How and Why Zener Diode Regulate Voltage in Reverse Bias?

Zener Diode as Voltage Regulator

One of the most important applications of Zener Diode is Voltage Regulation. Zener Diode is used in electrical as well as electronic circuits for voltage regulation. To use a Zener Diode as a Voltage Regulator we must connect the Zener Diode in reverse. So there are some questions arise that How Zener Diode regulate voltage and why Zener diode regulate voltage in reverse bias, not in forward bias.

What is Voltage Regulation?

The method or process of keeping constant voltage in a circuit even if the power supply or load changes is called Voltage Regulation.

There are some electrical and electronic devices are available which cannot withstand voltage fluctuation. So for this purpose, Voltage Regulation is needed.

Why does Zener Diode Regulate Voltage in Reverse Bias, not in Forward Bias?

In forward bias, the Zener diode acts as a normal PN Junction diode. The Zener Diode has a very low threshold voltage(0.7V for Silicon and 0.3V for Germanium) for conducting current in forward bias.

That means when we connect a Zener Diode in forward bias, it will conduct current when the applied voltage goes above that threshold voltage.

The minimum reverse voltage after which the Zener diode conducts current in reverse bias is called Zener voltage. Generally, Zener diodes are designed to conduct current in reverse bias. So when we connect a Zener diode in reverse bias it will only conduct current when the applied voltage goes above the Zener Voltage.

Zener voltage ratings which are available in the market are 6V, 12V, 24V, etc.

Suppose we connect a Zener diode(rated 12V Zener voltage) in forward bias to regulate a 12V power supply. In this case, the Zener diode will not work. Because to regulate the 12V power supply, the current should be flowing through the Zener diode when the voltage goes above the 12V. But as the Zener diode is connected in forward bias and it has a very low threshold voltage, therefore, the current will flow through the Zener diode even if the voltage is less than 12V

When we connect that Zener diode in reverse it will work for voltage regulation because it will only conduct when the applied voltage goes above the 12V

What happens if we connect the Zener Diode in Forward Bias?

According to the above explanation, if we connect the Zener Diode in forward bias for voltage regulation, it will not work. And the Zener Diode will create a short circuit.

These were the reasons why we needed to connect the Zener Diode in reverse bias for voltage regulation.

How does a Zener Diode Regulate Voltage?

The concept of voltage regulation using a Zener Diode is very simple. Let's take a circuit to better understand.

As you see in the above circuit diagram we have taken a power supply which can vary 5V to 9V. But we need to keep constant the voltage at 6V. So we have connected a Zener diode in reverse bias which has a rated 6V Zener voltage.

Change in Input Voltage

(1) When the input voltage goes above 6V the Zener diode will start conducting current so a voltage drop will be produced in the series resistor and the output voltage across the load remains constant. The conduction of current through the Zener diode will be increased if the input voltage is increased. Thus the Zener diode keeps the constant voltage across the load.

(2) When the input voltage decreases, the flow of current through the Zener Diode will decrease therefore the voltage drop across the series resistor also decreases and the voltage across the load remains constant.

Change in Load

(1) When we increase the load, that means the load will draw more current, then the flow of current through the Zener Diode will decrease and it maintains a constant voltage across the load.

(2) When we decrease the load, that means the load will draw less current, then the flow of current through the Zener Diode will increase and the constant voltage across the load will be maintained.

Why a resistor is always connected in series with the Zener Diode?

We already know that when the applied voltage goes above the Zener Voltage, the Zener Diode starts conducting and a huge amount of current may flow. So to limit the flow of current through the Zener Diode, a resistor is always connected in series with the Zener Diode.

In the above circuit, the series resistor is used to limit the flow of current and to drop the voltage during the overvoltage condition.

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