Varistor

Varistor S14K385 photo

Varistor symbol 1

Western Electric Type 3B varistor manufactured in June 1952

Varistor circuit historical construction

Diac

Western Electric Type 44A varistor on U1 receiver

Varistor is an electronic component that is designed to protect electrical and electronic equipment against high voltage spikes. Its name is derived from the words "variable resistor," which describes its function: the resistance of a varistor changes with the voltage applied across it. Varistors are widely used in various applications to protect against overvoltage conditions, which can damage electronic circuits.

Overview

A varistor is a type of resistor with a non-ohmic current-voltage characteristic. The most common type of varistor is the metal-oxide varistor (MOV). These devices are made from a ceramic mass of zinc oxide grains, sandwiched between two metal plates (the electrodes), which act as the electrical contact to the rest of the circuit. The grain boundaries provide the non-linear electrical characteristics that make the varistor useful for overvoltage protection. When a low voltage is applied across the terminals of a varistor, it behaves like an insulator. However, when a high voltage is applied, the varistor's resistance decreases dramatically, allowing it to absorb and dissipate the excess energy as heat.

Applications

Varistors are used in a wide range of applications to protect against transient voltage spikes. These include:

• Power supply circuits
• Telecommunication equipment
• Automotive electronics
• Consumer electronics such as TVs, radios, and computers
• Lightning surge protectors

Types of Varistors

Besides the commonly used MOV, there are other types of varistors based on their material and construction, including:

• Silicon carbide (SiC) varistors - Used before the MOV became popular, but now largely obsolete due to their lower energy dissipation capabilities.
• Silicon Avalanche Diode (SAD) - A semiconductor device that operates in a similar manner to varistors but is used for lower voltage applications.

Selection Criteria

When selecting a varistor for a particular application, several factors must be considered, including:

• Maximum operating voltage
• Energy absorption capability
• Response time
• Operating temperature range

Limitations

While varistors are effective for protecting against voltage spikes, they have limitations, including:

• Degradation over time with repeated exposure to surges, leading to a reduced lifespan.
• Potential for catastrophic failure if exposed to voltages beyond their maximum rating, which can result in a short circuit.

Safety Considerations

To ensure safe operation, it is important to select a varistor with an appropriate rating for the specific application. Additionally, proper installation and periodic inspection are necessary to detect any signs of degradation or failure.