A diode is a semiconductor PN junction device that conducts current in the forward direction and blocks in the backward direction. However, the Zener diode is a type of diode that can conduct in both directions. Zener diodes are widely used in circuits as voltage regulators because they are less costly and perform well. In this tutorial we will learn how Zener Diode works as a voltage regulator in a circuit, its working principle, and circuit diagram.
What is Zener Diode?
A Zener diode is similar to the simple diode that is a semiconductor having a PN junction through which current flows in forward and backward directions. However, the difference between a simple diode and a Zener diode lies in its reverse operation.
A normal diode has a small leakage reverse current in reverse-biased conditions and if the reverse voltage is increased beyond the limit, the diode breaks down. The Zener diode has small reverse leakage current but when a reverse voltage is increased beyond a certain limit it does not break down, instead, it starts conducting as it does in forward-biased conditions. This conduction in reverse mode is known as Zener breakdown.
What is a Voltage Regulator?
A voltage regulator is used to balance voltage in a circuit and minimize fluctuations. It is used to prevent elements in the circuit from burning out. It maintains the level of voltage throughout the circuit operation, whether the load increases or decreases.
Therefore, the voltage regulator is used for two purposes:
- To keep the voltage constant when the source voltage is fluctuating
- To keep output voltage constant irrespective of load
How to Build a Circuit Where Zener Diode Works as a Voltage Regulator?
To build a circuit where the Zener diode works as a voltage regulator, the following steps should be followed:
- Calculation of Power Rating According to the Requirement of the Circuit
To use a Zener diode as a voltage regulator in the circuit, the power rating of the Zener diode must be taken into account because it tells how much current it can handle. The formula given below is used to calculate the power rating of the Zener diode.
- Select the appropriate Zener Diode according to Power Rating and Zener Voltage (VZ)
After the calculation of the power rating, select a Zener diode whose Zener Voltage corresponds to the output voltage required for the regulator. Similarly, consult the datasheet of the Zener diode because different Zener diodes having the same Zener voltage can be designed for different purposes.
- Select a Resistor to be in Series with the Zener Diode
For choosing a resistor, the breakdown current of diode Imin and the power rating of the diode would be considered that will give Imax.
Using Ohm’s Law, and maximum and minimum values of currents through the Zener diode, values of maximum and minimum resistances are to be calculated.
For minimum resistance:
For maximum resistance:
- Select the Ideal Value of Load resistance
By calculating the ideal value of load resistance, we can assess how much load this voltage regulator can work.
By using values of maximum and minimum resistances for resistor R1, minimum and maximum load currents can be calculated.
For maximum load current:
For minimum load current:
I-V Characteristics of Zener Diode
In the forward-biased condition, as the forward voltage is applied to the Zener diode, the forward current starts increasing steadily. After some time, the diode starts conducting and the current rises sharply.
In reverse-biased conditions, reverse leakage current flows for some time but when the reverse voltage exceeds the Zener voltage limit, the current rises sharply through the Zener diode in reverse-biased condition. At this point, the voltage becomes constant and the current increases to maximum.
Kirchhoff’s Current Law (KCL) says that the current exiting any junction must match the current entering the junction. The current flowing through the resistor (I.R) must be equal to the current flowing through the Zener diode plus the current flowing through the load in this case: IR=IZ+IL.
The I-V graph shown below explains the Current-Voltage characteristics of a Zener diode.
Conclusion
A Zener diode is a handy device that gives a regulated output voltage in reverse-biased conditions. It is used in circuits where certain elements with defined voltage ratings are used, such as LEDs. It prevents them from burning out by providing them with a constant voltage irrespective of fluctuations in supply voltage and is readily used in clipping circuits.
