The relationship between current, voltage, and resistance in a conventional resistor obeys Ohm's law. If the applied voltage is doubled, the current also doubles. Thus a 500-ohm resistor remains so, regardless of the current flowing through it or voltage drop across it, except at extreme temperatures and high frequencies.
A varistor, on the contrary, is a voltage-sensitive resistor. When the voltage applied to it is increased, the current does not increase in the same ratio but by a larger factor. The current might, for example, quadruple when the voltage is doubled. The resistance of the varistor therefore decreases nonlinearly inversely with voltage; that is, it is nonohmic.
A nonlinear resistor of this kind has many applications in electronic circuits. A simple two-terminal device, it can perform functions which otherwise would require several components and, in some instances, would call for transistors or integrated circuits. Its action is quick and automatic.
The varistor has become available in a variety of shapes, sizes, and ratings. The resistive material in the varistor can be a specially processed semiconductor, silicon carbide (carborundum). A common type of varistor is the metal-oxide varistor (MOV). This type is basically a ceramic-like material having small granules of zinc oxide suspended in a matrix of other metal oxides, such as bismuth. Electronic applications of varistors include limiting, expansion, harmonic generation, frequency multiplication, voltage regulation, and contact protection.
The figure below illustrates the current-voltage characteristic of a silicon carbide varistor. Instantaneous values of V and I are shown. The corresponding characteristic of a 1-megohm conventional (linear) resistor is plotted for comparison. This varistor has a maximum continuous power rating of 10 W. Note from the figure below that a 2:1 voltage increase (from 50 to 100 V) produces a 7.5:1 current change (from 0.04 mA to 0.3 mA). Other silicon carbide varistors can have different current, voltage, and power ratings, but their static characteristics resemble the curve shown here.
Operation of the varistor is symmetrical; any rectification effects are negligible. The figure below (view A) shows the nonlinear current-voltage characteristic. When an AC voltage is applied to a varistor, the current is in phase with the voltage, since the varistor is a resistance; however, the nonlinear condition alters the wave form of the current during each half-cycle.
The figure above (view B) illustrates the AC behavior with a sine-wave voltage. The large odd-harmonic content of the current wave suits the varistor to uses as a simple harmonic generator and frequency multiplier.