Varistor : Definition, Function, Symbol, Type, How It Works

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What is Varistor?


A varistor is a passive  electronic component that acts as a protector against voltage surges in electrical circuits. Varistors are made of semiconductor materials that have resistances that are sensitive to voltage.

When the voltage exceeds a certain threshold, the varistor will undergo drastic changes in resistivity, resulting in a sharp increase in its conductivity and dampening the voltage surge.

Simply put, a varistor is a semiconductor component that has a vital role in protecting electronic devices from unwanted voltage surges.

This component is also often known as a ‘voltage resistor’ because it has a resistance that can vary depending on the voltage applied to it.

This means that the varistor has a high resistance under normal conditions, but suddenly changes to a low resistance when there is a voltage surge.

How is the symbol of a varistor?

The varistor symbol in the electronic circuit scheme is represented by two arrows pointing up and down, with a vertical line in the middle representing the varistor’s semiconductor material.

This symbol describes the ability of the varistor to change its resistance depending on the voltage applied to it. Here is the representation of the varistor symbol in the electronic circuit scheme:


This symbol reflects the basic function of the varistor as a component that protects electronic devices from voltage surges  by passing excess  electric current through itself when the voltage exceeds a certain threshold.

Keep in mind that in practice, these varistor symbols can have variations depending on the conventions used in a particular circuit scheme or electronic diagram.

What is the function of a varistor?

The main function of the varistor is to protect electronic devices from sudden voltage surges or peaks. When the voltage exceeds a certain voltage threshold, the resistance of the varistor will change significantly from high resistance to low resistance.

This results in the varistor becoming an alternative channel to conduct higher electric currents, thus protecting the surrounding electronic components.

Thus, the varistor helps maintain the stability of the performance of electronic devices and prevents damage from unwanted voltage surges. The function of the varistor function in other electronic circuits is:

  • Protection Against Electronic Devices

Varistor helps protect electronic devices such as integrated circuit (IC) components, transistors, and capacitors from damage from voltage surges that can occur due to lightning, electrical interference, or other problems.

  • Increased Device Life

By preventing damage caused by voltage surges, the use of varistors can extend the service life of electronic devices and reduce maintenance and replacement costs.

  • Performance Stability

Uncontrolled voltage surges can disrupt the performance of electronic devices and result in operational errors. The varistor helps to maintain the stability of the voltage and performance of the device.

  • Security

In some critical applications such as medical equipment or security systems, varistors play a role in keeping devices operating safely and preventing the risk of fire or other serious damage.

How Does Varistor Work?


The varistor works on the principle of rapid change of resistance when a voltage surge occurs. To understand how it works, let’s look in more detail at how the varistor responds to sudden voltage surges.

When the voltage applied to the varistor is below a certain threshold, the varistor behaves like an insulator. This means that the flow of electricity through the varistor is very limited and almost no current flows. At this time, the semiconductor structure in the varistor is in a normal state.

However, when there is a voltage surge that exceeds the threshold, an interesting thing happens in the varistor. The structure of semiconductors in them undergoes sudden changes in behavior. The particles inside the varistor that previously had a certain structure become more mobile, causing an increase in conductivity.

The result is that varistor resistance decreases dramatically. In an instant, the varistor went from being an insulator to an excellent conductor. This change causes the voltage surge to be absorbed by the varistor and directed away from the electronic devices it protects.

This process allows the varistor to act as an effective barrier against overvoltage surges. Quickly responding to voltage changes, varistors help keep your electronic devices safe from potential damage caused by unexpected voltage surges.

In some cases, voltage surges can be very strong, such as as the result of lightning during a thunderstorm. Varistor is a lifesaver in situations like this, with the ability to absorb huge amounts of energy from such surges and prevent them from reaching more sensitive electronic devices.

When the voltage returns to normal, the varistor’s resistance returns to its original level, and the varistor returns to its insulator state. This process can occur repeatedly without compromising the efficiency of protection provided by the varistor.

In this way, varistors perform an important role in maintaining the stability and safety of electronic devices, and help ensure reliable performance in the long run.

What are the types of varistors?


In the world of electrical protection, there are two main types of varistors that are commonly used, each of which has different characteristics and uses. Let’s explore more about these types of varistors:

  1. Metal Oxide Varistor (MOV)

Metal Oxide Varistor, commonly known as MOV (Metal Oxide Varistor), is the most commonly used type of varistor.

This varistor is made of metal oxide ceramic material that has semiconductor properties. MOV works by dampening voltage surges that occur in electrical circuits.

When the voltage exceeds a certain threshold, the resistivity of the varistor material suddenly decreases, creating an easier conduction path for electric current.

This diverts voltage surges away from protected electronic devices. This type of varistor has a quick response to voltage surges, keeping electronic equipment safe from damage from sudden surges in electrical current.

MOV is widely used in a wide range of applications, from home appliances to industrial equipment.

  1. Silicon Carbide Varistor (SiC)

Silicon Carbide Varistor, also known as SiC Varistor, is a type of varistor designed to handle larger and more extreme voltage surges.

SiC Varistor is made of silicon carbide material which has stronger semiconductor characteristics than metal oxide materials.

This makes it suitable for heavy industrial applications and environments that demand maximum protection against strong voltage surges.

This type of varistor has high resistance to extreme temperatures and extremely high voltages, making it an ideal choice for protection in challenging environments.

Current-Voltage Characteristics (V-I) on Varistor


Read also Definition and Function of Resistors

A varistor is an important electronic component in electrical protection that has distinctive current-voltage (V-I) characteristics. This characteristic describes the relationship between the voltage applied to the varistor and the current flowing through it.

The following is an explanation of the characteristics of V-I in varistors:

When the voltage applied to the varistor is low, the varistor has a high resistance to current flow. This means less current flows through the varistor at low voltage.

However, when the voltage begins to increase past a certain threshold called the breakdown voltage or operating voltage, the varistor characteristics change. At this point, the resistance of the varistor suddenly decreases significantly, creating an easier conduction path for electric current. This is what allows the varistor to dampen voltage surges by diverting those surge currents and keeping the voltage relatively stable.

The higher the applied voltage above the fracture voltage, the lower the varistor resistance, resulting in greater current flowing through the component. This keeps electronic equipment connected along the varistor safe from damaging voltage surges.

However, keep in mind that varistors also have limits on the current and energy capacity that can be faced. If the voltage surge is too large or the energy is too high, the varistor may melt or suffer permanent damage.

In V-I varistor characteristics, this sudden change in resistance creates a very important protective effect. The varistor acts like a switch that will open automatically when a voltage surge crosses its threshold, ensuring equipment along the line is protected from damage.

It is important to understand the characteristics of V-I varistors when designing and using electrical protection circuits. This helps ensure the varistor is used correctly and provides effective protection against damaging voltage surges.

Code Meaning On Veristor

Here is a table that contains the various meanings of the code on the varistor:

Code Limbs
S Types of varistors made with “Square” technology
D Silicon type varistor
In Metal oxide type varistor (MOV)
J Diode type varistor, used for amplifier devices
With Varistor with fixed zener voltage
T Varistor with stable zener voltage
L Varistor with low voltage rating
H Varistor with high voltage rating
N Varistor with non-linear voltage characteristics
P Varistor with linear stress characteristics
R High Temperature Resistant Varistor
K Temperature Sensitive Varistor
F “Fast response” type varistor, responds quickly to voltage surges
M Varistor type “Medium response”, has a moderate response to voltage surges
S “Slow response” type varistor, has a slow response to voltage surges
X Varistor with higher voltage tolerance
And Varistor with lower voltage tolerance
A General type varistor (General Purpose)
C Special type varistor (Special Purpose)

The table above describes some codes commonly used on varistors to indicate certain characteristics and specifications. Next you can see examples of varistor code arrangements from various products:

Manufacturer Varistor Series Meaning of Model Code
Littelfuse Series V14 – V56 Voltage range from 14V to 56V
Panasonic Seri ZNR – ZNO Varistor type ZNR to ZNO
Bourns Evenings MOV-07D – MOV-20D Varistor MOV from 07D to 20D
EPCOS (TDK) Series S10K – S20K Varistor with rated voltage between S10K to S20K
Vishay Seri VDR – VDC VDR to VDC type varistor

Generally, these codes indicate the range of varistor voltage values in volts (V) followed by additional numbers or letters that provide more information about the varistor specifications, such as current, energy, or type of material used.

By understanding these codes, you can more easily choose a varistor that suits your application needs.

Varistor Usage Examples

Varistor has a wide range of important applications in various electronic circuits. With its ability to protect devices from damaging voltage surges, varistors are an indispensable component in maintaining the stability and reliability of electronic devices.

Here are some examples of the application of varistors in various electronic circuits:

  1. Power Supply System

In power supply systems, varistors are used to protect sensitive components such as voltage regulators, transformers, and capacitors. Voltage surges that occur during start-up or disruptions in the power grid can be handled well by varistors, thus keeping the transmitted voltage within safe limits.

  1. Consumer Electronic Devices

Varistors are often used in consumer electronic devices such as televisions, computers, and audio equipment. When lightning or voltage surges suddenly occur, the varistor will function to absorb and drain excess current, thus protecting sensitive components in the device.

  1. Industrial Equipment

In industrial environments, there are a variety of equipment that requires protection from voltage surges, such as processing machines, control systems, and automation equipment. Varistors are used in the electronic circuitry of this equipment to prevent damage that may arise due to unexpected voltage fluctuations.

  1. Medical Equipment

In medical equipment such as monitoring devices and diagnostic equipment, stability and accuracy are critical. Varistor helps protect sensitive components in these medical devices from voltage surges that can interfere with equipment functionality and reduce the risk of misdiagnosis.

  1. Security System

Security systems, including surveillance cameras and alarm systems, also utilize varistors to maintain smooth operations. In case of voltage surges or lightning, the varistor works quickly to protect the electronic circuit and prevent interference with the security system.

  1. Electric Vehicles

Varistors are also applied in electric vehicles to protect charging systems and other electronic components from voltage surges that can be damaging.

  1. Lightning Protection Device

Varistors are also used in lightning protection systems  to protect building structures and equipment from the impact of voltage surges caused by lightning strikes.

In this case, the varistor plays an important role in flowing lightning currents through a safe path, preventing damage to the building and the electronic devices inside.

Thus a series of materials about one of the electronic components called Varistor. Some examples of the application of varistors in various electronic circuits prove how important the role of this component in maintaining the performance and security of electronic devices.

With varistors, these devices can operate more reliably and avoid damage due to unexpected voltage surges.


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