11.16 Thermistor
A thermistor is a type of resistor whose resistance is highly dependent on temperature. The name is a portmanteau of "thermal" and "resistor." These passive electronic components are widely used as temperature sensors and for temperature control in a vast range of applications.
Types of Thermistors
Thermistors are primarily classified into two types based on how their resistance responds to changes in temperature.
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NTC (Negative Temperature Coefficient) Thermistor:
- Resistance decreases as temperature increases.
- Most common type; used for temperature sensing and measurement.
- Behaviour is due to increased charge carrier liberation in the semiconductor at higher temperatures.
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PTC (Positive Temperature Coefficient) Thermistor:
- Resistance increases as temperature increases.
- Used as self-regulating heating elements or resettable fuses for overcurrent protection.
Materials and Construction
Thermistors are made from semiconductor materials — specifically sintered metallic oxides of manganese, nickel, cobalt, or iron. The powders are compressed and heated (sintered) into ceramic-like shapes: beads, discs, or rods, then fitted with connecting leads.
Non-Linearity of Thermistors
The resistance–temperature relationship of a thermistor is highly non-linear. This means the change in resistance is not constant for each degree of temperature change. In practice, this requires:
- Calibration of the thermistor for the specific temperature range.
- Use of mathematical models such as the Steinhart-Hart equation to convert resistance readings into accurate temperatures.
Why Thermistors Are Preferred for Temperature Sensing
Thermistors have a very high temperature coefficient of resistance because they are made from semiconducting oxides. A small temperature change produces a large, easily measurable resistance change. By contrast, metallic resistors have a much smaller, nearly linear response and far lower sensitivity.
Applications of Thermistors
1. Temperature Sensing and Measurement
- Digital Thermometers: Used in medical and industrial thermometers for accurate temperature readings.
- Home Appliances: Found in ovens, refrigerators, and air conditioners to monitor and regulate temperature.
- Automotive: Used to measure engine coolant and oil temperature.
2. Temperature Control and Compensation
- Circuit Protection: A PTC thermistor acts as a fuse — if current gets too high, the thermistor heats up, resistance increases sharply, and current is effectively cut off.
- Temperature Compensation: Thermistors counteract temperature-dependent behaviour in electronic circuits, ensuring consistent performance over a wide temperature range.
3. Use in a Potential Divider Circuit
A common application of an NTC thermistor is in a potential divider circuit:
- Setup: The thermistor is connected in series with a fixed resistor .
- Function: As temperature changes, the thermistor resistance changes, altering the voltage ratio across the divider. The output voltage changes predictably with temperature.
- This changing voltage can be read by a microcontroller or used to trigger a component such as a fan or warning light.
