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Varistors - Basic Properties, Terminology and Theory

What is a Varistor?

Varistors are voltage dependent, nonlinear devices which have an electrical behavior similar to back-to-back zener diodes. The symmetrical, sharp breakdown characteristics shown in Figure 3.1 enable the varistor to provide excellent transient suppression performance. When exposed to high voltage transients the varistor impedance changes many orders of magnitude from a near open circuit to a highly conductive level, thus clamping the transient voltage to a safe level. The potentially destructive energy of the incoming transient pulse is absorbed by the varistor, thereby protecting vulnerable circuit components.

varistor characteristic
Figure 3.1 Typical Varistor I-V Characteristic
The varistor is composed primarily of zinc oxide with small additions of bismuth, cobalt, manganese and other metal oxides. The structure of the body consists of a matrix of conductive zinc oxide grains separated by grain boundaries providing P-N junction semiconductor characteristics. These boundaries are responsible for blocking conduction at low voltages and are the source of the nonlinear electrical conduction at higher voltages.
Varistors are available with ac operating voltages from 4V to 2800V. Higher voltages are limited only by packaging ability. Peak current handling exceeds 50,000A and energy capability extends beyond 6500J for the larger units. Package styles include the axial device series for automatic insertion and progress in size up to the rugged high energy device line.
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