Choosing temperature sensing products may seem like a small matter, but due to the wide variety of available products, this task can be quite daunting. In this blog post, the author will introduce four types of temperature sensors (resistance temperature detectors (RTDs), thermocouples, thermistors, and integrated circuit (IC) sensors with digital and analog interfaces) and discuss the advantages and disadvantages of each sensor.
The reference address for this article is: http://www.eepw.com.cn/article/201601/285709.htm
From a system level perspective, whether a temperature sensor is suitable for your application will depend on the required temperature range, accuracy, linearity, solution cost, functionality, power consumption, solution size, installation method (surface mount method, through-hole insertion method, and circuit board external installation method), as well as the ease of circuit design support.
When measuring the resistance of an RTD while changing its temperature, the response is almost linear and behaves like a resistor. As shown in Figure 1, the resistance curve of the RTD is not completely linear, but has a deviation of several degrees (a straight line is shown as a reference) - but it is highly predictable and verifiable. To compensate for this slight nonlinearity, most designers digitize the measured resistance values and use a lookup table within the microcontroller to apply correction factors. The repeatability and stability within a wide temperature range (approximately -250 ℃ to+750 ℃) make RTDs extremely useful in high-precision applications, including measuring the temperature of liquids or gases in pipelines and large containers.
The complexity of circuits used to process RTD analog signals generally varies depending on the application. Components such as amplifiers and analog-to-digital converters (ADCs), which generate their own errors, are essential. Only power the sensor when necessary for measurement - this method can also achieve low-power operation, but it will make the circuit much more complex. Moreover, the power required to power on the sensor will also increase its internal temperature, thereby affecting measurement accuracy. With only a few milliamps of current, this self heating effect can generate temperature errors (which can be corrected, but require further consideration). Additionally, please note that the cost of wire wound platinum RTDs or thin-film RTDs may be quite high, especially when compared to the cost of IC sensors.
Thermistor
Thermistors are another type of resistive sensor. There are various types of thermistors available, ranging from cost-effective 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 are very inexpensive and have nonlinear resistance temperature properties, as 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 non-linear problem, but such arrays are usually less sensitive than a single thermistor.
Figure 2: Resistance and temperature of thermistor
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 contacts 2 and 3 are placed at different temperatures measured by an 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 generate a usable output voltage. Within the working range of thermocouples, nonlinearity in the temperature to voltage transfer function often requires compensation circuits or lookup tables, just like RTDs and thermocouples. However, despite these drawbacks, thermocouples are still very popular, especially for ovens, water heaters, kilns, testing equipment, and other industrial processes - due to their low thermal mass and wide operating temperature range (which can be extended to over 2300 ℃).
IC sensor
IC sensors can operate within a temperature range of -55 ° C to+150 ° C - selected IC sensors can operate at temperatures up to+200 ° C. There are various types of integrated IC sensors, but the four most common types of integrated IC sensors are analog output devices, digital interface devices, remote temperature sensors, and integrated IC sensors with temperature control functions (temperature switches). Analog output devices (usually voltage output, but some also have current output) are most like passive solutions when they require an ADC to digitize the output signal. Digital interface devices most commonly use two-wire interfaces (I2C or PMBus) and have built-in ADCs.
In addition to including a local temperature sensor, remote temperature sensors also have one or more inputs to monitor the temperature of remote diodes - they are most commonly placed in highly integrated digital ICs (such as processors or field programmable gate arrays [FPGAs]). When the temperature threshold is reached, the thermostat can provide a simple alarm.
There are many benefits to using IC sensors, including low power consumption; We can provide small packaged products (some sizes as small as 0.8mm × 0.8mm); Low device cost can also be achieved in certain 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 based on the final product requirements. Table 1 shows the relative advantages/disadvantages of each temperature sensor.
In the coming months, the author and my colleagues will write more blogs on basic temperature knowledge, and we welcome your continued attention. If you would like us to discuss any topic, please provide your opinion below to let us know.