Zener diode and its characteristics

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

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:

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.

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_z is known as the dynamic resistance of the Zener diode given as

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

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 V\;\geq V_z

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

{\displaystyle V_s\;=I_s\;R_s\;+\;V_z}

{\displaystyle 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:

The output V_L =V_z is a constant

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

and {\displaystyle 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.

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

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

The disadvantage of the Zener Voltage Regulator

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

Application of Zener diode

1. Zener diode is used as a voltage regulator.
2. It is used as a peak clipper
3. It is used as meter protection against damage from accidental application.
4. It is used for switching operations.
5. 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. Zener diode symbol is	3. The P-N junction diode symbol
4. Dynamic Zener resistance is very small in reverse breakdown conditions.	4. The diode resistance in reverse-biased conditions is very high.
5. It allows the current to flow in both the direction i.e forward and reverse direction.	5. It allows the current to flow only in one direction i.e in forward direction.
6. The V-I characteristic of the Zener diode is :	6. The V-I characteristic of P-N junction diode is:
7. It does not obey ohm’s law.	7. It obeys Ohm’s law.
8. Its breakdown voltage is higher than normal on the junction diode.	8. Its breakdown voltage is comparatively lower than the Zener diode.
9. At high reverse voltage Zener diode does not get damaged.	9. At high reverse voltage P-n junction diode can get damaged
10. Application of Zener diode are Voltage Regulator, peak Clipper, Protection Circuits, Reference Voltage, etc.	10. Application of P-N junction diode are Rectifiers, Voltage Multipliers, Clippers, Clampers, etc.