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 RL
across which constant output is desired. The series resistance RS
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 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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