Overview
Temperature sensors are among the most commonly used sensors. Their accuracy and response speed vary, and they are widely used in computers, automobiles, kitchen appliances, air conditioners, and household thermostats.
Currently, the four most common types of temperature sensors are:
- Thermistors
- Thermocouples
- RTDs (resistance temperature detectors)
- Semiconductor-based sensors
1. Thermistors
A thermistor is a temperature-sensing device whose resistance is a function of temperature.
There are two types of thermistors: PTC (positive temperature coefficient) and NTC (negative temperature coefficient). A PTC thermistor's resistance increases with rising temperature. Conversely, an NTC thermistor's resistance decreases as temperature rises; NTC types are the most commonly used thermistors. See Figure 1 below.
Figure 1. Electrical symbols for PTC and NTC thermistors
Note that the relationship between a thermistor's resistance and temperature is highly nonlinear. See Figure 2 below.
Figure 2. Resistance versus temperature for an NTC thermistor
The standard equation for NTC thermistor resistance versus temperature is:
R at 25°C is the nominal resistance of the thermistor at room temperature (25°C). This value is typically provided in the datasheet.
2. Thermocouples
Thermocouples are commonly used to measure higher temperatures and wide temperature ranges.
A thermocouple is formed by electrically connecting two wires made of different metals at two junctions. The voltage generated between the two different metals varies proportionally with temperature. Thermocouples are nonlinear, and conversions for temperature control and compensation typically use lookup tables. Accuracy is relatively low, ranging from about 0.5°C to 5°C, but thermocouples have the widest operating temperature range, from about -200°C to 1750°C.
3. Resistance Temperature Detectors (RTDs)
Resistance temperature detectors, also called resistance thermometers, are similar to thermistors in that their resistance changes with temperature. However, RTDs are made from wound wire on a core of ceramic or glass rather than special materials chosen for high temperature sensitivity.
RTD wires are made from pure materials, typically platinum, nickel, or copper, and these materials have precise resistance-to-temperature relationships used to determine measured temperature.
4. Semiconductor-based Temperature Sensors (Analog Temperature ICs)
Semiconductor-based temperature sensors are usually integrated into integrated circuits. These sensors often use two identical diodes whose voltage-to-current characteristics are temperature sensitive to monitor temperature changes. They provide a linear response but generally offer the lowest accuracy among the basic sensor types. Their operating temperature range is narrower, typically about -70°C to 150°C.
Importance of Temperature Sensors
Temperature sensors play a critical role in everyday life. These technologies measure the heat emitted by objects or systems, and the measured values allow us to detect temperature changes physically. One important function of temperature sensors is protection: they detect when a set threshold is reached so preventive actions can be taken.
