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Zener Diode Voltage Regulator


Zener Diode Voltage Regulator

When the input of a Zener diode is greater than the Zener voltage the Zener diode works in the breakdown region, provided it is connected in the reverse bias. This principle is used in the regulator along with its equivalent circuit is shown in fig. in this circuit the Zener diode is reversely connected across the load RLacross which constant output is desired. The series resistance RSabsorbs 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 VZ across the load so long as the input voltage does not fall below VZ.
When the circuit is properly designed, the load voltage VO remains essentially constant equal to VZ even though the input voltage Vi and load resistance RL may vary over a wide range.
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 the fig. it is clear that output voltage remains constant at VZ the excess voltage is dropped across the series resistance R. this will cause an increase in the value of total current I. the Zener will conduct the increase of current in I while load current remains constant. Hence the output voltage VO remains constant irrespective of the changes in the input voltage Vi.

(a) Equelent Circuit

      
                                                                                      (b) Zener Regulator

Now suppose that input voltage is constant but the load resistance RL decreases. This will causes an increase in load current. The extra current cannot come from the source because drop in R 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.




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