At very large currents, usually more than 100 A, the varistor characteristics are
dominated by the low value bulk resistance Rbulk of the device. Usually, varistors
are fabricated in the form of discs and hence have large parasitic capacitance values
in the range of 0.2–10 nF. Including the inductance of the varistor leads will complete
the equivalent circuit of the varistor, which is shown in Figure 5.11.
Varistors are fast acting devices with response times less than 0.5 ns, if parasitic
inductance due to leads can be avoided. The performance of varistors is affected by
temperature. Excessive leakage currents can raise the temperature of the varistor.
Because the varistor has a negative temperature coefficient, the current will increase
as it become hotter, which will increase the current even further, resulting finally in
a thermal runaway. Varistors are usually used in protecting electronic systems from
transient overvoltages that propagate on the mains. There are various varistor
models that have been developed in the recent past that are being used for various
applications and depend upon the type of varistor used.
Energy absorbed in the ceramic of a varistor is distributed throughout the ceramic at
numerous grain boundaries rather than a single junction as in other semiconductor
voltage clamping devices. Varistors can withstand single pulse transients up to
150 per cent of their rated current, but may fail at multipulse transients at 75 per
cent of peak rated current. When energized at power system operating voltage, they
could only withstand 40 per cent of the rated current in a multipulse environment [26].