Comparison of advantages and disadvantages of RTD, thermocouple, thermistor and IC sensor

Choosing a temperature sensing product may seem like a trivial matter, but because of the variety of products available, this task can be daunting. In this article, I will introduce four types of temperature sensors (resistive temperature detectors (RTDs), thermocouples, thermistors, and integrated circuit (IC) sensors with digital and analog interfaces) and discuss each The advantages and disadvantages of the sensor.

From a system-level standpoint, the suitability of the temperature sensor for your application will depend on the temperature range, accuracy, linearity, solution cost, function, power consumption, solution size, and mounting method (surface mount). Method and through-hole interpolation and off-board mounting methods are also necessary to support the ease of design of the circuit.

RTD

When the temperature of the RTD is measured while measuring 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 few degrees of deviation (showing a straight line used as a reference) - but it is highly predictable and retestable. To compensate for this slight nonlinearity, most designers digitize the measured resistance values ​​and use a lookup table within the microcontroller to apply the correction factor. The reproducibility and stability in this wide temperature range (approximately -250°C to +750°C) makes RTD extremely useful for high precision applications, including measuring the temperature of liquids or gases in pipes and large vessels.

Figure 1: Resistance and temperature of the RTD

The complexity of the circuitry used to process the RTD analog signal varies substantially depending on the application. Components such as amplifiers and analog-to-digital converters (ADCs) that generate their own errors are indispensable. The sensor is only powered when the measurement is necessary - this way you can also achieve low-power operation, but this makes the circuit much more complicated. Moreover, the power required to energize the sensor also increases the temperature inside it, which affects measurement accuracy. With just a few milliamps of current, this self-heating effect produces temperature errors (these errors are correctable, but further considerations are needed). Also, keep in mind that the cost of wire-wound platinum RTDs or thin film RTDs can be quite high, especially when compared to the cost of IC sensors.

Thermistor

Thermistors are another type of resistive sensor. There are a wide variety of thermistors available, ranging from affordable products to high-precision products. Low-cost, low-precision thermistors perform simple measurements or threshold detection – these resistors require multiple components (such as comparators, reference and discrete resistors) but are very inexpensive and non-volatile Linear resistance-temperature properties, as shown in Figure 2. If you need to measure a wide range of temperatures, you will need a lot of linearization work. Calibration of several temperature points may be necessary. To achieve higher accuracy, a more expensive and tighter thermistor array can be used to help solve this nonlinear problem, but such arrays are typically less sensitive than a single thermistor.

Figure 2: The resistance and temperature of the thermistor

Because multi-hop point systems add complexity and cost, low-cost thermistors are typically only used for 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 where their resistance is lower). As in the case of the RTD, the root cause of the inability to use the thermistor at low supply voltages has not been discovered - but keep in mind that the lower the full-scale output, the system that is directly converted to the ADC based on the analog-to-digital converter (ADC) characteristics. The lower the sensitivity. Low-power applications also need to increase circuit complexity so that they are very sensitive to noise-induced errors. Thermistors operate over 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

Thermocouples include joints of two wires made of different materials. For example, the J-type thermocouple is made of iron and constantan. As shown in Figure 3, junction 1 is at the temperature to be measured, while junction 2 and junction 3 are placed at different temperatures as measured by the LM35 analog temperature sensor. The output voltage is roughly proportional to the difference between the two temperature values.

Figure 3: Using the LM35 for Thermocouple Cold Junction Compensation

Because the sensitivity of the thermocouple is quite low (on the order of tens of microvolts per degree Celsius), you will need a low offset amplifier to produce the available output voltage. In the working range of thermocouples, the nonlinearity in the temperature-to-voltage transfer function often requires a compensation circuit or look-up table, just like an RTD and a thermocouple. However, despite these shortcomings, thermocouples are still very popular, especially for ovens, water heaters, kilns, test equipment and other industrial processes – because the thermocouples have very low thermal mass and operating temperature range (operating temperature can be Expanded to 2300 ° C or more) is very broad.

IC sensor

IC sensors operate over the -55°C to +150°C temperature range – several selected IC sensors can operate up to +200°C. There are various types of integrated IC sensors, but the four most common integrated IC sensors are analog output devices, digital interface devices, remote temperature sensors, and integrated IC sensors (temperature switches) with thermostat functions. Analog output devices (typically voltage outputs, but some also have current outputs) are most like passive solutions when they require an ADC to digitize the output signal. Digital interface devices most often use a two-wire interface (I2C or PMBus) with a built-in ADC.

In addition to including a local temperature sensor, the remote temperature sensor has one or more inputs to monitor remote diode temperatures - they are most often placed in highly integrated digital ICs (eg, processor or field programmable gate arrays [FPGA] 】)in. The thermostat provides a simple alarm when the temperature threshold is reached.

There are many benefits to using IC sensors, including: low power consumption; small package sizes (some sizes as small as 0.8mm & TImes; 0.8mm); low device cost in some applications. In addition, since the IC sensor is calibrated during the production test, no further calibration is necessary. They are commonly used in fitness tracking applications, wearable products, computing systems, data loggers, and automotive applications.

Experienced board designers will use the most appropriate solution based on the final product requirements. Table 1 shows the relative advantages/disadvantages of each temperature sensor.

Table 1: Relative advantages and disadvantages of RTDs, thermistors, thermocouples, and IC sensors

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