Varistors

The term varistor is a combination of variable and resistor. At low currents and voltages varistors have a high resistance but at higher voltages and currents the resistance drops dramatically. A varistor's I-V curve is very similar to a bidirectional TVS device as shown in Figure 1 . Varistors are usually made of a ceramic of zinc oxide grains in a matrix of other oxides as illustrated in Figure 2 . The grains form diodes with the surrounding matrix, creating a complex array of parallel and anti-parallel diodes. At low voltage across the varistor each miniature diode has a very low voltage across it and very little current flows. At higher voltages the individual diodes begin to conduct and the resistance of the varistor drops dramatically. Factors such as grain size, the nature of the matrix material between the grains, the thickness of the ceramic and the attachment of leads to the ceramic determine the properties of the varistor.

Figure 1 IV curve of a Varistor

Figure 2 Diagram of a Metal Oxide Varistor

The bulk nature of the varistor resistive material allows them to carry considerable current without catastrophic failure. They do suffer from degradation upon multiple stresses.

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