What is a thermistor and how do they work?

What is a thermistor?

A thermistor is a device that would change its resistance with temperature, its name has derived from words such as a thermal resistor. This device is a thermally sensitive resistor that would change its resistance for a change in temperature. Basically, thermistors are semiconductors that behaves like a resistor. These devices are really sensitive to temperature and due to this, they can be used for accurate temperature measurements.

How is a thermistor made?

Thermistors are mostly made up of semiconducting materials such as oxides of nickel and cobalt. These devices are available in different shapes and sizes, thermistors could be in the form of beads, rods, or discs. These devices are made by pressing metal under high pressure into cylindrical flat shapes. Thermistors would be mounted in stainless steel tubes like most temperature sensors so that they can be protected from the environment in which it is used. Grease would be used to improve the thermal contact between the sensor and the tube.

How does a thermistor work?

The thermistor would vary its resistance if there is a change in temperature. The thermometry of the thermistor is based on the fact that the metal oxides would vary their resistance during a change in the temperature. These devices are highly sensitive so they would vary their resistance very quickly for a small change in the temperature. This device would be connected to a meter and it would convert and displays the temperature readings. The voltage will be applied to the thermistor and this would increase its temperature and its resistance would decrease.

(R/R_O) = β (1/T – 1/T₀)

R is the resistance at temperature T

R_OIs the resistance at a reference temperature T₀

T₀Is the reference room temperature

β – is the constant characteristic of the material, if the β value is high then the resistance temperature relation is good.

We can measure the resistance of a thermistor by using an ohmmeter.

What are the major types of thermistors?

Negative temperature coefficient (NTC)

Most thermistors have a negative temperature coefficient and in this case, the resistance would decrease when the temperature increases. These types of thermistors are called NTC thermistors and they would be mostly made up of metal oxides. These types of thermistors are mostly used for temperature measurements, these thermistors are also used in power circuits for inrush current protection.

Positive temperature coefficient (PTC)

PTC thermistors have a positive temperature coefficient and these are mostly made up of barium and strontium titanate mixtures. In this type of thermistor, its resistance would increase with an increase in temperature. These types of thermistors are mostly used as a fuse. PTC thermistors are mostly used for electric current control, this type of thermistors can be used to limit the current to safe levels. So if the fault current is high then the PTC thermistor would switch off very quickly. PTC thermistors have limited use mostly they would be used to do the protection of motor and transformer windings.

What are the characteristics of thermistors?

• The temperature resistance coefficient of this device is really high when compared to other similar metals
• Thermistors resistance temperature coefficient is not constant, but with the inverse function of temperature
• Resistance of this device would decrease if there is an increase in temperature but in most metals, it is the opposite

What is the difference between resistors and thermistors?

All the resistors are thermistors, the major difference between a resistor and a thermistor is that in a resistor the resistance would remain constant over a wide temperature range.

How should we choose a thermistor?

• It should be selected according to the temperature coefficient like NTC or PTC
• Maximum operating temperature
• We should consider the power rating
• Resistance tolerance
• Beta tolerance
• It should be selected according to the resistance temperature characteristics
• Dissipation constant must be considered
• Thermal time constant

How to test a thermistor?

Basically, thermistors are resistors so we can measure the resistance with the help of a multimeter. So mostly the widely used thermistors are NTC so it would decrease with an increase in temperature. If we are testing the resistor in an AC room then it would show almost 10Kohm. If we are not testing in an AC room considering the room temperature, we could get a reading of about 8kohms. The measurement would vary according to the room temperature.

What are the various wiring methods in thermistors?

The thermistors are mostly wired in two-wire and four-wire configurations. In the two-wire method, the resistance in the lead wire would affect the measurement. So in order to solve this problem thermistors are mostly wired in a four-wire configuration. 

How to measure the resistance by using a thermistor?

We can measure the resistance if the current source is available if the current is known and voltage is measured then we can get the resistance.

R=V/I

In an Ohm meter, the current source and voltage measurement are inside the device and leads would be connected to the current source to the resistance element. A voltage divider is used in thermistors to do the linearization.

How can we linearize a thermistor?

Thermistor has high sensitivity but its temperature coefficient doesn’t decrease linearly for the rise in temperature. There is one way we can linearize the thermistor and it is by shunting it with a fixed resistor. Paralleling the thermistor with a fixed resistor would increase its linearity.

What is a Wheatstone bridge thermistor?

In the below image we can see a basic circuit that uses a thermistor for the temperature measurement. This is a Wheatstone bridge, we can see four resistors in it and one of them is a thermistor. These four resistors are balanced and due to this, there won’t be any current through the ammeter. If there is a change in temperature then there would be a change in resistance of the thermistor and thus current would flow through the ammeter. 

What are thermistor probes?

We can’t place the thermistor in a rugged environment, thermistor probes are thermistor elements that will be embedded in metal tubes. These probes would protect the thermistor from the industrial environment.

What are the advantages of a thermistor?

• They are small in size
• Economical
• It would work in any voltage
• Rugged
• There would be a huge variation of resistance for the changes in the given temperature
• Quick response
• Good stability

What are the disadvantages of thermistors?

• Resistance and temperature characteristics are nonlinear
• We can’t use this device for a wide range of temperature measurements
• It would require electric power for its operation
• During high temperatures, the chances of de-calibration of this device are really high
• It won’t work properly in excessive cold or hot temperatures
• Due to its high response to the variations in temperature, it could be susceptible to self-heating errors

What are the applications of thermistors?

• Temperature compensation
• Temperature compensation in complex electronic equipment, magnetic amplifier, and instrumentation
• Digital thermostats
• It is used in cars to determine the oil and coolant temperatures
• Surge suppression – when a circuit is energized the current needs to flow through the thermistor for a while to heat it so that it opens and acts again as a switch
• Industrial process control
• Solar energy equipment

Automotive and transport applications

• Engine temperature
• Emission controls
• Aircraft temperatures

Some useful questions related to thermistors?

What are the symbols of NTC and PTC thermistors?

What is the symbol of thermistor in circuits?

What is the thermal time constant in, a thermistor?

It is the time that is required to change the total difference between its initial and final body temperature when subjected to a step function change in temperature under zero power conditions.

What is the dissipation constant in the thermistor?

It is the ratio at a specified ambient temperature of a change in power dissipation in a thermistor to the resultant body temperature change.

What is the zero power resistance in a thermistor?

In a thermistor the zero power resistance is the DC resistance value which is measured at a specific temperature with the power dissipated by the thermistor low enough that the further decrease in power would result in .1% change in the resistance.