Zener Diode as Voltage Regulator: Circuit, Function, and Analysis

A Zener diode is a special type of diode designed to operate in the reverse breakdown region, which is beyond its normal reverse bias breakdown voltage.

Zener diodes are primarily used as voltage regulators, specifically in applications where a stable and precise voltage output is required despite variations in the input voltage or load conditions. Let’s analyze the circuit diagram of a Zener diode as a voltage regulator.

Once the diode is in breakdown mode, even a small change in voltage across the Zener diode results in a significant change in current through the diode. Therefore, a Zener diode with a known breakdown voltage can be used as a reference voltage when reverse biased. It’s used to regulate voltages. Voltage regulation measures the ability of a circuit to maintain a constant voltage output despite variations in either the input voltage or the load current.

The figure depicts a voltage regulator circuit and its Zener diode equivalent. As shown, a resistor $R_s$ is used to limit the reverse current through the diode to a safe value. Both $V_s$ and $R_s$ are selected such that the diode operates in the breakdown region. The series resistor $R_s$ absorbs output voltage fluctuations to maintain a constant voltage across the load.

The current $I_s$ can be described by the equation:

$I_s = \frac{V_s - V_z}{R_s}$

Where:

$I_s = I_z + I_L$

The Zener diode maintains a constant voltage across the load as long as the supply voltage is greater than the Zener voltage.

Merits and Demerits of Zener Diode as Voltage Regulator

Zener diodes are often preferred over other voltage regulator types because they are lighter, smaller, and have a long lifespan. Additionally, they are cheaper and simpler to implement.

Here are some drawbacks of Zener diode voltage regulators:

• Efficiency is low for high load currents.
• The output voltage slightly varies due to the Zener impedance ($r_z$).
• The output voltage depends on the breakdown voltage of the Zener diode and cannot be selected as a separate voltage.

Summary

In summary, a Zener diode is designed to operate in the reverse breakdown region. It maintains a stable voltage drop across its terminals when operated in this region. This controlled breakdown behavior makes Zener diodes useful for voltage regulation and protection in various electronic circuits.