Question

What is Zener diode? How is it used as a voltage regulator?

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Solution

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When any diode's p terminal is connected to the positive terminal of the power supply and n to negative, it is called as forward bias. When the applied voltage becomes greater than the barrier potential, (maybe 0.7V) the diode becomes a very good conductor.
Opposite connection is known as reverse bias. In this bias, the conductivity of the diode becomes very very less and it behaves like an insulator.
A special type of diode called Zener diode behaves like any diode in the forward bias mode of connection.
Whereas in the reverse bias, it initially works like an insulator, but when the applied voltage becomes high enough to reach a special voltage called zener breakdown  voltage, its conductivity suddenly becomes very high and it behaves like a very good conductor.
Zener diode is made specifically to work in the zener voltage region. 
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When any two resistances are connected in series, the the voltage gets divided across the resistances. The greater the resistance, greater is the voltage drop across a resistance. 
In the circuit diagram, the parallel combination of the Zener diode and the load resistance is in series combination with another series resistance  
A zener diode whose breakdown voltage is approx equal to the the desired voltage is chosen.
We know that in parallel combination, the devices get the same voltage. Hence in the circuit, the zener voltage is the output voltage.
Due to any fluctuations if the input voltage becomes more, then the zener's resistance decreases and the voltage across the remains almost same.
Due to any fluctuations if the input voltage becomes less, then the zener's resistance increases and the voltage across the remains almost same.
If the becomes less then the output voltage should otherwise decrease. But due to this the zener's resistance increases and the voltage across the remains almost same.
Vice-versa happens if becomes more and output voltage remains same

This is how a Zener Diode sucessfully maintains a stable voltage across the output and it works as a voltage regulator.  

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