Thermistor Working Principle & Characteristics

In this article, I am going to discuss the NTC thermistor working principle, its characteristics, and its 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 the 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 the 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 the precision measurement of temperature, control, and compensation. This is the basic thermistor working principle.

  • They 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.

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 configurations are Disk type, Bead type, and Rod type.

ntc thermistor working principle, working principle of thermistor

The disc type and the rod type thermistors have greater power dissipation capability. The rod type thermistor has high power handling capability. So they should be used in a particular application according to their power handling and temperature dissipation capabilities.

Out of these, the bead type configuration is the smallest one. Generally, its diameter is about 0.15 mm. In this case, the measurement element is encapsulated in a glass probe. It is commonly used for measuring the temperature of liquids.

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 the thermistor, typically its value ranges from 3500 to 4500.

thermistor characteristics, characteristics of thermistor

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.

working principle of thermistor

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 can 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.

thermistor characteristics

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.

voltage current characteristics of thermistor

In this arrangement when the ambient temperature of the thermistor increases, its resistance decreases, which increases 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 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 thermistors.
  • Measurement of level, flow, and 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 used to monitor the internal temperature of heating machines like microwaves, boilers, etc.
  • They are used to monitor the voltage and current output of power supplies and protect the connected devices in the case of abnormality.
  • Digital thermometers often use thermistors as the temperature sensing element.
  • They are also used in vehicles to monitor the temperature of different parts and sections.
  • Food & beverage, AC & refrigeration industry to monitor the temperature and control the process accordingly.

Advantages of Thermistor

  • 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.

Disadvantages of Thermistor

They have non-linear temperature resistance characteristics.

Thermistor Vs Thermocouple

Here we are making a brief comparison between thermistor and thermocouple so that one can differentiate between these two and can select them for a particular application easily.

  • The thermistors have a narrow sensing range. It is typically about 55 to +150oC. On the other hand, thermocouples have a wider temperature sensing range. For instance, Type T thermocouples have a sensing range of -200 to 350o
  • The thermistors have a nonlinear relationship between the sensing parameter (resistance) and temperature. Whereas, thermocouples have a linear relationship between the sensing parameter (voltage) and temperature.
  • The thermistors are good for sensing small temperature changes however they are not as accurate. Whereas, the thermocouples are comparatively more accurate.
  • Thermistors are cheaper, smaller in size, and easier to use than thermocouples.

Thermistor vs RTD

Resistance Temperature Detectors (i.e. RTD sensors) are very similar to thermistors resistance of the RTDs varies just like that of thermistors. But they differ due to construction materials. Generally thermistors are made up of ceramic or polymer materials whereas RTDs are made up of pure metals.

Thermistors are more accurate, cheaper, and faster in response than RTDs. The only demerit of the thermistor is its narrow operating temperature range which is wider for the RTDs. Aside from this, there is no reason to use a thermistor over an RTD.

Thanks for reading about “thermistor working principle.”

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