A temperature sensor is a device that detects temperature and produces a measurable output corresponding to thermal conditions. It may measure the temperature of air, liquids, solids, machinery, electronic components, biological systems or other environments. Temperature sensors use different physical principles, including changes in electrical resistance, semiconductor junction behaviour, thermoelectric effects and infrared radiation. Common examples include thermistors, resistance temperature detectors, thermocouples, semiconductor temperature sensors and infrared temperature sensors.
Some provide an analogue electrical output, while others contain integrated electronics and communicate a digital temperature value. Temperature sensors are widely used because temperature affects physical processes, chemical reactions, biological systems, mechanical dimensions, semiconductor behaviour and electronic reliability. Measurement may be required for monitoring, control, safety, calibration, process optimisation or fault detection.
Thermistors measure temperature through a strong temperature dependence of electrical resistance. Negative-temperature-coefficient thermistors generally decrease in resistance as temperature rises, while positive-temperature-coefficient thermistors can increase sharply over particular temperature ranges. Their sensitivity can be high, making them useful for detecting relatively small temperature changes, but their response is nonlinear and therefore often requires mathematical conversion or calibration. A resistance measurement can be obtained by applying a known current or placing the thermistor in a voltage-divider circuit. The resulting voltage is related to resistance and therefore temperature. Thermistors are common in appliances, battery packs, heating systems, automotive equipment and electronic devices because they are relatively small and inexpensive. Their accuracy depends on the thermistor's characteristics, circuit design, calibration and operating range.
Resistance temperature detectors, commonly called RTDs, use the predictable change in resistance of a material with temperature. Platinum is widely used because of its stable and relatively reproducible characteristics, with devices such as Pt100 and Pt1000 representing commonly encountered nominal resistance values at a reference temperature. RTDs are valued for accuracy, stability and repeatability in industrial and laboratory measurement. Because the resistance change can be relatively small, measurement circuits often use bridge configurations, precision current sources and differential instrumentation. Lead resistance can introduce errors, which is why two-wire, three-wire and four-wire connection arrangements have different accuracy characteristics. RTDs generally provide excellent measurement quality but can cost more and respond more slowly than some small thermistors.
Thermocouples operate using the thermoelectric effect. When two dissimilar conductors form junctions at different temperatures, a voltage is produced that is related to the temperature difference between the junctions. Because a thermocouple inherently measures a temperature difference, the measurement system must account for the temperature at the reference or cold junction, commonly through electronic cold-junction compensation. Different thermocouple types use different metal combinations and have different temperature ranges, sensitivities and environmental characteristics. Thermocouples can operate across very broad temperature ranges and can be physically robust, making them useful in furnaces, engines, industrial processes and scientific equipment. Their output voltage is relatively small, so amplification and careful noise management are often necessary.
Semiconductor temperature sensors exploit the temperature dependence of semiconductor electrical characteristics. Integrated temperature sensors can use transistor junction behaviour and associated circuits to generate an analogue voltage, current or digital measurement. Digital temperature sensors may include the sensing element, analogue circuitry, analogue-to-digital conversion, calibration and communication interface within a single integrated circuit. They can communicate with a microcontroller using standard digital buses, simplifying system design.
Semiconductor sensors are particularly useful for monitoring electronics because they can be physically small and integrated close to processors, power devices or other components. Their operating temperature range may be more limited than that of thermocouples or specialised RTDs, so selection depends on the required environment and accuracy.
Non-contact temperature sensing can be performed using infrared radiation. Objects emit electromagnetic radiation according to their temperature, with significant thermal radiation often occurring in the infrared region for ordinary environmental temperatures. An infrared sensor can detect this radiation and estimate the object's temperature without physical contact. Thermal cameras extend this principle by using arrays of detectors to create spatial temperature maps.
Non-contact measurement is useful when the target is moving, extremely hot, electrically isolated, contaminated, inaccessible or likely to disturb a contact sensor. However, infrared measurement depends on factors such as emissivity, distance, field of view, atmospheric absorption and reflections from surrounding objects. The sensor may therefore measure radiative conditions rather than simply reading an absolute temperature without assumptions.
Temperature sensors are used in control and protection systems throughout technology. Thermostats use temperature measurements to control heating and cooling, while computers and power electronics monitor temperatures to prevent overheating or adjust operating behaviour. Batteries use temperature sensing for charging control, safety and performance management. Industrial processes use temperature measurements to regulate furnaces, chemical reactions, food processing and manufacturing operations. Vehicles use them for engine management, exhaust monitoring, coolant measurement and thermal control. Medical equipment and environmental monitoring systems also depend on accurate temperature measurements. In many applications, the sensor is part of a feedback loop in which the measured temperature is compared with a desired value and a controller adjusts heating, cooling or system operation accordingly.
No temperature sensor measures temperature with perfect certainty. Accuracy can be affected by calibration, self-heating, thermal contact, heat conduction through mounting structures, electrical noise, response time and environmental gradients. A sensor mounted on a circuit board may measure the local board temperature rather than the temperature of nearby air or a particular component unless the thermal design is carefully considered.
A thermistor may provide high sensitivity but nonlinear output, an RTD may provide excellent stability but require more precise measurement circuitry, a thermocouple may tolerate very high temperatures but produce a small signal, and an infrared sensor may permit non-contact measurement but require emissivity considerations. The correct choice therefore depends on temperature range, required accuracy, response time, contact conditions, environmental exposure, physical size, cost and interface requirements.
