance R L
decreases. This will cause an increase in load current. The extra current
cannot come from the source because drop in R (and hence source
current I) will not change as the zener is within its regulating range. The
additional load current will come from a decrease in zener current Iz.
Consequently, the output voltage stays at constant value.
Voltage frop across R = E1 – E0
Current through R, I = Iz + I L
Applying Ohm’s law, we have,
R =( E1 – E0 )/( Iz + I L )





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