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Zener Diode - Definition, Working, Symbol, Specifications and Applications

A Zener diode reverses the flow of current when the voltage reaches Zener voltage by allowing the current to flow from cathode to anode. Because of this property, the Zener diode is mostly used in semiconductors. The main difference between a normal diode and a Zener diode is that a normal diode allows the current to flow in one direction. In contrast, a Zener diode allows the current to flow in both directions.

Zener diode is known as a breakdown diode. It is a heavily doped semiconducting device designed to work in reverse directions. When the potential across a Zener diode reaches Zener voltage, and the voltage is reversed, the Zener junction breaks, and the current starts to flow in the reverse direction. This effect is known as the Zener effect. The Zener diode is designed to sustain varying voltages.

Fun fact: The first person to describe the electrical properties of Zener Diode was Clarence Melvin Zener. He was a theoretical physicist who worked at Bell Labs. As a result of his work, the Zener diode was named after him. He first postulated the breakdown effect that bears his name in a paper that was published in 1934.

Working of a Zener diode in reverse bias

Zener diodes resemble the working of a normal diode when it is forward-biased. However, in reverse bias, a small leakage current flows through the Zener diode. The current starts flowing through the diode once the reverse voltage increases to a predetermined breakdown voltage Vz. Then, a series of resistors determine the maximum current. After this, the current stabilizes and remains constant irrespective of the voltage.

Avalanche breakdown vs. Zener breakdown

1) The avalanche effect takes over 5.6 volts, whereas the Zener effect is effective in voltages up to 5.6 volts.
2) They both show similar effects. The avalanche effect is the movement of electrons in the valence band like in an electric current, whereas the Zener effect is a quantum phenomenon.
3) Avalanche effect allows a larger current through the diode than what Zener breakdown allows.

Circuit symbol

Many symbols are used to pack a Zener diode. In addition, different operations use different symbols to represent a Zener diode. For example, some contain surface mount formats, while some use them for high-level power dissipation. The common Zener diode type is encapsulated in a small glass with a band and markings on it.

The above representation shows the Zener diode with a band and circuit symbols. This symbol is an easy way to remember which end belongs to which end. In addition, a Zener diode circuit symbol has two tags at the end. This helps to distinguish the Zener diode from other normal diodes in the electrical circuit.

VI characteristics
The following diagram shows the VI characteristics of a Zener diode-

The V-I characteristics of a Zener diode can be explained into two parts-

(i) Forward Characteristics – In the graph, we can see that the first quadrant represents a Zener diode's forward characteristics. It is mostly identical to the forward characteristics of a p-n junction diode.
(ii) Reverse Characteristics – A small saturation current flows across the diode when a reverse voltage is applied to the Zener voltage. As the reverse voltage increases up to a certain reverse voltage, the reverse current increases sharply and drastically, resulting in a breakdown. This breakdown voltage is called Zener voltage, represented by Vz.

Specifications

  • Zener/Breakdown Voltage – reverse breakdown voltage ranges from 2.4 V to 200 V; sometimes, it can go up to 1 kV while the maximum for the surface-mounted device is 47 V.
  • Current Iz (max) – maximum current at the rated Zener Voltage (Vz – 200μA to 200 A)
  • Current Iz (min) – minimum value of current required for the diode to break down.
  • Power Rating – maximum power the Zener diode can dissipate. It is given by the product of the voltage of the diode and the current flowing through it.
  • Temperature Stability – around 5 V is the best stability.
  • Voltage Tolerance – typically ±5%
  • Zener Resistance (Rz) – resistance to the Zener diode exhibits.

Applications

1. Zener diode as a voltage regulator – It is used as a shunt voltage regulator to regulate the voltage across small loads.          Even though the Zener diode is prone to vary currents, its breakdown voltage will remain the same. It is connected in            parallel to the load to make it reverse bias. Once the Zener diode exceeds the knee voltage, the voltage becomes constant.
2. Zener diode as an over-voltage protector – When the input voltage is higher than the Zener breakage voltage, the              voltage across the resistor drops resulting in a short circuit. This can be avoided by using the Zener diode.
3. In clipping circuits – Zener diodes are used for modifying AC waveform clipping circuits by limiting the parts of either     one or both the half cycles of an AC waveform.

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