Thermistor Working Principle | Advantages and Applications

Thermistor Working Principle | Advantages and Applications

Table of Contents

Thermistor i.e. THERMally sensitive variable resISTOR are semiconductor of  ceramic (non-metallic) materials that are sensitive to temperature, made by  sintering mixtures of metallic oxides such as maganese, nickel, cobalt, copper  and uranium.  

• Thermistors have negative temperature coefficient (NTC) i.e. resistance decreases as temperature increases. 
• Temperature resistance relationship is exponential (non-linear). 
• The resistance of thermistor ranges from 0.5 Ω to 0.75 mΩ. Thermistor is a highly sensitive device. 
• The negative temperature coefficient of resistance can be as large as several percent per degree celcius. 
• This allows the thermistor circuits to detect very small changes in temperature which could not be observed by RTD or a thermocouple. 

In some cases the resistance of thermistor at room temperature may decrease as much as 5% for each 1°C rise in temperature. This high sensitivity to temperature changes makes thermistors extremely useful for precision temperature measurements control and compensation.

Thermistors are widely used for measurement in the range of – 60°C to 15°C. 

Thermistor Working Principle

Resistance of certain metal oxides with negative temperature coefficient of resistance varies with temperature.

Construction of Thermistor 

Thermistors are composed of sintered oxides of metallic oxides such as manganese, nickel, cobalt, copper, iron and uranium. They are available in variety of sizes and shapes. 

The thermistors may be in the form of beads, rods probe and discs as shown in Fig. 

Thermistor Working Principle
Different Forms of Construction of Thermistor  

• A thermistor in the form of bead is smallest in size of diameter 0.015 mm to 1.25 mm. Beads are sealed in the tip of solid glass rods to form problems which are easier to mount than beads. 
• Glass probes have a diameter of 2.5 mm and length varies from 6 mm to 50 mm. 
• The probes are used for measuring the temperature of liquids. The resistance ranges from 300 Ω to 100 mΩ. 
• For greater power dissipation requirements, thermistors may be obtained in the form of disc, washer or rod forms. 
• Disc thermistors are made by pressing material under high pressure into cylindrical flat shapes with diameters ranging from 2.5 mm to 25 mm. It has resistance values of 1 Ω to 1 mΩ. These are sintered and coated with silver on two flat surfaces. 
• Rod thermistors are extruded through dies to make long cylindrical units of 1.25, 2.75 and 4.25 m in diameter and 1.25 – 50 mm long. Leads are attached to ends of rods. Their resistance varies from 1 – 50 kΩ. 
• The advantage of rod thermistor is the ability to produce high resistance with high power handling capability. 
• Thermistors can be connected in series or parallel combinations for applications which requires increased power handling capability. 
• Thermistors are chemically stable and can be used in nuclear environments. Their wide range of characteristics permits them to be used in limiting and regulation circuits, as time delays, for integration of power pulses and as memory units. 

Resistance-Temperature characteristics of Thermistors

The relationship between the resistance and absolute temperature of a thermistor is represented mathematically as – 

RT1 = Resistance of the thermistor at absolute temperature T1, °K 

RT2 = Resistance of the thermistor at absolute temperature T2, °K 

β = A constant depending upon the material of thermistor, typically 3500 to 4500 °K 

The resistance-temperature characteristics of a typical thermistor is shown in Fig. 

Resistance-Temperature  Characteristics of a Typical Thermistor and Platinum  

Also, figure shows the very high negative temperature co-efficient of resistance,  making it ideal temperature transducer.

  Thermistors are non-linear devices over a temperature range, with better than  0.2% linearity over the 0 –100°C temperature range are available. Typical  sensitivity of a thermistor is approximately 3 mV/°C at 200°C.  

Advantages of Thermistors

1. Small size and low cost.  
2. Fast response over narrow temperature range.  
3. Good sensitivity in NTC region.  
4. Contact and lead resistance problem not occurred due to large resistance.  

Disadvantages of Thermistors

1. Unsuitable for large (wide) temperature range.  
2. Resistance-temperature characteristic is non-linear.  
3. Very low excitation current to avoid self-heating.  
4. Need of shielding power lines, filters etc. due to high resistance.  

Thermistor Applications

1. Measurement of power at high frequencies.  
2. Measurement of thermal conductivity.  
3. Measurement of level, flow and pressure of liquids.  
4. Vacuum measurement.  
5. Measurement of composition of gases.