A thermistor is a type of temperature sensor that uses the principle of resistance change with temperature to measure and monitor temperature variations. It is a thermally sensitive resistor whose electrical resistance varies with temperature.
The most common type of thermistor is the negative temperature coefficient (NTC) thermistor. It means that as the temperature increases, the resistance of the thermistor decreases, and as the temperature decreases, the resistance increases. This relationship between resistance and temperature is not linear but follows a specific curve that is defined by the manufacturer.
The functioning of a thermistor in temperature sensing applications can be understood as follows:
Resistance-Temperature Relationship: The resistance of a thermistor changes with temperature according to its specified resistance-temperature curve. This curve is determined during the manufacturing process and is typically provided in the thermistor's datasheet.
Measurement Circuit: A measurement circuit is set up to utilize the change in resistance for temperature measurement. The thermistor is connected in a circuit where it is either part of a voltage divider or a current divider network.
Voltage Divider Configuration: In a voltage divider circuit, the thermistor is connected in series with a fixed resistor (or a set of fixed resistors) and an input voltage. The output voltage is taken from the junction between the thermistor and the fixed resistor(s). As the resistance of the thermistor changes with temperature, it affects the voltage at the output.
Current Divider Configuration: In a current divider circuit, the thermistor is connected in parallel with a fixed resistor (or a set of fixed resistors). A constant current source is passed through the circuit. The voltage across the thermistor is measured, and it changes as the resistance of the thermistor varies with temperature.
Temperature Calculation: The output voltage or the voltage across the thermistor is measured using an analog-to-digital converter (ADC) or a microcontroller. By using the resistance-temperature curve provided by the manufacturer, the corresponding temperature can be determined.
Calibration: In some cases, calibration may be required to ensure accuracy. This involves comparing the output of the thermistor-based temperature measurement with a known reference temperature to make necessary adjustments or corrections.
Thermistors are commonly used in various applications where temperature sensing is required, such as in thermostats, industrial control systems, automotive systems, medical devices, and temperature-compensating circuits. Their small size, fast response time, and high sensitivity make them popular choices for temperature measurement and control.