What is thermistor? How it is used in Wheatstone Bridge?

What is thermistor: It is a passive temperature transducer so battery must be used in its circuit. Thus a thermistor is a transducer which changes its resistance due to change in temperature.

There are two types of thermistors: NTC and PTC thermistors. In NTC type thermistor its resistance decreases when its body temperature increases. And PTC type thermistor, its resistance increases when its body temperature increases. Usually all electronics measurement circuits use NTC type of thermistor. The resistance of NTC thermistor decreases by , for a temperature rise of . This relation remains linear throughout the given range of NTC thermistor.

Construction of Thermistor

Thermistor is made up of mixtures of metallic oxides like Iron (Fe), Nickel (N), Cobalt (Co), Manganese (Mn). Thermister is manufactured in different shapes like in the form of bead, disk, probe, etc. It is very small in size with a diameter of about 2.5mm to 25mm.

Thermistor as Electronics Thermometer 

The circuit of electronic thermometer is given below. It uses a NTC thermistor connected in Wheatstone bridge circuit.

Working: As shown in above circuit, when R₁ / R₃ = R₂ / R₄ the Wheatstone bridge is balanced. So PD across A-B is zero, so the micro ammeter shows zero deflection.

Now suppose the temperature of R₄ (thermistor) increases. Then its resistance will decrease as it is NTC type. So voltage at point B is less than voltage at point A. So PD is produced across A-B and the micro ammeter deflects proportionally.

The scale of the micro ammeter is calibrated in terms of required unit of temperature like Celsius or Fahrenheit.

Applications of Thermistor

1. Electronic thermometer
2. Temperature controlled soldering iron
3. Industrial furnace temperature control
4. Medical heating pad used in therapeutic treatment
5. Heat detector system in LPG gas production system
6. Premature baby care unit in medical applications
7. Hot gas detector in industries