What Is A Varistor?

Overvoltage events, whether caused by natural occurrences like lightning strikes or internal factors like switching transients, can critically damage components in an electrical system. Varistors, also known as Voltage-Dependent Resistors (VDRs) help mitigate these threats by dynamically adjusting their resistance in response to voltage levels. In this blog, we will explain how varistors work and what purposes they serve in both industrial and consumer applications.

How Varistors Operate

Typically made of semiconductor materials, a varistor possesses a defined voltage threshold known as the clamping voltage. When applied voltage remains below this point, the varistor allows minimal current to flow and essentially acts as an insulator. Once this threshold is surpassed, the varistor’s resistance decreases dramatically, permitting a large surge to pass through and channeling it away from sensitive components.

Through this switch-like capability, varistors perform core protective functions like:

• Surge Suppression: The primary purpose of a varistor is to protect circuits from transient voltage spikes by absorbing and dissipating excess energy.
• Voltage Regulation: Varistors maintain voltage within safe operating ranges.
• Detection and Monitoring: Some circuits use varistors to detect irregular voltage activity, aiding in diagnostics and monitoring.
• Voltage Equalization: Varistors can balance voltages across multiple components in a circuit, especially in series-connected systems.

Compatible with AC and DC systems alike, varistors provide fast and reliable protection without requiring an external method of control or signal.

Metal Oxide Varistors (MOVs)

The most widely used type of varistor is the MOV. To make one, zinc oxide grains mixed with small amounts of other metal oxides are compressed into a ceramic disk and sandwiched between two conductive terminals. All the tiny connections between zinc oxide grains act like back-to-back diodes, giving the MOV its voltage-dependent resistance characteristics.

At normal operating voltages, the device only lets a tiny leakage current to pass. Upon being exposed to a high-voltage surge, the electric field across these junctions becomes strong enough to cause breakdown through quantum tunneling and localized electron heating, allowing a large current to flow and dissipate excess energy.

MOVs are favored for their:

• Fast reaction times, responding to voltage spikes in microseconds
• Capability to absorb large surges of energy without degradation
• Compact and versatile form, fit for modern circuit board integration
• Simple design that is economical to manufacture and implement

Typical Varistor Use Cases

Varistors are integrated into numerous products and systems, including:

• Consumer electronics like TVs, microwaves, gaming consoles, and power strips
• Power supply units (PSUs) to protect transformers, rectifiers, and associated circuitry
• Telecommunications equipment to safeguard signal processing systems and transmission hardware
• Industrial machinery like programmable logic controllers (PLCs) and motor drives, and heavy-duty systems to absorb surges caused by switching or external interference
• Automotive electronics, such as battery management systems and electronic control units (ECUs)

Browse a Variety of Varistors on AFR Enterprises

Varistors are go-to solutions for circuit protection, their straightforward construction supporting broad applications. However, their lifespan typically ranges from a few hundred to several thousand surge events, depending on the intensity and frequency of exposure, so having access to backup varistors is a must to keep systems protected.

On AFR Enterprises, a website owned and operated by ASAP Semiconductor, you will find a comprehensive selection of high-quality varistors, circuit protection components, and various other products from trusted manufacturers from around the world. With a streamlined database, quick quotation processes, and curated procurement options, AFR Enterprises is set up so you can meet all your operational demands quickly and confidently. To see how we can surpass your expectations for fulfillment, connect with our team at your earliest convenience.