Varistors

Copper Oxide Varistors. The use of copper oxide varistors as rectifiers was discussed early in this chapter. They are widely used in communication for other purposes, such as modulators and demodulators, which will be considered in Chapter 11.

Silicon Carbide Varistors. The copper oxide varistor just discussed is a non-symmetrical varistor, its resistance being very low for one direction of applied voltage and very high for the other direction as shown by Fig. 43. In contrast, the silicon carbide varistor is a symmetrical varistor. For this varistor the resistance varies greatly with the magnitude of the applied voltage but is independent of the direction of the applied voltage.

Figure 43. Comparison of copper oxide and silicon carbide varistors. (Reference 42.)

These varistors consist of granules of silicon carbide firmly embedded in a vitreous ceramic matrix.^41,42 By a suitable manufacturing process they are formed into strong hard disks, provided with suitable electrodes and vacuum-impregnated with a moisture-resisting material.⁴¹ The change in resistance is not a characteristic of the silicon carbide granules but is caused by the many contacts between adjacent particles.⁴¹