Exhibition

Home > Exhibition > Content
RTD, thermocouple, thermistor, IC sensor advantages and disadvantages of comparison
Dec 26, 2017

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

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 describe four types of 

temperature sensors (resistive temperature detectors (RTDs), thermocouples, thermistors, and 

integrated circuit (IC) sensors with digital and analog interfaces) and discuss each Advantages 

and Disadvantages of Sensors.


From a system-level standpoint, the temperature sensor is suitable for your application will 

depend on the required temperature range, accuracy, linearity, solution cost, function, power, 

solution size, mounting method (surface mount Method and through-hole plug-in method and circuit

 board installation method) is also necessary to support the circuit easy to design.

RTD


While 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 resistance curve of the RTD is not 

fully linear, but has a few degrees of deviation (a straight line is shown for reference) - but

 highly predictable and retestable. To compensate for this slight non-linearity, most designers 

digitize the measured resistance value and use the look-up table in the microcontroller to apply 

the correction factor. The repeatability and stability in this wide temperature range (about 

-250 ° C to + 750 ° C) makes the RTD extremely useful in high-precision applications, including 

the measurement of liquid or gas temperatures in pipes and large vessels.

The complexity of the circuitry used to process the RTD analog signal varies substantially depending on the application. Amplifiers and analog-to-digital converters (ADCs), which produce their own errors, are indispensable. Power is supplied to the sensor only when the measurement is necessary - this way you can also achieve low power operation, but this makes the circuit much more complex. Moreover, the power required to power the sensor will also increase its internal temperature, thus affecting the measurement accuracy. This self-heating effect produces a temperature error (which is correctable but requires further consideration) due to only a few milliamperes of current. Also, keep in mind that wire-wound platinum RTDs or thin-film RTDs can be quite costly, especially when compared to the cost of IC sensors.

Thermistor


Thermistors are another type of resistive sensor. There are a variety of available thermistor, from

 inexpensive products to high-precision products, and so forth. 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 have

 non-reactive Linear resistance-temperature properties, as shown in Figure 2. If you need to 

measure a wide range of temperatures, you will need to do a lot of linearization work. It may be 

necessary to calibrate several temperature points. To achieve higher accuracy, more expensive and 

more thermally tolerable thermistor arrays can be used to help solve this nonlinear problem, but 

such arrays are generally less sensitive than a single thermistor.


Because multi-trip point systems add complexity and cost, low-cost thermistors are typically used

 only for applications with minimal functional requirements, including toasters, coffee makers, 

refrigerators, and hairdryers. In addition, the thermistors are subject to self-heating problems 

(usually at higher temperatures, where their resistance is lower). As with the RTD, the root cause

 of the inability to use a thermistor at low supply voltages has not yet been found - but remember

 that the lower the full-scale output, the more directly it is converted to a system based on the

 characteristics of an analog-to-digital converter The lower the sensitivity. Small-power 

applications also require increased circuit complexity to be sensitive to noise-induced errors. 

Thermistors can be operated over the 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


The thermocouple includes the contacts of two wires made of different materials. For example, 

J-type thermocouple is made of iron and constantan. As shown in Fig. 3, the contact 1 is located 

at the temperature to be measured, while the contact 2 and the contact 3 are placed at different 

temperatures as measured with the LM35 analog temperature sensor. The output voltage is

 approximately proportional to the difference between these two temperature values.



Figure 3: Using the LM35 for thermocouple cold junction compensation


Because thermocouples have very low sensitivity (on the order of tens of microvolts per degree

 Celsius), you will need low-offset amplifiers to produce usable output voltages. In the operating

 range of the thermocouple, the non-linearity in the temperature-to-voltage transfer function 

often requires a compensation circuit or look-up table, as is the RTD and thermocouple. However,

 despite these drawbacks, thermocouples are still very popular, especially for ovens, water heaters

, kilns, test equipment and other industrial processes - because of the low thermal mass of the 

thermocouple and the operating temperature range Extended to 2300 ℃ or more) is very broad.


IC sensor


The IC sensor operates over a temperature range of -55 ° C to + 150 ° C - several IC sensors are

 available for operation 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 the most passive solutions when they require an ADC to digitize the output signal. Digital 

Interface Devices Two-wire interfaces (I2C or PMBus) are most commonly used and have a built-in 

ADC.


In addition to including a local temperature sensor, the remote temperature sensor also has one or

 more inputs to monitor the remote diode temperature - they are most often placed in a highly 

integrated digital IC such as a processor or field programmable gate array 】)in. The thermostat 

provides a simple alarm when the temperature threshold is reached.


The use of IC sensors offers a number of benefits, including: low power consumption; small package

 sizes (some small 0.8mm x 0.8mm); and low device cost for 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 track applications, wearable products, computing systems, data 

loggers and automotive applications.


Experienced board designers will use the most appropriate solution based on the end product

 requirements. Table 1 shows the relative strengths / weaknesses of each temperature sensor.

Table 1: RTD, thermistor, thermocouple and IC sensor relative advantages and disadvantages




Add: No. 16, Road 3, Sanjiang Industry Zone, Shengzhou, Zhejiang, China

Contact:Boris Yue      

Email: boris@benoelectric.com

Contact:Charles Yu     

Email: charles@benoelectric.com

Tel: +86-575-83703068

Mob: +8613456528940

Fax: +86-575-83539727

Skype: live:borisyuecn

Wechat & Whatsapp: +8613456528940









Copyright © Shengzhou Beno Electric Appliance Co.,Ltd All Rights Reserved.Tel: +86-575-83539729