What Are Varistors?

Varistors are nonlinear, two-terminal passive electronic components whose resistance decreases sharply as the voltage across them increases beyond a threshold. The name is a contraction of "variable resistor," but the mechanism is fundamentally different from a conventional variable resistor: a varistor's resistance is not set by a mechanical adjustment but by the instantaneous applied voltage, following a highly nonlinear I-V characteristic. At normal operating voltages the varistor presents high resistance and draws negligible current; when a transient overvoltage appears, resistance drops by several orders of magnitude, diverting the surge current and clamping the voltage to a safe level. This clamping behavior makes varistors the predominant component class for transient voltage suppression in power circuits and electronic equipment.

Varistors draw on solid-state physics, materials science, and power systems protection. The practical development of metal-oxide varistors in the 1970s displaced earlier silicon carbide and selenium surge suppressors in most applications.

Metal-Oxide Varistors

Metal-oxide varistors (MOVs) are by far the most widely used varistor type in modern electronics and power systems. An MOV consists of a ceramic body made primarily of zinc oxide (ZnO) granules in a matrix of small quantities of bismuth, cobalt, and manganese oxides, pressed and sintered between two metal contacts. The grain boundaries within the ceramic form back-to-back diode junctions that collectively produce the nonlinear clamping behavior. Response time is in the nanosecond range, which is fast enough to protect against lightning-induced surges and power line transients. The Vishay application note on varistors as a surge protection solution documents MOV selection parameters including clamping voltage, energy absorption capacity, and peak current rating. When an MOV absorbs a transient, part of the energy is dissipated as heat, and repeated large surges can degrade the ceramic and raise the leakage current, a failure mode that must be accounted for in the design of long-lived systems.

Varistor Characteristics and Parameters

The key electrical parameters of a varistor are its varistor voltage (the clamping reference voltage at a specified test current), its energy rating in joules, and its peak surge current. The V-I curve follows an approximate power law, with the nonlinearity exponent typically ranging from 25 to 50 for ZnO-based devices. Higher nonlinearity exponents produce sharper transitions and tighter clamping but reduce the energy handling per unit volume. The IEEE C62 series of standards covers surge protective devices for low-voltage AC power circuits, including test methods and application guidelines for MOV-based surge suppressors and arrester assemblies.

Arresters and Surge Protection Systems

Varistors form the active element inside surge arresters, which are the devices mounted at service entrances, distribution panels, and equipment terminals in power systems. An arrester housing combines one or more MOV discs with mechanical and thermal protection to form a field-installable unit rated for a specific system voltage class. The IEEE C62.11 standard defines performance requirements for metal-oxide surge arresters used on AC power systems from distribution to transmission voltage levels, including energy handling and protective level tests.

Applications

Varistors have applications across a broad range of power and electronics domains, including:

• Surge suppression in residential, commercial, and industrial power distribution
• Transient protection for telecommunications and data communication equipment
• Motor control panels and variable speed drives
• Automotive electronics exposed to load-dump and ignition transients
• Lightning protection at transmission line insulator fittings