[Guide]Choosing a temperature sensing product may seem like a trivial matter, but due to the variety of products available, this task can be daunting. In this blog post, the author will introduce four types of temperature sensors (resistance temperature detectors (RTD), thermocouples, thermistors, and integrated Circuit (IC) sensors with digital and analog interfaces) and discuss each The advantages and disadvantages of this sensor.
From a system-level standpoint, whether a temperature sensor is suitable for your application will depend on the required temperature range, accuracy, linearity, solution cost, function, power consumption, solution size, installation method (surface mount Method and through-hole insertion method and external
Circuit board installation method) It is also necessary to support the ease of design of the circuit.
RTD
When measuring the resistance of the RTD while changing its temperature, the response is almost linear, behaving like a resistor. As shown in Figure 1, the RTD’s resistance curve is not completely linear, but has a deviation of a few degrees (a straight line used as a reference is shown) – but it is highly predictable and repeatable. To compensate for this slight non-linearity, most designers digitize the measured resistance value and use a look-up table in the microcontroller to apply correction factors. This wide temperature range (approximately -250°C to +750°C) of repeatability and stability makes RTDs extremely useful in high-precision applications, including measuring the temperature of liquids or gases in pipes and large vessels.
Figure 1: RTD resistance and temperature
The complexity of the circuit used to process the RTD analog signal basically varies according to the application. Components such as amplifiers and analog-to-digital converters (ADCs), which produce their own errors, are indispensable. Only when the measurement is necessary to power the sensor-through this method you can also achieve low-power operation, but this will make the circuit much more complicated. Moreover, the power required to energize the sensor will increase its internal temperature, thereby affecting the accuracy of the measurement. With only a few milliamps of current, this self-heating effect will produce temperature errors (these errors are correctable, but require further consideration). Also, keep in mind: the cost of wirewound platinum RTDs or thin-film RTDs can be quite high, especially when compared to the cost of IC sensors.
Thermistor
Thermistor is another type of resistive sensor. There are a wide variety of available thermistors, ranging from high-quality and inexpensive products to high-precision products. Low-cost, low-precision thermistors can perform simple measurement or threshold detection functions-these resistors require multiple components (such as comparators, references, and discrete resistors), but they are very cheap and have non-linear characteristics. The linear resistance-temperature properties are shown in Figure 2. If you need to measure a wide range of temperatures, you will need to perform a lot of linearization work. It may be necessary to calibrate several temperature points. To achieve higher accuracy, more expensive and tighter tolerance thermistor arrays can be used to help solve this nonlinear problem, but such arrays are usually less sensitive than a single thermistor.
Figure 2: Resistance and temperature of the thermistor
Because multi-trip point systems increase complexity and cost, low-cost thermistors are generally only used in applications with minimal functional requirements, including toasters, coffee makers, refrigerators, and hair dryers. In addition, thermistors suffer from self-heating problems (usually at higher temperatures, when their resistance is lower). As in the case of RTDs, the fundamental reason why the thermistor cannot be used under low power supply voltage has not yet been discovered-but remember, the lower the full-scale output, the system directly converted into a system based on the characteristics of the analog-to-digital converter (ADC) The lower the sensitivity. Low-power applications also need to increase the circuit complexity in order to be very sensitive to errors caused by noise. Thermistors can operate in a temperature range of -100°C to +500°C, although most thermistors are rated for a maximum operating temperature range of +100°C to +150°C.
Thermocouple
A thermocouple consists of the junction of two wires made of different materials. For example, J-type thermocouples are made of iron and constantan. As shown in Figure 3, contact 1 is located at the temperature to be measured, while contact 2 and contact 3 are placed at different temperatures measured by the LM35 analog temperature sensor. The output voltage is roughly proportional to the difference between these two temperature values.
Figure 3: Using LM35 for thermocouple cold junction compensation
Because the sensitivity of thermocouples is quite low (on the order of tens of microvolts per degree Celsius), you will need a low-offset amplifier to produce a usable output voltage. Within the operating range of thermocouples, non-linearities in the temperature-to-voltage transfer function often require compensation
Circuits or look-up tables, just like RTDs and thermocouples. However, despite these shortcomings, thermocouples are still very popular, especially suitable for ovens, water heaters, kilns, test equipment and other industrial processes-because the thermal mass of thermocouples is very low and the operating temperature range (the operating temperature can be Extend to above 2300℃) is very broad.
IC sensor
IC sensors can work in a temperature range of -55°C to +150°C-selected several IC sensors can work at temperatures up to +200°C. There are various types of integrated IC sensors, but the four most common integrated IC sensors are undoubtedly analog output devices, digital interface devices, remote temperature sensors, and those integrated IC sensors (temperature switches) that have a thermostat function. Analog output devices (usually voltage outputs, but some also have current outputs) are most like passive solutions when they need an ADC to digitize the output signal. Digital interface devices most often use a two-wire interface (I2C or PMBus) and have a built-in ADC.
In addition to including a local temperature sensor, remote temperature sensors also have one or more inputs to monitor remote diode temperature-they are most often placed in highly integrated digital ICs (for example, processors or field programmable gate arrays[FPGA】)middleWhenthetemperaturethresholdisreachedthethermostatcanprovideasimplealarm
There are many advantages to using IC sensors, including: low power consumption; small packaged products (some sizes as small as 0.8mm×0.8mm) can be provided; and low device costs can also be achieved in some applications. In addition, since IC sensors are calibrated during production testing, there is no need for further calibration. They are commonly used in fitness tracking applications, wearable products, computing systems, data loggers, and automotive applications.
Experienced circuit board designers will use the most suitable solution according to the requirements of the final product. Table 1 shows the relative advantages/disadvantages of each temperature sensor.
