A thermistor is a type of temperature sensor that measures temperature by exploiting the principle of changes in electrical resistance with temperature. The term "thermistor" is a combination of "thermal" and "resistor," which reflects its functionality. Thermistors are made from semiconductor materials with a high temperature coefficient of resistance (TCR), meaning their electrical resistance changes significantly with temperature variations.
There are two main types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). Here's how they work:
NTC Thermistor (Negative Temperature Coefficient):
In an NTC thermistor, as the temperature increases, its electrical resistance decreases. This is because the semiconductor material used in the thermistor becomes more conductive at higher temperatures. Conversely, as the temperature decreases, the resistance increases.
The relationship between temperature and resistance in an NTC thermistor is generally described by the Steinhart-Hart equation or a simplified form of it. This equation is not linear and requires calibration to accurately convert resistance readings into temperature values.
PTC Thermistor (Positive Temperature Coefficient):
In a PTC thermistor, the relationship between temperature and resistance is opposite to that of an NTC thermistor. As the temperature increases, the resistance of the PTC thermistor also increases. This behavior is due to the specific properties of the semiconductor material used in PTC thermistors.
To measure temperature using a thermistor, you would typically set up a simple electrical circuit. The thermistor is connected in series with a known resistor, forming a voltage divider circuit. A voltage is applied across the circuit, and the output voltage is taken from the junction between the thermistor and the resistor. As the temperature changes, the resistance of the thermistor changes, altering the voltage division ratio and resulting in a change in the output voltage.
By measuring the output voltage and knowing the characteristics of the thermistor (such as its resistance at a reference temperature), you can infer the temperature using a calibration curve or mathematical equations specific to the type of thermistor you're using.
Keep in mind that thermistors have some limitations, including nonlinearity, self-heating effects, and a limited temperature range. Calibration and compensation techniques are often used to improve the accuracy of temperature measurements taken using thermistors.