Thermistor
- 2. PRINCIPLE: Thermistors are non-metallic resistors , i.e , semiconductors of ceramic material having negative coefficient of resistance. The change in temperature to which thermistor is subjected to causes a change in the resistance which in turn gives the measure of change in temperature. Resistance of thermistor decreases with an increase in temperature & vice-versa.
- 3. CONSTRUCTION: A metal tube houses a thermistor sensing element & an insulation separates thermistor sensing element from metal tube . Lead wires are drawn out from thermistor & connected to the wheatstone bridge. Metal tube sensing element + leads = thermistor. OPERATION: Thermistor works in similar way as that of RTD but as the temperature rises the resistance of the sensing element of the thermistor decreases. Change in resistance of sensing element of thermistor is measured using the wheatstone bridge.
- 6. APPLICATIONS: • Thermistors can be used to measure varying temperatures as they have good sensitivity. • Used for temperature compensation in electronic equipment. • Used in time delay circuits. • Used to measure thermal conductivity. • Used to measure pressure and flow of liquids. • Used in precision temperature measurement.
- 7. ADVANTAGES: • Low cost. • High accuracy. • For 1°C change in temperature ,the resistance changes as far as 6% in some cases. • They possess the ability to withstand mechanical & electrical stresses. • Thermistors can be manufactured to very small sizes as the resistivity of thermistors are very high. • Simple electric circuits can be used to measure change in resistance .
- 8. LIMITATIONS: • They have non-linear scale over its range of operation. • Resistance of thermistor increases when time elapses which is called “aging effect”. • When current passes through the thermistor , it gets heated which is called “self heating effect”