Nonlinear Resistors

Those elements whose V - I curves are not straight lines are called nonlinear elements because
their resistances are nonlinear resistances. Their V - I characteristics can be represented by an
equation of the form 1= kV = b where n is usually not equal to one and the constant b mayor may not
be equal to zero.
Examples of nonlinear elements are filaments of incandescent lamps, diodes, thermistors and
varistors. A varistor is a special resistor made of carborundum crystals held together by a binder.
Fig. 1.37(a) shows how current through a varistor increase rapidly when the applied voltage increases
beyond a certain amount (nearly 100V in the present case).There is a corresponding rapid decrease in resistance when the current increases. Hence, varistors
are generally used to provide over-voltage protection in certain circuits.
A thermistor is made of metallic oxides in a suitable binder and has a large negative coefficient
of resistance i.e. its resistance decreases with increase in temperature as shown in Fig. 1.30 (b). Fig.
1.30 (c) shows how the resistance of an incandescent lamp increases with voltage whereas Fig. 1.30
(d) shows the V-I characteristics of a typical silicon diode. For a germanium diode, current is related
to its voltage by the relation.