Metal Oxide Varistors - Basic Properties, Terminology, and Theory
What is a Metal Oxide Varistor?
Metal oxide 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 metal oxide varistor to provide excellent transient suppression performance.
When exposed to high voltage transients the metal oxide 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 metal oxide varistor, thereby protecting vulnerable circuit components.
The metal oxide 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.
Since electrical conduction occurs, in effect, between zinc oxide grains distributed throughout the bulk of the device, the metal oxide varistor is inherently more rugged than its single P-N junction counterparts, such as zener diodes. In the metal oxide varistor, energy is absorbed uniformly throughout the body of the device with the resultant heating spread evenly through its volume. Electrical properties are controlled mainly by the physical dimensions of the metal oxide varistor body which generally is sintered in the shape of a disc. The energy rating is determined by volume, voltage rating by thickness or current flow path length, and current capability by area measured normal to the direction of current flow.
Metal oxide 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.
