Thermistor is a contraction of a term “thermal resistors”. Thermistors are essentially composed of semiconductor materials. Although positive temperature coefficient of units are available, most thermistors have a large negative coefficient of temperature resistance i.e. their resistance decreases with increase of temperature. The negative temperature coefficient of resistance can be as large as several percent per degree celcius . This high sensitivity to temperature change makes the thermistors extremely suitable to precision temperature measurement, control and compensation.
Construction of Thermistors
Thermistors are composed of sintered mixture of metallic oxides, such as manganese, nickel, cobalt, copper, iron and uranium. They are available in variety of sizes and shapes. They have resistance ranging from 0.5Ω to 0.075MΩ. A smallest size of thermistor is 0.015mm to 1.25mm diameter in the shape of beads. The beads may be seal in the tips of solid glass rods to form probes which are somewhat easier to mount than beads. They are also available in the shape of rod and discs. The disc type thermistor with long vertical leads are most common in use. The discs are made by pressing thermistor material under high pressure into a flat cylindrical shapes with diameters varying from 2.5mm to 25mm.Resistance-Temperature Characteristics of Thermistors
As already discussed, the resistance of a thermistor decrease with increase in temperature non-linearly. The relation between resistance and absolute temperature is given by the mathematical expression;
Where RT1 = resistance of the thermistor at absolute temperature T1 (in K)
RT2 = resistance of the thermistor at absolute temperature T2 ( in K)
β = a constant which depends upon the thermistor material typically its value varies from 3500 to 4500 K.