We all use thermocouples, RTDs, infrared sensors, and thermistors every day, but have you ever wondered why these temperature sensors accurately measure temperature? It's simply because they are designed for precise measurements and temperature regulations in HVAC (Heating, Ventilation, and Air Conditioning) systems.
We all use thermocouples, RTDs, infrared sensors, and thermistors every day, but have you ever wondered why these temperature sensors accurately measure temperature? It's simply because they are designed for precise measurements and temperature regulations in HVAC (Heating, Ventilation, and Air Conditioning) systems.
But actually, there is more to that. In this article, we'll discuss the different types of temperature sensors available, including their working principles and differences.
Figure 1. different types of temperature sensors Image source: gisli.mx
A temperature sensor, also known as a transducer, is a device that converts the heat in its surroundings into a measurable signal. To elaborate more on this general definition, let's take a look at the diagram below.
Figure 2. thermocouple measuring the temperature of a system Image source: youtube.com
This illustration demonstrates that a thermocouple is an example of a temperature sensor. The thermocouple is responsible for transforming a temperature into an electric signal. Based on different categories, the signal may have different electrical ranges. However, for industrial applications, the signals are usually limited to some globally accepted electrical signal ranges, such as 4-20mA, 1-5v, and 0-10v.
How is that possible? This is made possible with a temperature transmitter, which sends the sensor's output to an appropriate signal range.
There are many different types of temperature sensors, each with its own advantages and disadvantages. Some of the most common types of temperature sensors are:
Thermocouples
Resistance temperature detectors(RTDs)
Thermistors
Infrared temperature sensors
Figure 3. ttype K thermocouple Image source:onetemp
A thermocouple is a temperature sensor that generates a voltage proportional to the temperature difference between two junctions of different metals. These sensors are found in automotive industries, chemical manufacturing plants, and even in gas turbines.
Figure 4. overview of thermocouple working principle Image source:engineeringconcepts.com
The thermocouple operates based on the Seebeck effect, which was discovered by Thomas Johann Seebeck in . The Seebeck effect occurs when two different metals are joined at one end to form a junction. This junction experiences a temperature difference, then generates a voltage that is directly proportional to its temperature difference.
The generated voltage, also known as the Seebeck voltage, is measured across the open ends of the thermocouple. By calibrating the relationship between the voltage and temperature, the thermocouple provides an accurate temperature reading.
There are different types of thermocouples, each made from specific metal combinations. Some common types include type J, type K, and type T, which use different combinations of metals like iron, nickel, and copper.
Thermocouples can be used in diverse ways, but here are some of their primary uses:
They are used to measure the temperature range of -200°C to +°C.
They are used to measure exhaust gas temperature in vehicles.
They are used to determine the temperature of HVAC systems.
Figure 5. labelled structure of an RTD Image source:instrumentationtoolbox.com
RTDs are widely recognized for their precision in temperature measurement. It is a type of temperature sensor that measures temperature by correlating the resistance of a metal with temperature changes. RTDs are also known for their high accuracy and stability, making them suitable for applications that require precise temperature measurements.
Figure 6. overview of RTD working principle Image source:engineeringconcepts.com
RTDs are typically made from pure metals, with platinum being a popular choice due to its stable and linear relationship between resistance and temperature. As the temperature changes, the resistance of the metal in the RTD changes proportionally. This is because the RTD is connected to a circuit that supplies a specific amount of electric current. During this process, a voltmeter is used to measure the voltage drop across the RTD, and the formula below is used to calculate the corresponding temperature:
R= R0(1 + α ΔT)
In this formula, α stands for the temperature coefficient of resistance, R0 stands for resistance at 0°C, and ΔT stands for the change in temperature. The most common types of RTDs include PT100, PT500, and PT1,000.
RTDs have helped scientists in many ways, making their work much easier. Here are some relevant applications of RTDs:
They are used to measure the temperature range of -200°C to +500°C.
They are used as medical sensors to monitor the temperature of medical equipment.
Figure 7. symbols of NTC and PTC thermistors Image source:hoidla
A thermistor, short for 'thermal resistor', is a type of temperature sensor that exhibits significant changes in electrical resistance in response to temperature changes. Thermistors are known for their sensitivity, making them useful for applications where accurate temperature measurements are required.
Figure 8. overview of thermistor working principle Image source:slideshare.net
The thermistor operates based on the principles of inverse and direct proportionality between two physical quantities: temperature and resistance. Most thermistors fall into two categories: NTC and PTC. In NTC thermistors, the temperature is inversely proportional to resistance, meaning resistance decreases as temperature increases, and vice versa.
NTC thermistors are commonly used in applications that require high sensitivity to temperature changes. On the other hand, PTC thermistors work on the principle of direct proportionality because their resistance increases with temperature.
Figure 9. graph of NTC and PTC thermistors Image source:ametherm.com
Both NTC and PTC thermistors are the basic regulators of thermophysics. Here are some of their major applications:
They are used to measure the temperature range of 0°C to 100°C.
They are used to make thermostats in electronic circuits.
They are used to monitor and regulate the temperature in HVAC systems.
Figure 10. labelled structure of an IR sensor Image source:researchgate.net
An infrared temperature sensor, also known as an infrared thermometer or pyrometer, is a device that measures an object's temperature without making physical contact. It does so by detecting the infrared radiation emitted by the object, which is directly proportional to its temperature.
Figure 11. overview of how an IR sensor works Image source:easybom.com
An infrared sensor works by obeying the principle of black body radiation and Stefan-Boltzmann's law, which states that all objects with a temperature above absolute zero emit IR radiation with an intensity that is directly related to the object's temperature. The IR sensor consists of a lens that focuses its radiation onto a detector; the detector converts the IR radiation into an electrical signal.
The electrical signal is then processed by the sensor's electronics, which calculate the object's temperature based on the intensity of the detected radiation.
Having used an IR sensor for over a decade, I can attest to its versatility, demonstrated by its applications in:
Laboratories, to measure temperatures ranging from -50°C to +1,000°C.
Industrial settings, to control temperatures in manufacturing processes like metal and glass production.
Medical settings, for non-contact temperature measurements, particularly for patients in delicate conditions.
Integrated circuits, to detect overheating in electronic components.
The table below gives you a detailed comparison of the key features in thermocouples, thermistors, RTDs, and Infrared temperature sensors:
Features
Thermocouples
Thermistors
RTDs
IR temperature sensors
Working principle
Principle of Seebeck effect
Principle of inverse and direct proportionality
Principle of linear proportionality
Principle of black body radiation and Stefan Boltzmann's law
Temperature range
-200°C to +°C
0°C to 100°C
-200°C to +500°C
-50°C to +°C
Sensing material
Two different metals
Metal oxide
Single metal platinum material
Focus lens and detector
Accuracy
0.5°C to 5°C
0.1°C to 1.5°C
0.1°C to 1°C (highest)
0.1°C to 2°C
Speed of response
0.1 to 10mm/s
0.05 to 2.5mm/s
0.1 to 50mm/s
0.1 to 5mm/s
(Fastest)
Type of Transducer
Active transducer
Passive transducer
Passive transducer
Active transducer
Linearity
Non-linear
Exponential
Most linear
Non-linear
Cost
Least expensive
More expensive than thermocouples
Moderately expensive
Less expensive than RTDs
Temperature sensors are widely utilized in a variety of integrated circuits and electrical appliances. In this guide, we have explored the applications, differences, and working principles of:
If you're looking to purchase any of these temperature sensors, Quarktwin offers a wide selection and quick delivery. Simply click on the sensor that interests you from the list above, place your order, and we'll ensure it's delivered to you promptly!
This is the first article in a two-part series. Part 1 will first discuss the history and design challenges for a thermistor-based temperature measurement system and how it compares with a resistance temperature detector (RTD)-based temperature measurement system. It will also outline thermistor selection, configuration trade-offs, and the importance of sigma-delta analog-to-digital converters (ADCs) in this application space. Part 2 will detail how to optimize and how to evaluate the final thermistor-based measurement system.
As discussed in the article 'How to Select and Design the Best RTD Temperature Sensing System,' an RTD is a type of resistor with resistance that varies as a function of temperature. Thermistors operate in a similar manner to RTDs. Unlike RTDs that only have a positive temperature coefficient, thermistors can either have positive or negative temperature coefficients. A negative temperature coefficient (NTC) thermistor decreases its resistance as the temperature increases, while a positive temperature coefficient (PTC) thermistor increases its resistance as the temperature increases. Figure 1 shows the response characteristics of typical NTC and PTC thermistors and how they compare to an RTD curve.
Figure 1. The response characteristics of thermistors vs. RTD.
In terms of temperature range, the RTD curve is near linear, and the sensor covers a much wider temperature range (commonly '200°C to +850°C) than thermistors due to the thermistor's nonlinear (exponential) characteristics. RTDs are usually available in a well-known standardized curve while thermistor curves vary depending on the manufacturer. We will discuss this in detail in the Thermistor Selection Guide section of this article.
Thermistors are made up of composite materials, usually ceramic, polymer, or semiconductor (typically metal oxides), which are quite smaller and more inexpensive but not as rugged when compared with RTDs, which are made of a pure metal (platinum, nickel, or copper). Thermistors can detect changes in temperature much faster than RTDs, delivering faster feedback. Thus, thermistors are commonly used sensors in applications that require low cost, small size, faster response rate, higher sensitivity, and with restricted temperature range such as with monitoring electronic equipment, households and building controls, scientific laboratories, or the cold-junction compensation used for thermocouples in commercial or industrial applications.
In most cases, NTC thermistors rather than PTC thermistors are used in precision temperature measurement applications. There are a few available PTC thermistors that are used in overcurrent input protection circuits or as resettable fuses for safety applications. The resistance-to-temperature curve of a PTC thermistor exhibits a very small NTC region until its switching point (or Curie point) is reached, above which a dramatic increase in resistance of several orders of magnitude occurs within a span of a few degrees Celsius. So, during an overcurrent condition, the PTC thermistor will have a high amount of self-heating beyond the switching temperature and its resistance will increase dramatically, thereby resulting in a reduced current being input to the system and thereby preventing damage to occur. The switching point for a PTC thermistor is typically between 60°C and 120°C, which is not suitable for monitoring temperature measurements in a wide range application. This article focuses on NTC thermistors that can typically measure or monitor temperature from '80°C to +150°C. NTC thermistors are available with nominal resistances at 25°C, ranging from a few ohms to 10 M'. As shown in Figure 1, the resistance changes per degree Celsius is more significant for a thermistor vs. an RTD. The high sensitivity and high resistance values of thermistors make their front-end circuitry much simpler compared to RTDs as a thermistor does not need any special wiring configurations such as 3-wire or 4-wire to compensate for lead resistance. Thermistor design only uses a simple 2-wire configuration.
Table 1 shows the pros and cons of RTDs, NTC, and PTC thermistors.
Table 1. Thermistors vs. RTDs Parameters NTC Thermistors PTC Thermistors RTDs Temperature Range '80°C to +300°C 60°C to 120°C '200°C to +850°C Temperature Coefficient Negative Positive Positive Linearity Exponential Exponential Near linear Sensitivity High High Low Response Time Fast Fast Slow Excitation Required Required Required Self-Heating Yes Yes Yes Wiring Configuration 2-wire 2-wire 2-wire, 3-wire, 4-wire Cost Inexpensive to moderate Inexpensive Moderate to expensive Size Small Small MediumA high precision and accurate thermistor-based temperature measurement requires precise signal conditioning, analog-to-digital conversion, linearization, and compensation, as shown in Figure 2. Though the signal chain looks simple and straightforward, there are several complex factors involved that also impact overall system board size, cost, and performance. There are several integrated solutions available in ADI's precision ADC portfolio such as the AD-4/AD-8, which offer multiple benefits when designing a temperature system since most of the building blocks required in the application are built in. However, there are different challenges involved in designing and optimizing a thermistor-based temperature measurement solution.
Figure 2. A typical NTC thermistor measurement signal chain block.
Challenges include:
This article discusses each of these challenges and provides advice on how to resolve these issues and further ease the journey in designing such a system.
A broad selection of NTC thermistors is available on the market today, so selecting a specific thermistor for your application can be quite challenging. Note that thermistors are listed by their nominal value, which is the nominal resistance at 25°C. So, a 10 kΩ thermistor has a nominal resistance of 10 kΩ at 25°C. Thermistors are available with nominal or base resistance values from a few ohms to 10 MΩ. Thermistors with low nominal resistance (10 kΩ or less nominal resistance) typically support a lower temperature range such as '50°C to +70°C. Thermistors with higher nominal resistance support temperatures up to 300°C.
The thermistor element is made from metal oxides. Thermistors are available in bead, radial, and SMD form. Bead thermistors are epoxy coated or glass encapsulated for extra protection. Epoxy coated bead thermistors, radial, and SMD thermistors are suitable for temperatures up to 150°C. Glass coated bead thermistors are suitable for high temperature measurements. The coating/packaging for all types also prevents against corrosion. Some thermistors will also have additional housing to add further protection in harsh environments. Bead thermistors have a faster response time vs. radial/SMD thermistors. However, they are not as robust. So, the thermistor type to use depends on the end application and the environment in which the thermistor will reside. The long-term stability of a thermistor is dependent on the materials it is made from along with its packaging and construction. For example, an epoxy coated NTC thermistor can change by 0.2°C per year while a hermetically sealed one changes by only 0.02°C per year.
Thermistors have differing accuracy. Standard thermistors typically have an accuracy of 0.5°C to 1.5°C. Thermistors have a tolerance on their nominal resistance value and on their beta value (25°C to 50°C/85°C relationship). Note that the beta value of a thermistor is dependent on the manufacturer. For example, 10 kΩ NTC thermistors from different manufacturers will have different beta values. For higher accuracy systems, thermistors such as the Omega' 44xxx series can be used. These have an accuracy of 0.1°C or 0.2°C over a temperature range of 0°C to 70°C. So, the temperature range being measured along with the accuracy required over the temperature range determines whether a thermistor is suitable for the application. Note that the more accurate the Omega 44xxx series is, the higher its cost.
Therefore, the thermistor to be used depends on:
To convert from resistance to degrees Celsius, a beta value is commonly used. The beta value is determined by knowing two temperature points and the corresponding resistance at each temperature point.
Where:
RT1 = Resistance at Temperature 1
RT2 = Resistance at Temperature 2
T1 = Temperature 1 (K) T2 = Temperature 2 (K)
The data sheet for the thermistor normally lists the beta value for two cases:
The user uses the beta value that is closest to the temperature range being used in the design. Most thermistor data sheets list the beta value along with the tolerance of the resistance at 25°C and the tolerance of the beta value.
Higher accuracy thermistors such as the Omega 44xxx series and higher accuracy end solutions use the Steinhart-Hart equation to convert from resistance to degrees Celsius. From Equation 2, three constants'A, B, and C'are needed, and these are provided again by the sensor manufacturer. As the coefficients for the equation are generated using three temperature points, the resulting equation minimizes the error introduced due to linearization (error due to linearization is typically 0.02°C).
Where:
A, B, and C are constants derived from three temperature test points.
R = Thermistor's resistance in Ω
T = Temperature in degrees K
Figure 3 shows current excitation of the sensor. An excitation current is applied to the thermistor and the same current is applied to a precision resistor; the precision resistor being used as the reference for the measurement. The reference resistor must have a value greater than or equal to the highest resistance value of the thermistor (dependent on the minimum temperature being measured in the system). When choosing the magnitude of the excitation current, the maximum resistance of the thermistor must again be considered. This ensures that the voltage generated across the sensor and the reference resistor is always at an acceptable level for the electronics. Excitation current sources require some headroom or output compliance. If the thermistor has a large resistance at the minimum temperature being measured, this leads to a very low excitation current value. Therefore, the voltage generated across the thermistor at hot temperatures is small. To optimize the measurement of these low level signals, a programmable gain stage could be used. However, the gain needs to be programmed dynamically as the signal level from the thermistor changes significantly over temperature.
Figure 3. The current excitation of a thermistor.
Another option is to set the gain but use a dynamic excitation current. So, as the signal level from the thermistor changes, the excitation current value is changed dynamically so that the voltage generated across the thermistor is within the electronics' specified input range. The user must ensure that the voltage generated across the reference resistor is also at an acceptable level for the electronics. Both options require a high level of control, continuously monitoring the voltage across the thermistor to ensure that the signal can be measured by the electronics. Is there a simpler option? Let's look at voltage excitation.
Figure 4. The voltage excitation of a thermistor.
When the thermistor is excited by a constant voltage, the current through the thermistor will scale automatically as the thermistor resistance changes. Rather than using a reference resistor, a precision sense resistor is now used, its purpose being to calculate the current flowing through the thermistor so that the thermistor resistance can be calculated. With the excitation voltage being used as the ADC reference also, this eliminates the need for a gain stage. There is no workload for the processor in terms of monitoring the voltage across the thermistor, determining whether the signal level can be measured by the electronics, and calculating what the gain/excitation current value needs to be adjusted to. This is the approach used in this article.
If the thermistor nominal resistance and resistance range are small, either voltage or current excitation can be used. In this case, the excitation current and gain can be fixed. So, the circuit will be as shown in Figure 3. This method is useful as the current flowing through the sensor and reference resistor can be controlled, which is valuable in low power applications. Also, self-heating of the thermistor is minimized.
Using voltage excitation for a thermistor with low nominal resistance can also be used. However, the user must ensure that the current through the sensor is not too large for the sensor itself or for the application at any time.
When using thermistors with large nominal resistance and large temperature range, voltage excitation leads to an easier implementation. The larger nominal resistance ensures that the nominal current is at a reasonable level. However, the designer needs to ensure that the current is at an acceptable level over the complete temperature range being supported by the application.
Sigma-delta ADCs offer multiple benefits when designing thermistor measurement systems. Firstly, as sigma-delta ADCs oversample the analog input, external filtering is minimized, a simple RC filter being the only requirement. They offer flexibility in terms of choice of filter type and output data rate. The inbuilt digital filtering can be used to reject any interference from the mains power supply in mains operated designs. A 24-bit part such as the AD-4/AD-8 has a peak-to-peak resolution of 21.7 bits maximum, so they offer a high resolution.
Other benefits are:
Some sigma-delta ADCs are highly integrated and include:
The use of sigma-delta ADCs simplifies the thermistor design significantly along with reducing the BOM, system cost, board space, and time to market.
For this article, the AD-4/AD-8 are used as the ADC, as they are low noise, low current precision ADCs with an integrated PGA, embedded reference, analog input, and reference buffers.
Whether you use an excitation current or an excitation voltage, it is recommended to use a ratiometric configuration wherein the reference and sensor voltages are derived from the same excitation source. This means that any variations in the excitation source will not affect the accuracy of the measurement.
Figure 5 shows a constant excitation current supplying the thermistor and a precision resistor RREF, the voltage generated across RREF being the reference voltage for the thermistor measurement. The excitation current does not need to be accurate and could be less stable since any errors in the excitation current will be cancelled in this configuration. An excitation current is usually preferred over voltage excitation due to its superior control over sensitivity and its better noise immunity when the sensor is in a remote area. This type of biasing technique is commonly used for RTDs or thermistors with low resistance values. However, for thermistors with higher resistance values and with higher sensitivity, signal levels generated will be larger per change in temperature, so voltage excitation is used. For example, a 10 k' thermistor has a resistance of 10 k' at 25°C. At '50°C, the NTC thermistor resistance is 441.117 k'. The smallest excitation current of 50 µA provided by the AD-4/AD-8 generates a voltage of 441.117 k' × 50 µA = 22 V, which is too high and outside the operating range of most available ADCs used in this application space. Thermistors are also usually connected or located near the electronics, so the noise immunity advantage of an excitation current is not needed.
Figure 5. Configure with a constant current source.
Figure 6 shows a constant excitation voltage used to generate a voltage across the NTC thermistor. Adding a series sense resistor in the form of a voltage divider circuit will limit the current flow across the thermistor at its minimum resistance value. In this configuration, the value of the sense resistor, RSENSE, must be equal to the magnitude of the thermistor's resistance at the base temperature of 25°C so that the output voltage will be set at the midscale value of the reference voltage when it is at its nominal temperature of 25°C. So again, if a 10 k' thermistor that has a resistance of 10 k' at 25°C is used, the RSENSE must be equal to 10 k'. When the temperature changes, the resistance of the NTC thermistor also changes and the fraction of the excitation voltage across the thermistor also changes, producing an output voltage that is proportional to the NTC thermistor resistance.
Figure 6. Configure with a voltage divider circuit.
If the selected reference voltage that is used to supply the thermistor and/or RSENSE is the same reference as the ADC reference used for the measurement, then the system is configured in a ratiometric measurement (Figure 7) so that any errors associated with the excitation voltage source will be removed.
Figure 7. A thermistor ratiometric configuration measurement.
Note that the sense resistor (voltage excitation) or reference resistor (current excitation) need to have low initial tolerance and low drift as both variables contribute to the overall system accuracy.
When multiple thermistors are used, a single excitation voltage can be used. However, each thermistor must have its own precision sense resistor as shown in Figure 8. Another option is to use an external mux or switches with low on resistance, which allows sharing of a single precision sense resistor. When using this configuration, each thermistor requires some settling time in the measurement.
Figure 8. Multiple thermistors' analog input configuration measurements.
In summary, there are multiple concerns when designing thermistor-based temperature systems: sensor selection, sensor connectivity, trade-offs in terms of component selection, ADC configuration, and how these different variables contribute to the overall system accuracy. The next article in this series will explain how to optimize system design and the overall system error budget to achieve the target performance.
But actually, there is more to that. In this article, we'll discuss the different types of temperature sensors available, including their working principles and differences.
Figure 1. different types of temperature sensors Image source: gisli.mx
A temperature sensor, also known as a transducer, is a device that converts the heat in its surroundings into a measurable signal. To elaborate more on this general definition, let's take a look at the diagram below.
Figure 2. thermocouple measuring the temperature of a system Image source: youtube.com
This illustration demonstrates that a thermocouple is an example of a temperature sensor. The thermocouple is responsible for transforming a temperature into an electric signal. Based on different categories, the signal may have different electrical ranges. However, for industrial applications, the signals are usually limited to some globally accepted electrical signal ranges, such as 4-20mA, 1-5v, and 0-10v.
How is that possible? This is made possible with a temperature transmitter, which sends the sensor's output to an appropriate signal range.
There are many different types of temperature sensors, each with its own advantages and disadvantages. Some of the most common types of temperature sensors are:
Thermocouples
Resistance temperature detectors(RTDs)
Thermistors
Infrared temperature sensors
Figure 3. ttype K thermocouple Image source:onetemp
A thermocouple is a temperature sensor that generates a voltage proportional to the temperature difference between two junctions of different metals. These sensors are found in automotive industries, chemical manufacturing plants, and even in gas turbines.
Figure 4. overview of thermocouple working principle Image source:engineeringconcepts.com
The thermocouple operates based on the Seebeck effect, which was discovered by Thomas Johann Seebeck in . The Seebeck effect occurs when two different metals are joined at one end to form a junction. This junction experiences a temperature difference, then generates a voltage that is directly proportional to its temperature difference.
The generated voltage, also known as the Seebeck voltage, is measured across the open ends of the thermocouple. By calibrating the relationship between the voltage and temperature, the thermocouple provides an accurate temperature reading.
There are different types of thermocouples, each made from specific metal combinations. Some common types include type J, type K, and type T, which use different combinations of metals like iron, nickel, and copper.
Thermocouples can be used in diverse ways, but here are some of their primary uses:
They are used to measure the temperature range of -200°C to +°C.
They are used to measure exhaust gas temperature in vehicles.
They are used to determine the temperature of HVAC systems.
Figure 5. labelled structure of an RTD Image source:instrumentationtoolbox.com
RTDs are widely recognized for their precision in temperature measurement. It is a type of temperature sensor that measures temperature by correlating the resistance of a metal with temperature changes. RTDs are also known for their high accuracy and stability, making them suitable for applications that require precise temperature measurements.
Figure 6. overview of RTD working principle Image source:engineeringconcepts.com
RTDs are typically made from pure metals, with platinum being a popular choice due to its stable and linear relationship between resistance and temperature. As the temperature changes, the resistance of the metal in the RTD changes proportionally. This is because the RTD is connected to a circuit that supplies a specific amount of electric current. During this process, a voltmeter is used to measure the voltage drop across the RTD, and the formula below is used to calculate the corresponding temperature:
R= R0(1 + α ΔT)
In this formula, α stands for the temperature coefficient of resistance, R0 stands for resistance at 0°C, and ΔT stands for the change in temperature. The most common types of RTDs include PT100, PT500, and PT1,000.
RTDs have helped scientists in many ways, making their work much easier. Here are some relevant applications of RTDs:
They are used to measure the temperature range of -200°C to +500°C.
They are used as medical sensors to monitor the temperature of medical equipment.
Figure 7. symbols of NTC and PTC thermistors Image source:hoidla
A thermistor, short for 'thermal resistor', is a type of temperature sensor that exhibits significant changes in electrical resistance in response to temperature changes. Thermistors are known for their sensitivity, making them useful for applications where accurate temperature measurements are required.
Figure 8. overview of thermistor working principle Image source:slideshare.net
The thermistor operates based on the principles of inverse and direct proportionality between two physical quantities: temperature and resistance. Most thermistors fall into two categories: NTC and PTC. In NTC thermistors, the temperature is inversely proportional to resistance, meaning resistance decreases as temperature increases, and vice versa.
NTC thermistors are commonly used in applications that require high sensitivity to temperature changes. On the other hand, PTC thermistors work on the principle of direct proportionality because their resistance increases with temperature.
Figure 9. graph of NTC and PTC thermistors Image source:ametherm.com
Both NTC and PTC thermistors are the basic regulators of thermophysics. Here are some of their major applications:
They are used to measure the temperature range of 0°C to 100°C.
They are used to make thermostats in electronic circuits.
They are used to monitor and regulate the temperature in HVAC systems.
Figure 10. labelled structure of an IR sensor Image source:researchgate.net
An infrared temperature sensor, also known as an infrared thermometer or pyrometer, is a device that measures an object's temperature without making physical contact. It does so by detecting the infrared radiation emitted by the object, which is directly proportional to its temperature.
Figure 11. overview of how an IR sensor works Image source:easybom.com
An infrared sensor works by obeying the principle of black body radiation and Stefan-Boltzmann's law, which states that all objects with a temperature above absolute zero emit IR radiation with an intensity that is directly related to the object's temperature. The IR sensor consists of a lens that focuses its radiation onto a detector; the detector converts the IR radiation into an electrical signal.
The electrical signal is then processed by the sensor's electronics, which calculate the object's temperature based on the intensity of the detected radiation.
Having used an IR sensor for over a decade, I can attest to its versatility, demonstrated by its applications in:
Laboratories, to measure temperatures ranging from -50°C to +1,000°C.
Industrial settings, to control temperatures in manufacturing processes like metal and glass production.
Medical settings, for non-contact temperature measurements, particularly for patients in delicate conditions.
Integrated circuits, to detect overheating in electronic components.
The table below gives you a detailed comparison of the key features in thermocouples, thermistors, RTDs, and Infrared temperature sensors:
Features
Thermocouples
Thermistors
RTDs
IR temperature sensors
Working principle
Principle of Seebeck effect
Principle of inverse and direct proportionality
Principle of linear proportionality
Principle of black body radiation and Stefan Boltzmann's law
Temperature range
-200°C to +°C
0°C to 100°C
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-200°C to +500°C
-50°C to +°C
Sensing material
Two different metals
Metal oxide
Single metal platinum material
Focus lens and detector
Accuracy
0.5°C to 5°C
0.1°C to 1.5°C
0.1°C to 1°C (highest)
0.1°C to 2°C
Speed of response
0.1 to 10mm/s
0.05 to 2.5mm/s
0.1 to 50mm/s
0.1 to 5mm/s
(Fastest)
Type of Transducer
Active transducer
Passive transducer
Passive transducer
Active transducer
Linearity
Non-linear
Exponential
Most linear
Non-linear
Cost
Least expensive
More expensive than thermocouples
Moderately expensive
Less expensive than RTDs
Temperature sensors are widely utilized in a variety of integrated circuits and electrical appliances. In this guide, we have explored the applications, differences, and working principles of:
If you're looking to purchase any of these temperature sensors, Quarktwin offers a wide selection and quick delivery. Simply click on the sensor that interests you from the list above, place your order, and we'll ensure it's delivered to you promptly!
This is the first article in a two-part series. Part 1 will first discuss the history and design challenges for a thermistor-based temperature measurement system and how it compares with a resistance temperature detector (RTD)-based temperature measurement system. It will also outline thermistor selection, configuration trade-offs, and the importance of sigma-delta analog-to-digital converters (ADCs) in this application space. Part 2 will detail how to optimize and how to evaluate the final thermistor-based measurement system.
As discussed in the article 'How to Select and Design the Best RTD Temperature Sensing System,' an RTD is a type of resistor with resistance that varies as a function of temperature. Thermistors operate in a similar manner to RTDs. Unlike RTDs that only have a positive temperature coefficient, thermistors can either have positive or negative temperature coefficients. A negative temperature coefficient (NTC) thermistor decreases its resistance as the temperature increases, while a positive temperature coefficient (PTC) thermistor increases its resistance as the temperature increases. Figure 1 shows the response characteristics of typical NTC and PTC thermistors and how they compare to an RTD curve.
Figure 1. The response characteristics of thermistors vs. RTD.
In terms of temperature range, the RTD curve is near linear, and the sensor covers a much wider temperature range (commonly '200°C to +850°C) than thermistors due to the thermistor's nonlinear (exponential) characteristics. RTDs are usually available in a well-known standardized curve while thermistor curves vary depending on the manufacturer. We will discuss this in detail in the Thermistor Selection Guide section of this article.
Thermistors are made up of composite materials, usually ceramic, polymer, or semiconductor (typically metal oxides), which are quite smaller and more inexpensive but not as rugged when compared with RTDs, which are made of a pure metal (platinum, nickel, or copper). Thermistors can detect changes in temperature much faster than RTDs, delivering faster feedback. Thus, thermistors are commonly used sensors in applications that require low cost, small size, faster response rate, higher sensitivity, and with restricted temperature range such as with monitoring electronic equipment, households and building controls, scientific laboratories, or the cold-junction compensation used for thermocouples in commercial or industrial applications.
In most cases, NTC thermistors rather than PTC thermistors are used in precision temperature measurement applications. There are a few available PTC thermistors that are used in overcurrent input protection circuits or as resettable fuses for safety applications. The resistance-to-temperature curve of a PTC thermistor exhibits a very small NTC region until its switching point (or Curie point) is reached, above which a dramatic increase in resistance of several orders of magnitude occurs within a span of a few degrees Celsius. So, during an overcurrent condition, the PTC thermistor will have a high amount of self-heating beyond the switching temperature and its resistance will increase dramatically, thereby resulting in a reduced current being input to the system and thereby preventing damage to occur. The switching point for a PTC thermistor is typically between 60°C and 120°C, which is not suitable for monitoring temperature measurements in a wide range application. This article focuses on NTC thermistors that can typically measure or monitor temperature from '80°C to +150°C. NTC thermistors are available with nominal resistances at 25°C, ranging from a few ohms to 10 M'. As shown in Figure 1, the resistance changes per degree Celsius is more significant for a thermistor vs. an RTD. The high sensitivity and high resistance values of thermistors make their front-end circuitry much simpler compared to RTDs as a thermistor does not need any special wiring configurations such as 3-wire or 4-wire to compensate for lead resistance. Thermistor design only uses a simple 2-wire configuration.
Table 1 shows the pros and cons of RTDs, NTC, and PTC thermistors.
Table 1. Thermistors vs. RTDs Parameters NTC Thermistors PTC Thermistors RTDs Temperature Range '80°C to +300°C 60°C to 120°C '200°C to +850°C Temperature Coefficient Negative Positive Positive Linearity Exponential Exponential Near linear Sensitivity High High Low Response Time Fast Fast Slow Excitation Required Required Required Self-Heating Yes Yes Yes Wiring Configuration 2-wire 2-wire 2-wire, 3-wire, 4-wire Cost Inexpensive to moderate Inexpensive Moderate to expensive Size Small Small MediumA high precision and accurate thermistor-based temperature measurement requires precise signal conditioning, analog-to-digital conversion, linearization, and compensation, as shown in Figure 2. Though the signal chain looks simple and straightforward, there are several complex factors involved that also impact overall system board size, cost, and performance. There are several integrated solutions available in ADI's precision ADC portfolio such as the AD-4/AD-8, which offer multiple benefits when designing a temperature system since most of the building blocks required in the application are built in. However, there are different challenges involved in designing and optimizing a thermistor-based temperature measurement solution.
Figure 2. A typical NTC thermistor measurement signal chain block.
Challenges include:
This article discusses each of these challenges and provides advice on how to resolve these issues and further ease the journey in designing such a system.
A broad selection of NTC thermistors is available on the market today, so selecting a specific thermistor for your application can be quite challenging. Note that thermistors are listed by their nominal value, which is the nominal resistance at 25°C. So, a 10 kΩ thermistor has a nominal resistance of 10 kΩ at 25°C. Thermistors are available with nominal or base resistance values from a few ohms to 10 MΩ. Thermistors with low nominal resistance (10 kΩ or less nominal resistance) typically support a lower temperature range such as '50°C to +70°C. Thermistors with higher nominal resistance support temperatures up to 300°C.
The thermistor element is made from metal oxides. Thermistors are available in bead, radial, and SMD form. Bead thermistors are epoxy coated or glass encapsulated for extra protection. Epoxy coated bead thermistors, radial, and SMD thermistors are suitable for temperatures up to 150°C. Glass coated bead thermistors are suitable for high temperature measurements. The coating/packaging for all types also prevents against corrosion. Some thermistors will also have additional housing to add further protection in harsh environments. Bead thermistors have a faster response time vs. radial/SMD thermistors. However, they are not as robust. So, the thermistor type to use depends on the end application and the environment in which the thermistor will reside. The long-term stability of a thermistor is dependent on the materials it is made from along with its packaging and construction. For example, an epoxy coated NTC thermistor can change by 0.2°C per year while a hermetically sealed one changes by only 0.02°C per year.
Thermistors have differing accuracy. Standard thermistors typically have an accuracy of 0.5°C to 1.5°C. Thermistors have a tolerance on their nominal resistance value and on their beta value (25°C to 50°C/85°C relationship). Note that the beta value of a thermistor is dependent on the manufacturer. For example, 10 kΩ NTC thermistors from different manufacturers will have different beta values. For higher accuracy systems, thermistors such as the Omega' 44xxx series can be used. These have an accuracy of 0.1°C or 0.2°C over a temperature range of 0°C to 70°C. So, the temperature range being measured along with the accuracy required over the temperature range determines whether a thermistor is suitable for the application. Note that the more accurate the Omega 44xxx series is, the higher its cost.
Therefore, the thermistor to be used depends on:
To convert from resistance to degrees Celsius, a beta value is commonly used. The beta value is determined by knowing two temperature points and the corresponding resistance at each temperature point.
Where:
RT1 = Resistance at Temperature 1
RT2 = Resistance at Temperature 2
T1 = Temperature 1 (K) T2 = Temperature 2 (K)
The data sheet for the thermistor normally lists the beta value for two cases:
The user uses the beta value that is closest to the temperature range being used in the design. Most thermistor data sheets list the beta value along with the tolerance of the resistance at 25°C and the tolerance of the beta value.
Higher accuracy thermistors such as the Omega 44xxx series and higher accuracy end solutions use the Steinhart-Hart equation to convert from resistance to degrees Celsius. From Equation 2, three constants'A, B, and C'are needed, and these are provided again by the sensor manufacturer. As the coefficients for the equation are generated using three temperature points, the resulting equation minimizes the error introduced due to linearization (error due to linearization is typically 0.02°C).
Where:
A, B, and C are constants derived from three temperature test points.
R = Thermistor's resistance in Ω
T = Temperature in degrees K
Figure 3 shows current excitation of the sensor. An excitation current is applied to the thermistor and the same current is applied to a precision resistor; the precision resistor being used as the reference for the measurement. The reference resistor must have a value greater than or equal to the highest resistance value of the thermistor (dependent on the minimum temperature being measured in the system). When choosing the magnitude of the excitation current, the maximum resistance of the thermistor must again be considered. This ensures that the voltage generated across the sensor and the reference resistor is always at an acceptable level for the electronics. Excitation current sources require some headroom or output compliance. If the thermistor has a large resistance at the minimum temperature being measured, this leads to a very low excitation current value. Therefore, the voltage generated across the thermistor at hot temperatures is small. To optimize the measurement of these low level signals, a programmable gain stage could be used. However, the gain needs to be programmed dynamically as the signal level from the thermistor changes significantly over temperature.
Figure 3. The current excitation of a thermistor.
Another option is to set the gain but use a dynamic excitation current. So, as the signal level from the thermistor changes, the excitation current value is changed dynamically so that the voltage generated across the thermistor is within the electronics' specified input range. The user must ensure that the voltage generated across the reference resistor is also at an acceptable level for the electronics. Both options require a high level of control, continuously monitoring the voltage across the thermistor to ensure that the signal can be measured by the electronics. Is there a simpler option? Let's look at voltage excitation.
Figure 4. The voltage excitation of a thermistor.
When the thermistor is excited by a constant voltage, the current through the thermistor will scale automatically as the thermistor resistance changes. Rather than using a reference resistor, a precision sense resistor is now used, its purpose being to calculate the current flowing through the thermistor so that the thermistor resistance can be calculated. With the excitation voltage being used as the ADC reference also, this eliminates the need for a gain stage. There is no workload for the processor in terms of monitoring the voltage across the thermistor, determining whether the signal level can be measured by the electronics, and calculating what the gain/excitation current value needs to be adjusted to. This is the approach used in this article.
If the thermistor nominal resistance and resistance range are small, either voltage or current excitation can be used. In this case, the excitation current and gain can be fixed. So, the circuit will be as shown in Figure 3. This method is useful as the current flowing through the sensor and reference resistor can be controlled, which is valuable in low power applications. Also, self-heating of the thermistor is minimized.
Using voltage excitation for a thermistor with low nominal resistance can also be used. However, the user must ensure that the current through the sensor is not too large for the sensor itself or for the application at any time.
When using thermistors with large nominal resistance and large temperature range, voltage excitation leads to an easier implementation. The larger nominal resistance ensures that the nominal current is at a reasonable level. However, the designer needs to ensure that the current is at an acceptable level over the complete temperature range being supported by the application.
Sigma-delta ADCs offer multiple benefits when designing thermistor measurement systems. Firstly, as sigma-delta ADCs oversample the analog input, external filtering is minimized, a simple RC filter being the only requirement. They offer flexibility in terms of choice of filter type and output data rate. The inbuilt digital filtering can be used to reject any interference from the mains power supply in mains operated designs. A 24-bit part such as the AD-4/AD-8 has a peak-to-peak resolution of 21.7 bits maximum, so they offer a high resolution.
Other benefits are:
Some sigma-delta ADCs are highly integrated and include:
The use of sigma-delta ADCs simplifies the thermistor design significantly along with reducing the BOM, system cost, board space, and time to market.
For this article, the AD-4/AD-8 are used as the ADC, as they are low noise, low current precision ADCs with an integrated PGA, embedded reference, analog input, and reference buffers.
Whether you use an excitation current or an excitation voltage, it is recommended to use a ratiometric configuration wherein the reference and sensor voltages are derived from the same excitation source. This means that any variations in the excitation source will not affect the accuracy of the measurement.
Figure 5 shows a constant excitation current supplying the thermistor and a precision resistor RREF, the voltage generated across RREF being the reference voltage for the thermistor measurement. The excitation current does not need to be accurate and could be less stable since any errors in the excitation current will be cancelled in this configuration. An excitation current is usually preferred over voltage excitation due to its superior control over sensitivity and its better noise immunity when the sensor is in a remote area. This type of biasing technique is commonly used for RTDs or thermistors with low resistance values. However, for thermistors with higher resistance values and with higher sensitivity, signal levels generated will be larger per change in temperature, so voltage excitation is used. For example, a 10 k' thermistor has a resistance of 10 k' at 25°C. At '50°C, the NTC thermistor resistance is 441.117 k'. The smallest excitation current of 50 µA provided by the AD-4/AD-8 generates a voltage of 441.117 k' × 50 µA = 22 V, which is too high and outside the operating range of most available ADCs used in this application space. Thermistors are also usually connected or located near the electronics, so the noise immunity advantage of an excitation current is not needed.
Figure 5. Configure with a constant current source.
Figure 6 shows a constant excitation voltage used to generate a voltage across the NTC thermistor. Adding a series sense resistor in the form of a voltage divider circuit will limit the current flow across the thermistor at its minimum resistance value. In this configuration, the value of the sense resistor, RSENSE, must be equal to the magnitude of the thermistor's resistance at the base temperature of 25°C so that the output voltage will be set at the midscale value of the reference voltage when it is at its nominal temperature of 25°C. So again, if a 10 k' thermistor that has a resistance of 10 k' at 25°C is used, the RSENSE must be equal to 10 k'. When the temperature changes, the resistance of the NTC thermistor also changes and the fraction of the excitation voltage across the thermistor also changes, producing an output voltage that is proportional to the NTC thermistor resistance.
Figure 6. Configure with a voltage divider circuit.
If the selected reference voltage that is used to supply the thermistor and/or RSENSE is the same reference as the ADC reference used for the measurement, then the system is configured in a ratiometric measurement (Figure 7) so that any errors associated with the excitation voltage source will be removed.
Figure 7. A thermistor ratiometric configuration measurement.
Note that the sense resistor (voltage excitation) or reference resistor (current excitation) need to have low initial tolerance and low drift as both variables contribute to the overall system accuracy.
When multiple thermistors are used, a single excitation voltage can be used. However, each thermistor must have its own precision sense resistor as shown in Figure 8. Another option is to use an external mux or switches with low on resistance, which allows sharing of a single precision sense resistor. When using this configuration, each thermistor requires some settling time in the measurement.
Figure 8. Multiple thermistors' analog input configuration measurements.
In summary, there are multiple concerns when designing thermistor-based temperature systems: sensor selection, sensor connectivity, trade-offs in terms of component selection, ADC configuration, and how these different variables contribute to the overall system accuracy. The next article in this series will explain how to optimize system design and the overall system error budget to achieve the target performance.
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