# Thermistor Working Principle

Hello Friends,

In this article, I am going to discuss the NTC thermistor working principle, its characteristics and applications. So let us start.

A thermistor is a special type of resistor whose resistance changes with the change in its body temperature. These are of two types:

• PTC Thermistor
• NTC Thermistor

## PTC Thermistor

Positive Temperature Coefficient (PTC) thermistor, is made up of a material having a positive temperature coefficient of resistance.

In case of a material having a positive temperature coefficient of resistance, the resistance of the material increases with an increase in temperature. Therefore, the resistance of PTC thermistor increases with its body temperature.

## NTC Thermistor

Negative Temperature Coefficient (NTC) thermistor is made up of a material having a negative temperature coefficient of resistance like manganese, nickel, cobalt, copper, iron and uranium, therefore, their resistance decreases with the increase in body temperature. They are available in various sizes and shapes.

As NTC thermistors are widely used in engineering applications, therefore, I will discuss them only and the further discussion belongs to NTC thermistors only.

# NTC Thermistor Working Principle

When the ambient temperature of a thermistor increases, its resistance decreases significantly. Typically, for every 1oC rise in temperature, there will be a 5% decrease in their resistance. So their sensitivity is very high.

In simple words we can say, they can observe even a very small change in temperature which could not be observed by a thermocouple or an RTD. This makes them very useful for precision measurement of temperature, control and compensation. This is the basic thermistor working principle.

• These can be used in the temperature range of – 60oC to 300o
• They have resistance in the range of 0.5 ohms to 0.75 M ohms.

## Characteristics of Thermistors

Three important characteristics of thermistors are:

• the resistance – temperature characteristics,
• the voltage – current characteristics,
• the current – time characteristics.

## Resistance Temperature Characteristics

As the temperature of a thermistor increases its resistance decreases exponentially. The mathematical expression for the relationship between resistance of thermistor and temperature is

RT1 = RT2 e [β(1/T1 – 1/T2)]

Where RT1 = resistance of the thermistor at temperature T1
RT2 = resistance of the thermistor at temperature T2
β = is a constant, its value depends upon the material used in the construction of thermistor, typically its value ranges from 3500 to 4500.

## Voltage Current Characteristics

The voltage drop across a thermistor increases with an increase in current. It increases until it reaches the peak value after the peak value, it decreases with the increase in temperature.

This is so because, initially when an increase in the current is small, it is not able to produce a change in the temperature of the thermistor, therefore, the voltage drop across it increases. But after the peak value, the value of the current is able to change the temperature of the thermistor.  It increases its temperature. It results in a decrease in thermistor resistance. And hence voltage drop across thermistor decreases.

## Current and Time Characteristics

The current – time characteristics are shown in Figure. It is obvious from the figure that the time delay to reach maximum current is the function of the applied voltage. When we are decreasing the applied voltage, the time delay to reach the maximum current is also decreasing.

This happens because, when the heating effect occurs in a thermistor, a certain finite time is required for the thermistor to heat and the current to build up to a maximum steady-state value.

## Applications of Thermistors

Measurement of temperature: The schematic diagram for the measurement of temperature with the help of a thermistor is shown in Figure.

In this arrangement when the ambient temperature of the thermistor increases, its resistance decreases, which results in an increase of current. In other words, we can say that a change in circuit current is proportional to the ambient temperature of the thermistor. Hence, the micro-ammeter can show the change in temperature in terms of micro-amperes and can be calibrated directly in temperature readings.

Temperature Compensation: As we know thermistors have a negative temperature coefficient of resistance whereas mostly electronic circuit elements have a positive temperature coefficient of resistance. Being opposite in magnitude, they can compensate for the effect of temperature. So the thermistors are widely used in electronic circuits to compensate for the effects of temperature.

• These can be used for the measurement of power at high frequencies.
• Measurement of the thermal conductivity can also be done with the help of the thermistors.
• Measurement of level, flow, pressure can be done.
• Measurement of vacuum can be done.
• Time delay can be provided in the operation of electronic devices with the help of thermistors.

• They are compact, rugged and inexpensive.
• They have good stability and high sensitivity.
• Their response is very fast.
• They are not affected by stray magnetic and electric fields.
• Due to all these advantages, thermistors are preferred over other temperature detecting devices like RTDs and thermocouples.