Name: Need for Modulation
Uploaded: 2019-07-19
Duration: 6 min 14 s
Description: Is converting audio signals into electrical signal enough for proper transmission? No. In order to transmit signals, Modulation is also required. Let us unde...

Question

Drawing circuit diagram, explain the role of Zener diode as a voltage regulator.

Accepted Answer

A Zener diode is a diode, which works in a breakdown region, and it is used in the reversed biased condition. Let us draw a reverse characteristic of the Zener diode for a better understanding.

As we can see that for the range $I_{Z_{m i n}} t o I_{z_{m a x}}$

Zener voltage is approximately constant and this region for which zener voltage is constant is the breakdown region.

$I_{Z_{m i n}}$ is also called knee voltage, or we can say it is the minimum value of zener current whereas $I_{Z_{m a x}}$ is the maximum value of zener current beyond this value the zener diode will burst. Therefore, zener diode is used within the range of $I_{Z_{m i n}} t o I_{z_{m a x}}$ .

Now, as Zener voltage remains the same for the range $I_{Z_{m i n}} t o I_{z_{m a x}}$

It can be used as a voltage regulator when it is operated in the breakdown region. Because the work of a voltage regulator is to keep the output voltage the same for varying input. To understand how Zener diode works as a voltage regulator let us draw a circuit diagram for the voltage regulator using Zener diode.

Here we can see from the diagram, there are two resistors R and RL

Where R is connected in series to absorb all the fluctuation of the output voltage and

RL is the load resistance across which output voltage is obtained.

VZ is the Zener voltage and $I_{1}$ is the current flowing through the circuit which divides into $I_{Z}$ and $I_{2}$ .

$I_{z}$ is the Zener current flowing through the Zener diode and I2 is the current flowing through load RL.
Now if we apply Kirchoff's current law at node A we can write

$I_{1} = I_{z} + I_{2}$

Now there can be two cases when the source is varying or input is varying and when the load resistance is varying. Let’s take the first case where input is varying, in this case, the extra voltage or the fluctuation is absorbed by series resistance R due to which total current or $I_{1}$ will increase, but this increase is balanced by Zener current $I_{z}$ and so $I_{2}$ doesn’t change resulting constant output voltage across load resistance.
In the second case that is when load resistance changes and input is fixed then there should be an increase in load current $I_{2}$ . As input is constant $I_{1}$ remains the same and the change is compensated by Zener current $I_{z}$ .
For example, if load resistance decreases then load current should increase so that output remains the same, therefore Zener current decreases to provide the amount of current required at load resistance to keep the voltage constant (because total current $I_{1}$ remains constant).

Hence, this way Zener diodes can be used as a voltage regulator.

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