Before entering the topic we should briefly know about the diodes, what is diode? It is basically made of semiconductors which have two characteristics p - type and n - type. The p - type and n - type semiconductors represent positive and negative type semi conductors. In p – type semiconductor number of holes is excess and in n - type number of electrons are greater than holes. When both of these types of characteristics are present in a single crystal then it can be termed as diode. The positive terminal of the, battery is connected with the p - side and the negative side is connected with the n-side. Now coming to zener diode it is nothing but a simple diode connecting in reverse bias. It is mainly a special property of the diode rather than any special type of equipment. Clearance Zener invented this property of the diode that’s why it is named after him. The main principle of this special property is that there is a breakdown in the circuit if the voltage applied across a reversely biased it does not allow the electric current to flow across it. Now as the voltage across the diode is increased, the temperature increases and the crystal ions vibrate with greater amplitude and all these leads to the breakdown of the depletion layer(i.e, the layer at the junction of p - type and n - type). And when the applied voltage exceeds an specific amount zener breakdown takes place.
Zener Diode Circuit
The circuit symbol of Zener diode is also shown below. For convenience and understanding, it is used normally
Now, discussing about the diode circuits we should look through the graphical representation of the operation of the zener diode. Normally it is called the V-I characteristics of a general p - n junction diode.
Characteristics of a Zener Diode
The above diagram shows the V-I characteristics of the zener diode. When the diode is connected in forward bias, this diode acts as a normal diode but when the reverse bias voltage is greater than a predetermined voltage zener breakdown voltage takes place. To make the breakdown voltage sharp and distinct, the doping is controlled and the surface imperfections are avoided. In the V-I characteristics above Vz is the zener voltage, we can say. It is also the knee voltage because at this point the electric current is the electric current is very rapid.
Application of Zener Diode
As we have gone through the first part of the article we know what is zener diode and what is the basic principle of operation of it. Now the question arises where this type of diodes are used. The main application of this type of diodes are as voltage regulator.
Overvoltage protection is done by using Zener diodes because there is electric current flowing through the diode after the reverse bias voltage exceeds a certain value. This circuit provides safety for the equipment connected at the terminals. Normally the electric current should not exceed normal valve but if due to any fault in the circuit the electric current exceeds the maximum allowable limit, the equipment of the system can be damaged permanently. A SCR is used, by it the output voltage is quickly cut down and a fuse blows which disconnects the input source power. The circuit arrangement is shown below for better understanding,
Voltage references determine the constant supply of power, electric current or voltage as the zener voltage works if the supply of electric current is constant. That’s why to avoid unstable performance, zener diodes are used where voltage reference is required like ohmmeters, ammeters and voltmeters.
Zener Diode as Voltage Regulator
The term regulator means which regulates or controls. Zener diode can work as a voltage regulator if it is introduced in a circuit. The output across the diode will be constant. It is driven by a current source. As we know if the voltage across the diode exceeds a certain value it would draw excessive electric current from the supply. The basic diagram of zener diode as voltage regulator is given below,
To limit the electric current through the Zener diode series resistance R is introduced whose value can be chosen from the following equation
Resistor value (ohms) = (V1 - V2) / (zener electric current + load current)
The above diagram is of a shunt regulators because the regulating element is parallel to the load. The Zener diode produce a stable reference voltage across the load which fulfills the criteria of regulator.