The Apple iPhone brought the benefits of integrated multi-sensor technology to the masses, and while the application of sensors and their associated algorithms has multiplied and changed the world forever, the fundamentals of the main sensor types and how they work has not. If you've forgotten those fundamentals, are new to sensor applications, or just need a handy list for reference, this article offers a review of major sensor types and applications.
Temperature sensors
Temperature is the most common of all physical measurements. We have temperature measurement-and-control units, called thermostats, in our home heating systems, refrigerators, air conditioners, and ovens. Temperature sensors are used on circuit boards, as part of thermal tests, in industrial controls, and in room controls such as in calibration labs and data centers. Though there are many types of temperature sensors, most are passive devices: Thermocouples, RTDs (resistance temperature detectors), and thermistors.
Thermocouples
(T/Cs) are the most common type of sensor because they don’t require an excitation signal. They consist of two wires made of dissimilar metals joined at the point of measurement. Based on the Seebeck effect, T/Cs operate on the premise that each metal develops a voltage differential across its length based on the type of metal and the difference in temperature between the ends of the wire. By using two metals, you get two different voltages V1 and V2. The difference (VT) represents temperature. Note that there is no voltage across the thermocouple junction. You will often hear that a thermocouple develops a voltage across the junction, which is incorrect. The voltage is developed over the length of each wire.
Thermocouples are designated using letters. For example, a Type-J T/C has iron and constantan (a copper-nickel alloy) wires. Most thermocouple wire is color coded.
Thermocouples require that the far ends of the wire be at the same temperature and that temperature must be known. Thus, instruments that use thermocouples will have an isothermal block with an embedded sensor to measure the temperature at that point. This is called cold-junction compensation. With one end of the wires at an equal and known temperature, a circuit can measure VT and calculate the unknown temperature.
Thermocouple curves are nonlinear and thus require linearization. That can be done in hardware of software, tho mostly in software with today's digital instruments through an equation or reference table.
Thermocouples are common because of their wide temperature range (type J can run up to 760°C), low cost, robustness, and simple signal-conditioning circuit. Wires can be run over long distances with proper shielding because the voltages are in microvolts/°C. They're often used in industrial applications such as ovens and furnaces.
Resistance-temperature detectors (RTDs) have a smaller range, typically a few hundred degrees Centigrade, but they have better accuracy and resolution than thermocouples. RTDs use precision wire, usually made of platinum, as the sense element. The element needs a known excitation current, typically 1mA. RTDs come in two-, three-, and four-wire configurations. Four-wire configurations, usually used as reference probes in calibration labs, have the best accuracy because two wires carry current and two are used for measuring the resistance across the element.
RTDs are specified with a base resistance, typically 100.0Ω at 0°C for platinum wire, and a resistance slope. For example a so-called 385 Pt100 RTD has a slope of 0.00385Ω/Ω/°C from 0°C to 100°C. At 100°C, the resistance is 138.5Ω. For applications between 0°C and 100°C, RTDs may be considered linear. RTDs are often used in regulated industries such as food processing where the temperature ranges aren’t as wide as for thermocuples, but a higher accuracy is needed.
Because RTDs produce resistance as a function of temperature, the instrumentation often uses them in bridge circuits to maximize resolution. From there, the bridge output is digitized and linearized in software.
Thermistors are also resistance-based temperature sensors, but their resistance/temperature curve has a negative slop and is highly nonlinear. But, they produce a higher change in resistance for a given change in temperature than RTDs. They're often used where the highest resolution is needed, though over a relatively narrow temperature range. As a result, thermistors are often used in medical devices, home thermostats, and machines. Engineers often use thermistors to measure temperature in circuit such as power supplies.
Thermistors produce a significantly higher resistance than RTDs, typically 2000Ω to 10,000Ω. Thus, they can operate at a significantly smaller excitation current, which reduces loss in wires. As a result, thermistors are often used in two-wire circuits.
Some temperature applications:
Soldering iron driver bridge controls temperature
Simple diode serves as a sensor for a thermal probe
Platinum-RTD-based circuit provides high performance with few components
