Thermistor: Definition,working principle and applications

What is a Thermistor?

A thermistor is a type of thermal resistor whose electrical resistance increase or decrease with change in temperature.Thus, the thermistors are thermally sensitive resistance whose resistance changes with  change in the thermistor body temperature. We will discuss about the working principle of thermistor. How  PTC and NTC thermistor function and  what are the applications of thermistor will be discussed in the later sections.

Thermistor symbol

The symbol of thermistor is as depicted in below figure.

thermistor symbol

The symbol of thermistor is as given below. The thermistor is a passive circuit element. The passive component does not require any external power supply for its operation. The thermistor is very sturdy and cheap and can be used to measure the temperature accurately. The thermistor does not function reliably in extremely hot or cold temperature. Therefore,  thermistors are most suitable for the applications where precision temperature measurement is required.

Working Principle of Thermistor

The resistance of the thermistor changes with change in thermistor body temperature. The resistance of the thermistor does not vary linearly with change in temperature. The thermistor has non-linear resistance temperature curve. The resistance of the thermistor can be measured using resistance meter(Ohm-meter). By knowing the exact relationship between the change in the resistance with temperature,the temperature can be derived by measuring the resistance of thermistor at particular temperature. The change in the thermistor resistance with temperature depends on the type of material used for thermistor construction. The plot between temperature and resistance of a thermistor is as given below.  

NTC thermistor temperature Vs resistance graph Vs tempera

From above graph we can measure the temperature by measuring the resistance of the thermistor. The procedure of temperature measurement is as follows.  

  • Measure the resistance of the thermistor by Ohm-meter.
  • Draw a vertical line across from the resistance on the y-axis and drawing a vertical line down from where this horizontal line intersects with the graph, we can hence derive the temperature.

Types of Thermistor 

The thermistors are classified according to increase or decrease in thermistor resistance with variation in temperature. There are two types of thermistors.

  • Negative Temperature Coefficient (NTC) Thermistor
  • Positive Temperature Coefficient (PTC) Thermisto

NTC Thermistor

NTC thermistor has negative temperature coefficient and the resistance of the thermistor decrease with an increase in the temperaure and, the thermistor resiatnce increase when temperature decrease. Thus, the resistance and temperature are inversely proportional in NTC thermistor. The electric current flow tthrough the NTC thermistor increase with increase in temperature.

With increase in temperature,  a large  number of charge carriers or free electron collides with valence electron of other atom.The valence electrons which gains sufficient energy will breaks the bonding with the parent atom and moves freely from one place to another place. The electrons that move freely from one place to another place are called free electrons. Thus, the free electrons increase due to rapid collision of the free electrons with the atom.The small increase in temperature produce millions of free electrons. The more free electrons cause rapid increase in electric current. Thus, the small increase in temperature cause  rapid decrease in the temperature and allows a large current flow through the thermistor.

Resistance-temperature equation of NTC Thermistor

The resistance of NTC thermistor decrease with an increase in temperature. The relationship between the temperature and resistance is governed in NTC thermistor by the following mathematical expression.

resistance temperature equation of thermistor

The equation of the thermistor is highly non linear. A standard NTC thermistor usually exhibits a negative thermal resistance temperature coefficient of about 0.0045/oK

The Beta (β) value of a thermistor is an indicator of the slope of the resist­ance-temperature curve characteristic.

slope of resistance temperature curve of thermistor

The higher value β shows good relationship between the resistance and temperature. For small increase in temperature will cause more decrease in the resistance if value of β is high. Thus higher sensitivity and more accuracy can be achieved if the value of β is high.

From equation (1) the temperature coefficient of the thermistor can be given as;

temperature coefficient formula of thermistor

The  equation (2) shows that the thermistor has negative temperature coefficient.
If β= 4000 K and T = 298 K, then the αT = 0.0045/∘K temperature coefficient. The temperature coefficient of the thermistor is much higher than the sensitivity of the platinum RTD.

PTC Thermistor

The resistance of the PTC thermistor increase with increase in temperature. The PTC thermistors are made from doped polycrystalline ceramic. Thermistors with Positive Temperature Co-efficient (PTC) are also called posistors. The plot between resistance and temperaure of PTC thermistor is as given below.

PTC thermistor working principle

The PTC thermistors are not as popular as NTC thermistors. The PTC thermistors are used in circuit protection. When current pass through PTC thermistor, it cause heating.In a PTC thermistor, this heating up will also cause increase in  resistance. This creates a self-reinforcing effect that drives the resistance upwards, therefore limiting the current and thus PTC thermistor is used as  a current limiting device.

Thermistor Construction

The thermistors are made with the sintered mixture of metallic oxides like maganese, cobalt, copper, iron, nickle, uranium etc. These oxides are blended in a suitable proportion and compressed into the desired shape and it is heat treated to recrystalize.

Types of Thermistor

There are different shapes and sizes of the thermistor are available in the market.The thermistors are available in shape of beads,disks and washers. The size of the thermistor in the form of beads are in the range of 0.15 millimetre to 1.5 millimetre diameter. Thermistors in the form of disks and washers are made by pressing the thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimetres to 25 millimetres.

The typical size of a thermistor is 0.125mm to 1.5 mm. The thermistors are available in the nominal values of 1K, 2K, 10K, 20K, 100K, etc. The resistance value of thermistor is at a temperature of 25oC. The major advantages of thermistors are their small size and relatively low cost.

Thermistor Applications

The thermistors are widely used in many temperature measurement applications including air and liquid temperature measurement. The some of the applications are as given below.

  • Used in medical instruments
  • Digital thermometers
  • In home appliances such as oven,hair dryers,toasters ,microwave,fire alarm and refrigerators
  • To measure oil and coolant temperature in car
  • Measurement of thermal conductivity of electrical materials
  • For surge protection
  • As inrush current limiter
  • Used in basic electronics circuit
  • Used in temperature measurement of motor
  • For temperature compensation circuit
  • Used in Wheatstone bridge  
  • Digital thermometer
  • Used in aerospace, communication and instrumentation field

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