A basic temperature sensor, also known as a thermistor, measures temperature based on its electrical resistance. A thermistor is a type of resistor whose resistance changes with temperature. The word "thermistor" comes from "thermally sensitive resistor."
There are two common types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). The NTC thermistor's resistance decreases as the temperature rises, while the PTC thermistor's resistance increases with temperature.
Here's how a basic thermistor measures temperature:
Resistance-Temperature Relationship: Thermistors are made from materials with a known and predictable resistance-temperature relationship. The resistance of the thermistor is characterized by a resistance value at a specific reference temperature (usually 25°C or 77°F) and a "Beta" value (β). The Beta value represents the rate of resistance change concerning temperature.
Connection in a Circuit: To measure temperature, the thermistor is connected in a simple electronic circuit. Typically, it is connected in series or parallel with a known resistor. The thermistor's resistance forms a voltage divider with the known resistor, creating a voltage across the thermistor.
Voltage Measurement: An analog-to-digital converter (ADC) or an operational amplifier (op-amp) circuit is used to convert the analog voltage across the thermistor into a digital value that represents the temperature.
Calibration: Before using the thermistor for temperature measurement, the system might require calibration. This calibration ensures that the output temperature values correspond accurately to the actual temperature range being measured. It involves comparing the sensor's output at specific reference temperatures to known and accurate temperature values.
Temperature Calculation: Once calibrated, the temperature is calculated using the resistance-temperature relationship for the specific thermistor being used. By comparing the thermistor's resistance value to the reference resistance value and the Beta value, the temperature can be determined.
It's essential to consider factors like self-heating effects and ambient temperature variations, which can influence the accuracy and response time of the thermistor-based temperature measurement system. Nevertheless, thermistors are widely used due to their simplicity, low cost, and reasonable accuracy in various applications, such as weather stations, thermostats, and temperature control systems.