Thermistor Definition Properties Construction Characteristics and Applications
ThermistorThe word Themistor can be termed as Thermal Resistor. So as the name indicates it is a device whose resistance changes with the change of the temperature. Due to there high sensitivity they are widely used for the measurements of the temperature. They are usually called the Ideal Temperature Transducer.
Properties of Thermistors
- They have Negative Thermal Coefficient i.e. resistance of the thermistor decreases with increase in temperature
- They are made up of the semiconductor materials
- They are made sensitive than RTD (Resistance Thermometres Detector) and Thermocouples
- There resistance lies between 0.5Ω to 0.75Ω
- They are generally used in applications where measurement range of temperature -60oC to 15oC
Construction of Thermistor
- Thermistors are generally composed of mixture of metallic oxides such as manganese, nickel, cobalt, copper etc.
- Smaller thermistors are in the form of beads of diameter from 0.15 millimeters to 1.5 millimeters.
- Thermistor may be in the form of disks and washers made by pressing thermistor material under high pressure into flat cylindrical shapes with diameter from 3 millimeters to 25 millimeters.
Characteristics of ThermistorsThe relationship governing the characteristics of thermistors is given below as-: R1 = resistance of thermistor at absolute temperature; T1oK. R2 = resistance of thermistor at temperature T2oK. β = constant depending upon material of transducer.
From the above equation it can concluded that relationship between temperature and resistance is highly non linear. A thermistor exhibits a negative thermal resistance temperature coefficient of about 0.05/oC
Applications of ThermistorsThermistors are used for the measurement of temperature as change of temperature produces large change in there resistance. They are used for the control of the temperature in any circuit. Thermal Compensation- They can be used for the temperature compensation in any circuit. For example in any circuit a simple carbon resistor is connected which has a positive thermal coefficient. So, if resistance of carbon resistor is to be made unaffected by the temperature so a thermistor having same negative thermal coefficient is connected in series with it. So, as due to rise in temperature resistance of resistor increases and resistance of thermistor decreases by the same amount as magnitude of there thermal coefficients is same. So, as the resistor and thermistor are in series, thus net resistance of resistance remains same. Measurement of thermal conductivity of electrical materials.