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.

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 4. Reverse Biasing of Zener diode
Fig 4. Reverse Biasing of Zener diode
Fig 5. Reverse characteristic of Zener diode
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.

Fig 6. Complete V I characteristic of Zener diode
Fig 6. 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 rz , Resistance rz is 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
Fig 7. Practical equivalent circuit of Zener diode

In the ideal equivalent circuit of the Zener diode, the value of Zener resistance rz 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 Vz 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<Vz

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.

Fig 10. Zener diode Shunt Regulator
Fig 10. Zener diode Shunt Regulator

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 RL is fixed and the input voltage Vs 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 VL =Vz 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 Vs increases then total current Is increases. Hence the current Iz increases to keep IL constant. If Vs decreases then total current Is decreases. Hence the current Iz decreases to keep IL constant.

(ii) Regulation with Varying Load

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

Fig 13. Shunt regulator with the varied load resistance
Fig 13. Shunt regulator with the varied load resistance.

As Vs is constant so the total current Is will also be constant.

Since , Is = Iz +IL = constant

Now if RL decreases, So IL increases, and Iz decreases to keep IR constant. Similarly if RL increases. so IL decreases and Iz increases to keep IR constant. Therefore the load voltage remains constant.

VL = 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.
  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.
  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 DIODEP-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.

Leave a Comment

10 Foods to Help Fight Insomnia Health Alert -New Infectious Fungus Strain TMVII Detected in New York. Best Sources of Prebiotics for Gut Health. Key Health Benefits of Milk for Women. World Milk Day: Celebacrating Dairy’s Global Impact