Thermistor: Definition, Features, Uses, Working Principle, Advantages

What is a thermistor?

A thermistor is a negative temperature co-efficient substance whose resistivity varies disproportionately. It is commonly used to measure temperature. It is a type of semiconductor made up of the right combination of metallic oxides, namely manganese, nickel, cobalt, copper and utram. It is chemically inert. Thermistor can be used in series parallel management.

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Characteristics of thermistors:

(i) The resistance of the thermistor ranges from 0.5Ω to 0.75Ω. Beads type thermistor is the smallest in size. It is usually 0.15mm in diameter. It is sealed inside the glass coating.

(ii) Probe type thermistor is bound to glass probe. Glass probe diameter 2.5mm and length (6–50) mm probe type thermistor is used to measure the temperature of the liquid. Their resistance is 300Ω to 100MΩ.

(iii) Disc type thermistor is made in flat cylindrical shape by pressing metal at very high pressure. Their diameter is 2.5mm to 25mm.

(iv) Rod type It is a long cylindrical shape through a mold (Dies). Its leads are attached to the ends of the rods. They are 1.25, 27.5 and 4.25mm in diameter and length (12.5-50) mm. Their resistance (1-50) kΩ.

Rod type thermistors have more advantages because they have the highest power handling capacity for very high resistivity.

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Working Principle of Thermistors:

Here the base bias of a driver transistor is controlled by a thermistor. That is, the voltage drop across its landscape acts as the base bias. The thermistor is kept in contact with the heater or furnace. When the temperature of the oven increases, the temperature of the thermistor also increases, resulting in a decrease in its resistivity. As the voltage across the thermistor or the base bias of the transistor decreases, less current flows through the heater coil and the oven temperature decreases. If for some reason the temperature decreases then the resistivity of the thermistor increases and as the base bias of the transistor increases, more current flows through the heater coil, resulting in an increase in temperature. Thus the temperature of a heater can be measured and controlled automatically through a thermistor.

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Advantages and Disadvantages of Thermistor

Advantages: 

1. The resistance of the thermistor is very high (0.5Ω – 100) MΩ so its lead resistance is considered to be very negligible.
2. Its cost is low.
3. It is very suitable for measuring low quality temperature (- 100 ° C to 400 ° C)
4. Extremely small in thermistor size.
5. Its response is very good.
6. Its stability is very good.

Disadvantages:

1. It is not suitable for temperatures above 400 C.
2. Temperature vs. Resistance Card Non-linear.
3. Its use is very limited in processing

Use of thermistor:

Thermistors are widely used in the field of temperature measurement due to their good accuracy and resolution. In addition thermistors are used in the following fields-

• Thermistors are used to measure high-frequency power,
• Thermistors are used to measure thermal conductivity,
• Thermistor is used to measure liquid level, flow, pressure,
• Thermistors are used to control the temperature of the device in electronic circuits.

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