ZENER DIODE

 Construction:

                      Zener diode is a specially designed PN junction diode. A reverse biased heavily doped PN junction diode operated in the breakdown region is known as Zener diode. The breakdown voltage of a Zener diode is set by carefully controlling the doping level during manufacture.

             Figure shows the symbol of a Zener diode. It is similar to the symbol of an ordinary PN junction diode except that bar is just turned in to Z- shape. Figure shows a practical equivalent circuit of a Zener diode. In this circuit shows that a Zener diode is equivalent to a battery with voltage (Vz) in series with a resistance (R). This voltage is called Zener voltage and the resistance in series is called Zener resistance.

Working principle:

             The operation of a Zener diode is same as that of an ordinary PN junction diode in forward biased condition. But in revered biased condition breakdown may occurs in the junction of the diode. The breakdown voltage depends upon the amount of doping. Heavily doped diodes breakdown at low voltage levels. Similarly lower doped diodes breakdown at high voltage levels.

Forward bias:

             Figure shows the arrangement for forward biased Zener diode. The positive terminal of the battery is connected to the anode and negative terminal of the battery is  connected to the cathode. When the applied voltage is zero no current flows through the Zener diode. When the forward bias voltage is increased above the knee voltage the barrier potential is reduced and the current starts flowing in the circuit. Beyond this knee voltage the current increases sharply with increase in applied voltage. Thus under forward bias condition Zener diode acts like an ordinary PN junction diode.

Reverse bias:

            Figure shows the arrangement for reverse bias of Zener diode. The positive terminal of the battery is connected to cathode and the negative terminal of the battery is connected to anode. When the applied voltage is zero no current flows through the Zener diode. When the reverse voltage is increased the reverse current increases sharply. The specialty of the Zener diode lies in the reverse bias condition.

 V-I Characteristics:

             Figure shows the V-I characteristics of Zener diode. Under forward bias condition the characteristics curve of Zener diode is same as like PN junction diode. Under reverse bias condition it totally differs from the forward characteristics. When the applied reverse voltage is zero, no current flows through the Zener diode. When the reverse voltage (Vr) is increased the reverse current (Iz) remains negligible upto the knee of the curve 'point P'. At this point the effect of breakdown process begins. From the bottom of the knee the breakdown voltage remains essentially constant. This ability of a diode is called regulating ability. It maintains anessentially a constant voltage across its terminal over a specified range of Zener current values.

The following two points are important from the characteristics of a Zener diode;

Ø  There is minimum value of Zener current called break over current designed as Iz(min) which must be maintained in order to keep the diode in breakdown region. When the current is reduced below the knee of the curve the voltage changes drastically and the regulation is lost.

Ø  There is a maximum value of Zener current designated as Iz(max) above which the diode may be damaged. 

Applications of Zener diode:

·         It can be used as voltage regulator.

·         It can be used as a limiter in wave shaping circuits.

·         It can be used as a fixed reference voltage in transistor biasing circuits.

·         It is used for meter protection against damage from accidental over voltage.

·         It can be used as a fixed reference voltage in a network for calibrating voltmeters. 


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