Zener Diode And Its Characteristics

Table of Contents

Definition of Zener Diode

Zener diode is a reverse-biased heavily doped P-N junction diode that operates in the breakdown region.

Symbol of Zener Diode

Zener Diode Symbol — Fig 1. Symbol of Zener diode.

Biasing Of Zener Diode

(I) Forward Bias

When the anode of the Zener diode is connected to the positive terminal and the cathode is connected to the negative terminal of the battery then the diode is said to be forward-biased and behaves as the normal forward bias P-N junction Diode as shown in fig below:

Forward biasing of Zener Diode — Fig 2. Forward Biasing Of Zener Diode

Forward characteristic of Zener Diode — Fig 3. Forward Characteristic Of Zener Diode

Reverse Biasing Of Zener Diode

When the cathode of the Zener diode is connected to the positive terminal and Anode is connected to the negative terminal of the battery then the diode is said to be reverse biased. Zener Diode in Reverse bias condition act as a voltage regulator.

Fig 5. Reverse characteristic of Zener diode.

As we increase the reverse bias, initially a very small reverse current will flow but at a certain reverse voltage the reverse current rises sharply and that voltage is called breakdown Voltage or Zener Voltage and is denoted By Vz and the diode is said to be in Zener region where the Zener voltage remains constant but current changes depending upon the applied voltage.

Complete V-I Characteristic of Zener Diode.

Equivalent circuit of Zener Diode

The practical Zener diode is equivalent to a battery of voltage Vz in series with a resistance r_z , Resistance r_zis known as the dynamic resistance of the Zener diode given as

$r_z\;=\frac{\triangle V_z}{\triangle I_z}$

Fig 7. Practical equivalent circuit of Zener diode

In the ideal equivalent circuit of the Zener diode, the value of Zener resistance r_z is zero. During the voltage from 0 V to Vz in reverse bias, the Zener diode behaves as an open circuit as shown in 9. The Zener diode is said to be “ON” when the reverse bias voltage exceeds Vz as shown in fig 8 while it remains “OFF” if the voltage across the Zener is less than V_z in reverse bias. The “ON” & “OFF” states are shown below:

Fig 8.Zener diode is ON when <span class="wp-katex-eq" data-display="false">V\;\geq V_z</span> — Fig 8.Zener diode is ON when $V\;\geq V_z$

Fig 9. Zener diode is OFF when — Fig 9. Zener diode is ‘OFF’ when 0<V<V_z

Zener Diode application as a Shunt regulator /Voltage regulator.

A voltage regulator is a circuit whose function is to maintain a constant output d.c voltage in spite of the a.c input voltage fluctuations or changes in load resistance values.

The input current is calculated by

$V_s\;=I_s\;R_s\;+\;V_z$

$I_s\;=\frac{V_s\;-V_z}{R_s}$

Here $V_s$ = Input d.c unregulated voltage
$V_z$ = Voltage across Zener diode.
The load Voltage is
$V_L=V_z$

Working of Zener diode Shunt regulator

(I) Regulation with varying Load

In this case, the load resistance R_L is fixed and the input voltage V_s is varied as shown in fig below:

Fig 8. Shunt regulator with input voltage varied. — Fig 11. Shunt regulator with input voltage varied.

The output V_L =V_z is a constant

$I_L\;=\;\frac{V_L}{R_L}=\;\frac{V_z}{R_L}$ = Constant

and $I_s\;=\;I_z\;+\;I_L$

Now if V_s increases then total current I_s increases. Hence the current I_z increases to keep I_L constant. If V_s decreases then total current I_s decreases. Hence the current I_z decreases to keep I_L constant.

(ii) Regulation with Varying Load

In this case, the input voltage is constant and the load resistance R_L is varied.

Fig 13. Shunt regulator with the varied load resistance.

As V_s is constant so the total current I_s will also be constant.

Since , I_s = I_z +I_L = constant

Now if R_L decreases, So I_L increases, and I_z decreases to keep I_R constant. Similarly if R_L increases. so I_L decreases and I_z increases to keep I_R constant. Therefore the load voltage remains constant.

V_L = Constant

Advantages of Zener diode voltage regulator

It is a simple circuit
It is more reliable
It provides regulation over a wide range of current.
It is smaller, lighter, and more rugged.
It has a longer life.
It is cheaper.

The disadvantage of the Zener Voltage Regulator

This voltage regulator cannot be made adjustable.
Due to large changes in the load current, large power wastage occurs.
The output voltage varies slightly due to Zener impedance rz.
Large power gets dissipated in the series resistance Rs.
The output volatge cannot be chosen independently but depends upon the breakdown voltage of the Zener diode.

Application of Zener diode

Zener diode is used as a voltage regulator.
It is used as a peak clipper
It is used as meter protection against damage from accidental application.
It is used for switching operations.
It is used as a reference voltage for comparing other voltages against it.

Comaprision of zener diode and P-N junction diode

ZENER DIODE	P-N JUNCTION DIODE
1. It operates in reverse bias condition.	1. It operates in Forward bias condition.
2. It has a high doping level.	2. It has a low doping level.
3. Dynamic Zener resistance is very small in reverse breakdown conditions.	3. The diode resistance in reverse-biased conditions is very high.
4. It allows the current to flow in both the direction i.e forward and reverse direction.	4. It allows the current to flow only in one direction i.e in forward direction.
5. It does not obey ohm’s law.	5. It obeys Ohm’s law.
6. Its breakdown voltage is higher than normal on the junction diode.	6. Its breakdown voltage is comparatively lower than the Zener diode.
7. At high reverse voltage Zener diode does not get damaged.	7. At high reverse voltage P-n junction diode can get damaged
8. Application of Zener diode are Voltage Regulator, peak Clipper, Protection Circuits, Reference Voltage, etc.	8. Application of P-N junction diode are Rectifiers, Voltage Multipliers, Clippers, Clampers, etc.

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Zener diode and its characteristics