Thermistors (Electrical Method)

• Thermistors are temperature sensitive resistors.

• All resistors vary with temperature, but thermistors are constructed of semiconductor material with a resistivity that is especially sensitive to temperature.

• However, unlike most other resistive devices, the resistance of a thermistor decreases with increasing temperature.

• That’s due to the properties of the semiconductor material that the thermistor is made from.

• Here is a graph of resistance as a function as a function of temperature for a typical thermistor.

• One of the good qualities of thermistors is a predictable, accurately known resistance value at every temperature in its operating range.

• Thermistors can be classified into two types:

Positive Temperature Coefficient (PTC) Thermistor:

Resistance increases with increase in temperature.

Negative Temperature Coefficient (NTC) Thermistor:

Resistance decreases with increase in temperature.

• Thermistors are widely used in applications which involve measurement in the range of minus 60˚C to 15˚C.

• The resistance of thermistors range from 0.5 ohm to 0.75 mega ohm.

• Thermistor is highly sensitive device.

• Thermistor exhibits highly non-linear characteristics of resistance versus temperature

The dependence of the resistance on temperature can be approximated by following equation R = R0e^β($\frac{1}{T}$-$\frac{1}{Tβ}$)

R is the resistance of thermistor at the temperature T (in K) R0 is the resistance at given temperature T0 (in K) β is the material specific-constant.

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 various sizes and shapes. The thermistors may be in the form of beads, rods and discs, as shown in the figure below.

Advantages

• They are small and compact having high sensitivity.

• It does not take very much heat away i.e. it absorbs less heat as it has low specific heat.

• It has rugged construction.

• Can be used for extremely low temperature measurement with accuracy.

• They are small in size and cheaper.

Disadvantages

• It has non-linear characteristics.

• It requires external energy source.

• Unsuitable for wide range of temperatures.