The Characteristics and Some Applications of Varistors

Varistors, circuit elements whose resistance is a function of the voltage applied, represent one important commercial application of semiconductors. They may be divided into two classifications: nonsymmetrical and symmetrical varistors. The first classification includes both metallic rectifiers such as copper oxide, selenium, and copper sulfide, and point contact rectifiers such as silicon and germanium. The only commercial varistor of the symmetrical class is the silicon carbide varistor, although a symmetrical characteristic may be obtained by connecting two nonsymmetrical varistors in parallel with proper polarity. Each varistor has its volt-ampere characteristic and at each point on this characteristic two different values of resistance may be defined, namely the dc resistance, defined as the ratio of voltage to current, and the dynamic or ac resistance, defined as the ratio of dE to dI. The former is important in problems dealing with steady-state dc or large-signal applications, while the latter is important when dealing with small applied signals. Because of the state of the art, varistors as manufactured commercially are less uniform than many other circuit elements and required uniformity is often obtained by special selection. Economical use of these elements therefore requires the circuit engineer to recognize clearly which of the several properties are important in his application and to specify special selection for only those properties and to the extent necessary for his application.