Electrical Engineering ⇒ Topic : Thermistors

Thermistors

Thermistors (thermal resistors) are semiconductor oxides like Cu0₂ having high temperature coefficient of resistance which is negative. Because of high α, they are very sensitive - ten times that of metal. Their resistance may decrease as much as 6% for each degree rise in temperature.Hence, they are used for precision temperature measurement, control, and compensation.

    The other semiconductor materials used as thermistors are oxides of manganese, nickel, cobalt, iron, and uranium. Their resistivities range from 150 to 450,000 ohm-cm or resistances from 0.5 Ω to 100 Ω. They are manufactured in the form of disks, rods, washers, and beads.Beads are of the smallest size 0.15 mm to 1.25 mm to 25 mm diameter while washers and disks range from 2.5 mm to 25 mm diameter.

    The range of measurement of temperature with thermistors is 100° to 300°C. Their chief defect is their nonlinear characteristic which is exponential.

The resistance of thermistor may be stated as:

                                                                 .......... (1)

where R_o is resistance of thermistor at reference temperature T_o K, R(T) is resistance at any temperature T K, and β is a material constant given in Kelvin, β = 3800 K.

   The three important characteristics of thermistors are: (i) resistance-temperature, (ii) voltage current-time, and (iii) current-time. The resistance of grade I thermistor falls from 10⁴ ohm-cm at 0°C to 1 ohm-cm at 300°C. This can be compared with platinum whose resistivity increases from 10^-5 ohm-cm at 0°C to 10^-5.5 at 300°C. This steep fall makes for the high sensitivity thermistors.