Below shown is the characteristics of a zener diode in
negative voltage region.
As shown above in characteristics, if we make voltage
of K positive with respect to A i.e VAK
negative, then, upto a certain voltage VZ , almost zero current
flows from K to A.
When negative VAK (or positive VKA)
reaches a particular value called zener voltage VZ , Imin
current starts flowing from K to A and the zener is said to be turned ON.
Now, for any reason like variation of R, let current
changes (upto Imax ) , then, there would not be any appreciable
change in voltage across the zener, because in this region (Zener ON region),
zener diode offers very small resistance (1/slope of VI curve).
Thus, if zener diode is operated in “Zener ON” region, it maintains almost a constant
voltage across it and therefore, it is used as shunt regulator or as voltage regulator.
Thus, below two conditions must be
strictly followed to operate zener as voltage regulator.
1. The zener current must not reduce below Imin , else zener will turn off.
2. The current through zener must not exceed Imax
, else zener will be damaged permanently due to excessive heating.
Zener
diode as Voltage Regulator Example
Q. Figure below shows zener diode voltage regulator.
The zener diode requires a minimum current of 25mA for satisfactory operation.
The value of R required for satisfactory voltage regulation of the circuit is ?
Sol :-
As given, VZ=10V, which remains
almost constant. Therefore, current through 100 Ω remains almost constant at 0.1A (10V/100
Ω)
or 100mA.
For zener to operate as shunt regulator a minimum 25mA
current must flow through it, as given.
Therefore, a minimum 125mA (100+25) current should pass through R.
This is the maximum value of R for satisfactory operation of
zener. If R is made greater than 80 Ω. IZ would fall below 25mA and zener
will turn OFF.
Related :-
(1) Zener diode basics
(2) Working of MOSFET
(3) Difference between depletion MOSFET and enhancement MOSFET
