5.3.2.2 Varistors
Varistors are non-linear semiconductor devices whose resistance decreases as the magnitude
of the voltage increases. Modern varistors are fabricated from metal oxides,
with zinc oxide the primary ingredient. A typical V–I curve of a metal oxide varistor
is shown in Figure 5.10. Under normal voltages there is a small leakage current of less
than 0.1 mA and the varistor behaves like a simple high value resistor Rleak. During
overvoltage the current through the varistor increases and the voltage is clamped at
a level close to the normal voltage. This is the operating region of the varistor
(Figure 5.10) and the voltage–current relationship in this region is given by
I ¼ kVa (5:1)
In equation (5.1) a is a coefficient with values between 25 and 60. The parameter k in
(5.1) has a value extremely small (,102100), therefore I is expressed in terms of
logarithms as in equation (5.2):