Table of Contents

Introduction to Zener Diode

What are the Applications of Zener Diode? Zener diode is the type of the diode that is designed for special purpose which is regulation. By the regulation we mean, maintaining constant voltages across its terminals. So Zener diode is designed in such a way that it has the ability to provide the constant voltages that is called (Zener voltages). Just like small signal diodes, Zener diode operates normally in the forward region but in the reverse region Zener goes into breakdown. Two types of breakdown occur in Zener diode:

Zener diodes usually provide voltage regulation from 2V to 1000v.The doping level in Zener diode is set high while designing which causes a very thin depletion region. As we know that

E= v/d

Where v is equal to applied voltages, d is the width of depletion region and E is the produced electric field. So, according to this formula the thin depletion region causes strong electric field to exist between its terminals.

This strong electric field in reverse mode causes the Zener breakdown. While the avalanche breakdown happens because of the collision of high energy electrons. Diodes having breakdown voltages are said to operate in Zener breakdown region while those having breakdown voltages greater than 5v are said to operate in avalanche breakdown.

Characteristic curve

If we have the closer look of characteristic curve of Zener diode, we will notice its characteristic curve can be divided into three regions: forward region, reverse leakage region, breakdown region.

Forward Region

By the forward region we mean when the diode is forward biased. So it acts just like the normal diode causing approximately voltage drop of 0.7V. In this region it can be used as a rectifier diode.

Reverse Leakage Region

This region can be identified from 0v to knee point (before breakdown point). In this region very small current flows through the Zener diode and usually diode is not operated specifically in this region.

Breakdown Region

As mentioned in the introduction, this region is very important and the property of maintaining constant voltages is utilized in different applications. There is the specific range of the reverse current where the Zener provides the regulation i.e., from to . causes the reduction in Zener voltages while causes increase in marked as shown in the figure below.

Fig 1: characteristics curve of Zener diode

Applications of Zener diode

Regulation

Two types of the regulation that can be achieved through the zener diode are:

line regulation
load regulation

The detail of these regulations is given below.

1.1 Line Regulation

In line regulation, we have to determine the range of the input voltages for which Zener will be providing constant voltages across its terminals. Almost all electronic devices of our daily use need constant supply of voltages so any change in the line voltage may cause some damage to our expensive devices. That’s here we can use Zener diode. Zener provide the line regulation by controlling the current through it.

1.2 Load Regulation

By load regulation means if we change different loads whose current rating is different then how Zener will maintain constant voltages. We generally do that by considering from no load (infinite resistance) to full load condition (very small resistance).

Both regulation phenomenon are explained in figure below.

Load Regulation by Zener Diode

Here is an example where the Zener diode provides the load regulation in the circuit.

2. Preregulator

Another application of zener diode is that it can be used as preregulator circuit. If we notice the following figure we can see is acting as a preregulator for This type of approach is usually taken when we have to provide well regulated input so that we can get well-regulated output.

3. Non-standard output voltages:

A very interesting application of Zener diode is to produce non-standard output voltages. The following figure explains well how we can get regulated output voltages from the fixed power supply.

4. Protection of relay

Zener can also be used when we want to operate a relay of 5v from a source of 10v dc. In such case we need to drop 5v somewhere else. Here comes Zener connected in series as shown below.

Analysis of the Circuit Containing Zener Diode

Whenever a zener diode is present in the circuit, we can analyze it by following these two steps:

Remove the Zener diode from the circuit and determine its state by replacing it with open circuit. Find voltages available at the open circuit terminals.

V=V_L=\frac{R_L V_i}{R+R_L}

If V > or = VZ, then Zener diode will be in ON state and we will replace Zener diode with its equivalent model providing constant voltages Vz. If V<Vz, then Zener will be in OFF state and we will replace it with an open circuit.

OFF State Analysis of Zener Diode

In that case,

V_i=V_R+V_L

V_R = V_i– V_L since

I_R= V_R /R

I_Z=0

I_L= V_L / R_L

Both these currents should be same.

ON State Analysis of Zener Diode

V_i=V_R+V_Z

V_R = V_i– V_Z since