Non linear resistors

Thermistors and varistors are the most common type of non linear resistors. Thermistors are thermally sensitive resistors with very high temperature co-efficient of resistance. There are two types of thermistors. They are negative temperature co-efficient of resistance thermistors (NTC) and positive temperature co-efficient of resistance thermistors (PTC). Mixtures of metal oxides are preferred for the construction of thermistors. Commonly used metal oxides are Nickel, Manganese, Iron, Copper, Cobalt, Titanium and Lithium. In NTC thermistors, resistance decreases exponentially with increase in temperature. They can be prepared in four shapes. They are rod, disc, bead and washer. Above Curie temperature, resistivity of PTC thermistor increases appreciably. Varistors are voltage dependent resistors. There are two types of varistors. They are silicon carbide varistors which are made from silicon carbide mixed with a suitable ceramic binder material and metal oxide varistors which are made from mixtures of zinc oxide, bismuth oxide and other powdered metals.