What are Varistors ?

A Varistor is an electronic component designed to protect electrical circuits from excessive voltage surges or transients. It is a nonlinear resistor whose electrical resistance varies significantly with changes in voltage. Varistors are commonly used to safeguard sensitive electronic devices and equipment by diverting excess voltage away from them. Varistors are frequently employed in parallel with the devices or circuits they are designed to safeguard. When there is a voltage spike, the varistor effectively “clamps” the voltage at a safe level, keeping it from reaching the protected components. The varistor recovers to its high-resistance state after the surge has passed.

What is a Varistor?

A varistor is an electronic component that protects electrical circuits against overvoltage. Because its electrical resistance varies with applied voltage, it is commonly referred to as a voltage-dependent resistor (VDR). Surge protection devices often use varistors to protect sensitive electronics and equipment from voltage spikes and transient voltage occurrences.

Varistor

Understanding the Reason for Evolution

The Varistors evolved in response to the need for effective surge protection in electronic circuits. As electronic devices became more prevalent and delicate, they became susceptible to damage from voltage spikes and surges. The varistors were developed to provide a cost-effective and reliable means of voltage clamping and transient suppression ensuring the safety and functionality of the electronic equipment.

Definition

Varistors are nonlinear electronic component designed to protect electrical circuits from excessive voltage surges or transients. It is a resistor whose electrical resistance varies significantly with changes in voltage. Basically, there are two types of varistors named as Metal Oxide Varistors and Silicon Carbide Varistors.

Protected_Circuit

Varistor Symbol

The varistor symbol consists of two arrows pointing in opposite directions and indicating bidirectional voltage protection. It signifies that varistors can handle both positive and negative voltage surges.

IEEE and IEC Standard Symbol of Varistor

Packages

The Varistors come in various package styles, including disc, cylindrical and surface-mount packages.

Here’s an example of a cylindrical package:

Cylindrical varistors are encapsulated in a ceramic or plastic cylindrical body. They have two wire leads for electrical connection.

Specifications

The Varistors have several specifications to consider when selecting them for a specific application:

• Voltage Rating (VAC/VDC): The maximum continuous voltage the varistor can withstand without the breakdown.
• Maximum Clamping Voltage (VCL): The voltage at which the varistor conducts significantly.
• Peak Current Rating (IPEAK): The maximum surge current the varistor can handle.
• Energy Rating (Joules): The energy absorption capability of the varistor.
• Response Time: The time it takes for varistor to respond to an overvoltage condition.

Construction of Varistor

The Varistors are typically constructed using a ceramic material often zinc oxide (ZnO) with the small amounts of other metal oxides. The ceramic material is sintered into a compact structure and which creates a network of grain boundaries. When a voltage surge occurs the varistor’s resistance decreases due to movement of charge carriers within these grain boundaries.

Metal Oxide Varistor (MOV)

The MOV Explanation: Metal oxide varistors commonly known as MOVs are a type of varistor made from the zinc oxide. They exhibit excellent voltage-clamping characteristics and are widely used for the surge protection in electronic circuits. The diagram shows a typical disc-shaped MOV.

Metal Oxide Varistor (MOV)

MOV varistors are designed to limit voltage spikes by conducting when the voltage exceeds a certain threshold. They absorb energy from surge preventing it from the damaging connected components.

Role of Varistor in Electrical Circuits

The Varistors have several important effects and functions in electrical circuits:

• Voltage Clamping: The Varistors limit the voltage across a circuit by the rapidly decreasing their resistance when voltage exceeds a certain threshold.
• Transient Suppression: They suppress transient voltage spikes and surges and preventing damage to connected components.
• Bidirectional Protection: The Varistors can protect against both the positive and negative voltage transients.
• Fast Response Time: They respond rapidly to the voltage changes offering near-instantaneous protection.
• Nonlinear Response: The resistance of the varistors decreases as voltage increases and resulting in a nonlinear voltage-current characteristic.
• Self-Healing: The Varistors can recover from the transient events and return to their normal state once the excessive voltage is removed.

Types of Varistor

There are primarily two types of varistors:

1. Metal Oxide Varistors (MOVs)
2. Silicon Carbide Varistors (SiC)

1. Metal Oxide Varistors (MOVs)

• Composition: The MOVs consist of a ceramic semiconductor material typically zinc oxide mixed with the small amounts of other metal oxides.
• Operation: The MOV’s resistance decreases as the voltage across it increases. It clamps the voltage to safe level by conducting excess current.
• Applications: The Surge protection in power lines, surge arresters in electrical distribution systems and transient voltage suppression in the electronic circuits.

2. Silicon Carbide Varistors (SiC)

• Composition: The SiC varistors use silicon carbide as the semiconductor material.
• Operation: To Similar to MOVs, SiC varistors exhibit a nonlinear voltage-current characteristic conducting when voltage surges exceed a certain threshold.
• Applications: The Surge suppression in high-voltage systems such as power transmission lines and substations.

Working of Varistor

The working principle of varistor is based on its nonlinear voltage-current characteristic. When the voltage across a varistor exceeds its specified threshold and its resistance decreases significantly allowing it to conduct current and shunt the excess voltage away from protected circuit. This effect is achieved due to semiconductor properties of varistor material.

• Normal Operation: The Under normal operating conditions. where voltage remains below the varistor’s threshold and it behaves as a high-resistance component effectively isolating the circuit from the voltage spikes.
• Surge Conditions: When a voltage surge or transient occurs exceeding the varistor’s clamping voltage the varistor undergoes a rapid transition from a high-resistance state to low-resistance state.
• Conduction: The varistor effectively shorts the excess voltage across its terminals is allowing a significant surge current to flow through it.

Operation and Characteristics Graph

Operation and Characteristics Graph

The characteristics graph of a varistor typically shows its voltage-current relationship. It illustrates how the varistor’s resistance changes with the applied voltage. At low voltages, the resistance is high but as voltage increases beyond the clamping voltage and resistance drops significantly.

Properties of Varistors

The Common properties of varistors include:

• The Voltage Rating
• Clamping Voltage
• The Energy Absorption Capacity
• The Response Time
• Leakage Current
• The Operating Temperature Range

Characteristics of Varistors

The Key characteristics of varistors are:

• The Nonlinear Voltage-Current Curve
• The High Surge Current Handling Capability
• Repeatability
• The Bidirectional Protection
• Fast Response
• The Wear-Out Mechanism

Applications of Varistor

The Varistors find applications in various industries and devices including:

• The Surge Protection in Power Supplies
• The Overvoltage Protection in the Electronic Devices
• The Lightning Surge Arrestors
• The Motor Drive and Control Systems
• The Telecommunication Equipment
• The Automotive Electronics

Advantages and Disadvantages of Varistor

There are some list of Advantages and Disadvantages of Varistor given below :

Advantages of Varistor

• The Effective protection against voltage surges.
• Quick response time.
• The Bidirectional protection.
• Reliability and long lifespan.
• The Cost-effective solution.

Disadvantages of Varistor

• The Limited lifespan in the high-energy surge scenarios.
• May generate heat during operation.
• To Requires proper sizing for the effective protection.

Conclusion

The Varistors play a crucial role in the safeguarding electronic circuits and devices from the voltage transients and surges. Their nonlinear characteristics and bidirectional protection make them an indispensable component in the modern electronics.

FAQs on Varistors

1. Why do varistors have a limited lifespan?

The Varistors can wear out over time, especially when exposed to frequent and high-energy surge events. This wear is due to varistor’s ability to dissipate excess energy leading to gradual degradation.

2. Can varistors protect against lightning strikes?

The Varistors can provide some level of the protection against lightning-induced surges but are often used in conjunction with the other surge protection devices.

3. Do varistors consume power when they are not conducting?

The Varistors typically have a low leakage current when they are not conducting. While this current is minimal and it may result in a slight power dissipation but it is generally negligible in the most applications.