The Thermistor or simply Thermally Sensitive Resistor is a temperature sensor that works on the principle of varying resistance with temperature. They are made of semiconducting materials. The circuit symbol of the thermistor is shown in the figure.
Construction of Thermistor
A thermistor is made of oxides of metals such as Nickel, Manganese, Cobalt, Copper, Uranium etc. It is available in a variety of shapes and sizes. Commonly used for configurations are Disk type, Bead type and Rod type.
The disc type thermistor and rod type thermistor is used when greater power dissipation is required. The rod type thermistor has high power handling capacity.
The smallest thermistor in these configurations is the bead type thermistor. its diameter is low as 0.15 mm. The measurement element is typically encapsulated in a glass probe. It is commonly used for measuring the temperature of liquids.
Working Principle of Thermistors
The thermistor works on the simple principle of change in resistance due to a change in temperature. When the ambient temperature changes the thermistor starts self-heating its elements. its resistance value is changed with respect to this change in temperature. This change depends on the type of thermistor used. The resistance temperature characteristics of different types of thermistors are given in the following section.
Types of Thermistors
The two basic types of thermistors available are the NTC and PTC types.
NTC stands for Negative Temperature coefficient. They are ceramic semiconductors that have a high Negative Temperature Coefficient of resistance. The resistance of an NTC will decrease with increasing temperature in a non-linear manner.
Circuit symbols of NTC and PTC thermistors are shown in the following figure.
PTC thermistors are Positive Temperature Coefficient resistors and are made of polycrystalline ceramic materials. The resistance of a PTC will increase with increasing temperature in a non-linear manner. The PTC thermistor shows only a small change of resistance with temperature until the switching point(TR) is reached.
The temperature resistance characteristics of an NTC and a PTC is shown in the following figure.
Advantages of thermistors
- Less expensive.
- More sensitive than other sensors.
- Fast response.
- Small in size.
Dis-advantages of thermistors
- Limited Temperature range.
- Resistance to temperature ratio correlation is non-linear.
- An inaccurate measurement may be obtained due to the self-heating effect.
Applications of thermistors
NTC Thermistor Application
- Digital Thermostats.
- Battery pack temperature monitors.
- In-rush-current limiting devices
PTC Thermistor Application
- Over-current protection
- In-rush-current protection