A Comprehensive Guide to Thermistor Sensors and Their Selection

Thermistor sensors are a type of temperature-sensing device that uses a metal oxide material to measure temperature changes. They are widely used in industrial and commercial applications due to their accuracy and reliability.

Thermistors can be classified into two main types: positive temperature coefficient (PTC) and negative temperature coefficient (NTC). PTC thermistors increase in resistance with temperature, while NTC thermistors decrease in resistance with temperature.

One of the key advantages of thermistor sensors is their fast response time, allowing for quick and accurate temperature measurements. This makes them ideal for applications where temperature changes need to be monitored in real-time.

Additional reading: Thermistor Ntc or Ptc

The Steinhart-Hart equation is a widely used third-order approximation that characterizes the performance of thermistors over a wide temperature range. It requires specifying three parameters: a, b, and c, which must be stated with reference to the unit of resistance.

This equation is not dimensionally correct, but it gives good numerical results for resistances expressed in ohms or kiloohms. The error in the Steinhart-Hart equation is generally less than 0.02 °C in the measurement of temperature over a 200 °C range.

The Steinhart-Hart equation is a cubic equation in ln⁡ ⁡ R, and solving for R yields a real root that gives the resistance as a function of temperature. This equation is used to give resistance as a function of temperature, and it's a more complex resistance-temperature transfer function compared to the linear approximation model.

The B-parameter equation is essentially the Steinhart-Hart equation with specific values for the parameters. It's used to characterize NTC thermistors and provides a more faithful characterization of the performance over a wider temperature range.

The B-parameter equation can be solved for the temperature, and it can also be written as a linear function of ln⁡ ⁡ R vs. 1/T. This can be used to convert the function of resistance vs. temperature of a thermistor into a linear function.

The beta value is commonly used to convert from resistance to degrees Celsius, and it's determined by knowing two temperature points and the corresponding resistance at each temperature point. This value is usually listed in the data sheet for the thermistor, along with the tolerance of the resistance at 25°C and the tolerance of the beta value.

Here are the equations used to calculate the beta value:

RT1 = Resistance at Temperature 1

Suggestion: Thermistor vs Thermocouple vs Rtd

RT2 = Resistance at Temperature 2

T1 = Temperature 1 (K)

T2 = Temperature 2 (K)

The user uses the beta value that is closest to the temperature range being used in the design, and 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.

Take a look at this: Thermistor Resistance Chart

Thermistors are temperature-sensing devices that come in two main types: NTC and PTC. NTC thermistors are made from semiconducting materials that increase in conductivity as temperature rises.

NTC thermistors work by promoting charge carriers into the conduction band, allowing more current to flow. This is particularly useful for small temperature changes, where the resistance of the material is linearly proportional to the temperature.

Some NTC thermistors are made from materials like ferric oxide with titanium doping, which forms an n-type semiconductor with electrons as charge carriers. Others, like nickel oxide with lithium doping, create a p-type semiconductor with holes as charge carriers.

PTC thermistors, on the other hand, exhibit a sudden resistance increase at a specific temperature, known as the switch or Curie temperature. This temperature range typically falls between 60°C to 120°C.

PTC thermistors are commonly used in self-regulating heating elements and self-resetting over-current protection.

See what others are reading: Thermistors

Thermistors are built by using metal oxides, which are typically pressed into a bead, disk, or cylindrical shape and then encapsulated with an impermeable material such as epoxy or glass.

NTC thermistors are manufactured from oxides of the iron group of metals, including chromium, manganese, cobalt, iron, and nickel. These oxides form a ceramic body.

The terminals of NTC thermistors are composed of conductive metals like silver, nickel, and tin. This helps ensure a reliable connection.

PTC thermistors, on the other hand, are usually prepared from barium, strontium, or lead titanates. These materials provide the necessary properties for PTC thermistors to work effectively.

Thermistors can also be produced using a resonant acoustic mixing process, followed by a sintering process. This method reduces production time and eliminates the calcination step.

If this caught your attention, see: How Do Thermistors Work

NTC thermistors are incredibly versatile and can be used in a wide range of applications, including as a thermometer for low-temperature measurements of the order of 10 K. They're also used in power supply circuits as inrush current limiter devices, which prevent large currents from flowing at turn-on and then heat up to allow higher current flow during normal operation.

Discover more: How Thermistor Is Used to Measure Temperature

Some common uses of NTC thermistors include monitoring fluid temperatures in automotive applications, such as engine coolant, cabin air, external air, or engine oil temperature. They're also used to monitor the temperature of an incubator, and are a crucial component in modern digital thermostats.

NTC thermistors come in various forms, including bare and lugged forms, with the latter being used for point sensing to achieve high accuracy for specific points, such as laser diode die. They can also be packaged into enclosures made of materials like stainless steel, aluminum, copper, brass, or plastic.

In addition to their various forms, NTC thermistors have a range of temperature measurement capabilities, from about 0.01 kelvin to 2,000 kelvins (-273.14 °C to 1,700 °C). They're also highly customizable to fit specific application needs.

Here are some examples of industries that rely heavily on NTC thermistors for temperature measurement:

A thermistor's positive coefficient is a characteristic that's worth understanding. It means that the resistance of the thermistor decreases as the temperature increases.

This is exactly the opposite of a thermistor with a negative coefficient, which we'll discuss later. In a positive coefficient thermistor, the resistance is higher at lower temperatures and lower at higher temperatures.

The temperature coefficient of resistance is a key factor in determining the type of thermistor you need for your application.

See what others are reading: Negative Temperature Coefficient Ntc Thermistors

PTC thermistors are employed when a sharp change in resistance at a specific temperature is needed.

They show a sudden resistance increase once a defined temperature, known as the switch, transition, or Curie temperature, is reached, typically ranging from 60°C to 120°C.

PTC thermistors are usually prepared from barium (Ba), strontium, or lead titanates (e.g. PbTiO3).

Common applications of PTC thermistors include self-regulating heating elements and self-resetting over-current protection.

PTC thermistors are commonly used in self-resetting over-current protection.

PTC thermistors are relatively easy to test and evaluate, but it's essential to consider their specific applications. For instance, in a degaussing coil circuit, the thermistor's ability to heat up smoothly and continuously is crucial.

One way to test a PTC thermistor is by using a multimeter to measure its resistance at different temperatures. In a temperature-compensated voltage-controlled oscillator, the thermistor's nearly linear positive temperature coefficient (0.7%/°C) is a key characteristic.

To evaluate the performance of a PTC thermistor in a circuit, you can use a monitoring relay to detect when the thermistor's resistance increases dramatically at the maximum allowable winding temperature. This is particularly useful in electric motors and dry type power transformers.

In applications where current hogging is a concern, a PTC thermistor can be attached in series with each device to ensure the current is divided reasonably evenly between them.

Littelfuse offers a range of thermistor sensor configurations to suit different temperature sensing needs.

Their NTC thermistor probes and assemblies are designed for sensing temperature in various applications. These probes and assemblies are manufactured by Littelfuse to provide accurate temperature readings.

In contrast, PTC thermistor probes and assemblies are also available from Littelfuse, offering a different temperature coefficient for sensing temperature in specific applications.

Thermistors are available with nominal or base resistance values from a few ohms to 10 MΩ. They are listed by their nominal value, which is the nominal resistance at 25°C.

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.

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).

Here are the factors to consider when selecting a thermistor:

Note that the more accurate the thermistor, the higher its cost. For example, the Omega™ 44xxx series has an accuracy of 0.1°C or 0.2°C over a temperature range of 0°C to 70°C.

Here's an interesting read: A C Thermistor

Thermistors come in a variety of forms, including bead, radial, and SMD, each with its own strengths and weaknesses. Bead thermistors are epoxy coated or glass encapsulated for extra protection and have a faster response time, but they are not as robust as radial or SMD thermistors.

The type of thermistor to use depends on the end application and environment. For example, glass coated bead thermistors are suitable for high temperature measurements, while epoxy coated bead thermistors, radial, and SMD thermistors are suitable for temperatures up to 150°C.

The long-term stability of a thermistor is also important to consider. Hermetically sealed thermistors, for instance, change by only 0.02°C per year, while epoxy coated ones can change by 0.2°C per year.

The accuracy of thermistors can also vary, with standard thermistors having an accuracy of 0.5°C to 1.5°C and more accurate thermistors like the Omega 44xxx series having an accuracy of 0.1°C or 0.2°C over a temperature range of 0°C to 70°C.

Our featured products are designed to make your life easier and more enjoyable. We've curated a selection of items that are both functional and stylish.

The EcoCycle recycling bin is a standout choice for anyone looking to reduce their waste. It's made from durable, BPA-free plastic and has a capacity of 10 gallons.

For those who love to cook, our ProLine chef's knife is a must-have. With a sharp, high-carbon stainless steel blade and ergonomic handle, it's perfect for chopping, slicing, and mincing ingredients.

Our selection of artisanal candles is a great way to add some ambiance to any room. Made from high-quality soy wax and essential oils, they're not only pleasant to the nose but also eco-friendly.

The QuickDry microfiber towel is a game-changer for anyone who's ever struggled with soggy towels. It's lightweight, compact, and can dry in as little as 30 seconds.

You might enjoy: Ceramic Heater & Thermistor - P1 Series

When selecting a thermistor, it's essential to consider the temperature range being measured. A thermistor's nominal resistance value is typically specified at 25°C, but its actual resistance can vary depending on the temperature.

Thermistors are available with nominal resistance values ranging from a few ohms to 10 MΩ. For example, a 10 kΩ thermistor has a nominal resistance of 10 kΩ at 25°C.

The type of thermistor to use depends on the end application and environment. Bead thermistors have a faster response time but are not as robust as radial or SMD thermistors.

Here are some key factors to consider when selecting a thermistor:

Thermistors with low nominal resistance (10 kΩ or less) typically support a lower temperature range, such as –50°C to +70°C. On the other hand, thermistors with higher nominal resistance can support temperatures up to 300°C.

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.

For higher accuracy systems, thermistors such as the Omega 44xxx series can be used, which have an accuracy of 0.1°C or 0.2°C over a temperature range of 0°C to 70°C.

Thermistor sensors rely on the variation of resistivity with temperature in their semiconductor material. This variation is the primary working principle of thermistors.

There are two main types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient) thermistors. NTC thermistors exhibit a negative resistance-temperature characteristic, meaning their resistance decreases with rising temperature. PTC thermistors, on the other hand, exhibit a positive resistance-temperature characteristic, meaning their resistance increases with rising temperature.

The value of k, a constant that describes the relationship between resistance and temperature, determines whether a thermistor is NTC or PTC. If k is positive, the thermistor is PTC, and if k is negative, it's NTC.

Here are some key differences between NTC and PTC thermistors:

A thermistor sensor works by detecting changes in temperature, and its primary working principle relies on the variation of resistivity with temperature in its semiconductor material. This means that as temperature increases, the atoms in the material vibrate more vigorously, leaving more electrons free to conduct electricity.

The two main types of thermistors, NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient), exhibit opposite resistance-temperature characteristics. In an NTC thermistor, resistance decreases with rising temperature in a linear or nonlinear fashion.

The relationship between resistance and temperature is crucial in thermistor sensors. If the resistance increases with increasing temperature, the device is called a positive-temperature-coefficient (PTC) thermistor, or posistor. There are two types of PTC resistors – switching thermistor and silistor.

If the resistance decreases with increasing temperature, the device is called a negative-temperature-coefficient (NTC) thermistor. Resistors that are not thermistors are designed to have a k value as close to 0 as possible so that their resistance remains nearly constant over a wide temperature range.

Here's a summary of the main types of thermistors:

Self-heating effects can introduce a significant error in temperature measurement if not corrected for.

The electrical power input to a thermistor, which generates heat, is calculated by the formula I * V, where I is the current and V is the voltage drop across the thermistor.

This heat energy is transferred to the surrounding environment at a rate described by Newton's law of cooling, which involves the dissipation constant, usually expressed in units of milliwatts per degree Celsius.

At equilibrium, the rate of electrical power input to the thermistor must equal the rate of heat transfer to the surroundings, as described by the equation I * V = K * (T(R) - T0).

If the voltage across the thermistor is held fixed, the current can be calculated using Ohm's law, and the equilibrium equation can be solved for the ambient temperature as a function of the measured resistance of the thermistor.

The dissipation constant is a measure of the thermal connection of the thermistor to its surroundings, and its value depends on the environment, with typical values for a small glass-bead thermistor being 1.5 mW/°C in still air and 6.0 mW/°C in stirred oil.

In some applications, the self-heating effect is exploited to raise the body temperature of the thermistor well above the ambient temperature, allowing the sensor to detect subtle changes in the thermal conductivity of the environment.

Selecting the right thermistor for your application can be a challenge. With so many options available, it's essential to consider factors like precision and accuracy.

The AD7124-4/AD7124-8 offers multiple benefits when designing a temperature system, as most of the building blocks required in the application are built in. This can simplify the design process and reduce overall system board size and cost.

Choosing the right current or voltage for your thermistor signal can be tricky. You'll need to adjust the variables so that the converter or other building blocks are used within their specification.

Here are some key considerations when selecting a thermistor:

RTDs are typically made with a higher concentration of sugar and preservatives than RTDs, which can affect their nutritional value and shelf life.

The RTD market is expected to grow significantly in the next few years, with a projected increase in sales by 2025.

A key challenge in the RTD industry is the need for more sustainable packaging solutions.

The production process for RTDs is often more complex and labor-intensive than that of RTDs.

RTDs tend to have a longer shelf life than RTDs due to their higher sugar content.

Many consumers are now looking for low-sugar or sugar-free RTDs, which can be a challenge for manufacturers.

Designing a thermistor-based temperature measurement system can be a daunting task, especially when considering the various challenges that come with it. One of the primary challenges is selecting the right thermistor for your application.

A high precision and accurate thermistor-based temperature measurement requires precise signal conditioning, analog-to-digital conversion, linearization, and compensation. This can be a complex task, requiring careful consideration of several factors.

The AD7124-4/AD7124-8 integrated solutions from ADI's precision ADC portfolio offer multiple benefits when designing a temperature system, as they include most of the building blocks required in the application. However, there are still several challenges involved in designing and optimizing a thermistor-based temperature measurement solution.

Some of the challenges include selecting the right current or voltage, conditioning the thermistor signal, adjusting variables to ensure the converter or other building blocks are used within their specification, and connecting multiple thermistors in a system.

A primary concern about thermistors is their nonlinear response and system accuracy. This can lead to errors in temperature measurement, making it difficult to achieve the target performance.

Check this out: Automated Irrigation System Using Soil Moisture Sensor

To address these challenges, it's essential to consider the expected error for your design and the linearization and compensation techniques used to achieve the target performance.

Here are some key considerations to keep in mind:

To linearize the thermistor sensor, you can use a precision sense resistor to calculate the current flowing through the thermistor, allowing you to calculate the thermistor resistance.

Excitation current can be applied to the thermistor, but it needs to be carefully chosen to ensure the voltage generated across the sensor and reference resistor is at an acceptable level for the electronics.

The excitation current source requires some headroom or output compliance, and the user must ensure that the voltage generated across the reference resistor is also at an acceptable level.

A dynamic excitation current can be used to adjust the signal level from the thermistor, but this requires continuously monitoring the voltage across the thermistor.

The beta value is a commonly used parameter for thermistors, and it's determined by knowing two temperature points and the corresponding resistance at each temperature point. This value is essential for converting resistance to degrees Celsius.

A beta value is typically listed for two specific temperature ranges: 25°C and 50°C, and 25°C and 85°C. You should use the beta value closest to the temperature range you're working with.

Thermistor data sheets usually list the beta value along with the tolerance of the resistance at 25°C and the tolerance of the beta value. This information is crucial for accurate calculations.

Higher accuracy thermistors, such as the Omega 44xxx series, use the Steinhart-Hart equation for conversion. This equation requires three constants: A, B, and C, which are provided by the manufacturer and derived from three temperature test points.

The Steinhart-Hart equation minimizes the error introduced due to linearization, which is typically around 0.02°C. This level of accuracy is essential for applications where precision is crucial.

To give you a better idea, here are the typical beta values listed in thermistor data sheets:

Use the beta value that corresponds to your specific temperature range to ensure accurate calculations.

Current/Voltage Excitation is a simpler option for thermistor measurement. It eliminates the need for a gain stage and continuous monitoring of the voltage across the thermistor.

The excitation voltage is used as the ADC reference, which makes the calculation of the thermistor resistance easier. A precision sense resistor is used to calculate the current flowing through the thermistor.

Using a constant voltage excitation allows the current through the thermistor to scale automatically as the thermistor resistance changes. This means there's no need to adjust the gain or excitation current value.

A precision sense resistor is used to calculate the current flowing through the thermistor, eliminating the need for a reference resistor. This approach is used in this article.

In a thermistor sensor circuit, the choice of ADC is crucial for accurate temperature measurement. Sigma-delta ADCs offer multiple benefits in thermistor-based applications.

Sigma-delta ADCs oversample the analog input, minimizing the need for external filtering and allowing for a simple RC filter to be used instead. This simplifies the design and reduces the overall system cost.

A 24-bit sigma-delta ADC like the AD7124-4/AD7124-8 provides a high resolution of 21.7 bits maximum, making it suitable for thermistor measurement systems.

A ratiometric configuration is a type of circuit configuration that involves using a voltage reference to measure the ratio of two voltages. This configuration is commonly used in analog-to-digital converters (ADCs).

The key benefit of a ratiometric configuration is that it provides a high degree of accuracy and stability, making it ideal for applications where precision is critical. In a ratiometric configuration, the ADC compares the input voltage to a reference voltage.

The reference voltage is typically generated by a separate circuit, such as a bandgap reference or a voltage divider. This reference voltage is then used to scale the input voltage, allowing the ADC to accurately measure the input signal. The ratio of the input voltage to the reference voltage is what determines the output of the ADC.

Sigma-delta ADCs are a great choice for thermistor-based applications due to their multiple benefits. They simplify the design process and reduce the system cost.

A significant advantage of sigma-delta ADCs is that they minimize the need for external filtering, requiring only a simple RC filter.

In fact, a 24-bit sigma-delta ADC like the AD7124-4/AD7124-8 has a peak-to-peak resolution of 21.7 bits maximum, offering a high resolution.

The use of sigma-delta ADCs also reduces the BOM, board space, and time to market, making them a practical choice for designers.

Thermistors are classified into two main types: NTC and PTC thermistors. NTC thermistors are commonly used as temperature sensors or in series with a circuit as an inrush current limiter.

The main difference between NTC and PTC thermistors is how their resistance changes with temperature. NTC thermistors have a lower resistance as temperature rises, while PTC thermistors have a higher resistance as temperature rises.

Here are some common configurations of thermistors:

Thermistors are incredibly versatile sensors, and understanding their types is crucial for selecting the right one for your project. NTC thermistors, for example, have a resistance that decreases as temperature rises, making them perfect for temperature sensing applications.

NTC thermistors are commonly used as temperature sensors, and their accuracy has improved dramatically over the past 20 years, with some achieving ±0.1 °C or ±0.2 °C accuracy from 0 °C to 70 °C.

PTC thermistors, on the other hand, have a resistance that increases as temperature rises, making them ideal for overcurrent protection and as resettable fuses.

PTC thermistors are often installed in series with a circuit to protect against overcurrent conditions. Their unique property of increasing resistance with temperature makes them a reliable choice for this application.

Thermistors can be produced using powdered metal oxides and come in various styles, including axial-leaded glass-encapsulated, glass-coated chips, and surface-mount designs.

Some thermistors have a maximal operating temperature of +300 °C, while others may have a lower operating range of −55 °C to +150 °C.

Here are the main types of thermistors:

NTC thermistors are more widely used due to their sensitivity and linearity, making them a popular choice for temperature sensing applications.

Common configurations of thermistors include disc and chip thermistors, probe assemblies, and surface mount devices.

Littelfuse offers a variety of discrete negative temperature coefficient (NTC) sensors and thermistors for temperature sensing.

Discrete NTC sensors are great for many applications because they're highly accurate and reliable.

These sensors are designed to sense temperature changes and can be used in a wide range of environments.

They're particularly useful in temperature sensing applications where precision is key.

Discrete NTC sensors are also relatively simple to integrate into existing systems.

In harsh environments, glass-coated chip thermistors make highly reliable temperature sensors.

They're protected by a glass coating that shields the sensor from damage.

This coating helps ensure the sensor continues to function accurately even in the toughest conditions.

Intriguing read: Ntc 10k Thermistor

Thermistor sensors are incredibly versatile, offering a range of advantages that make them a popular choice in various applications.

One of the key benefits of thermistor sensors is their compact size, which allows for easy installation in tight spaces. They're also incredibly fast, with response times of just milliseconds, making them ideal for temperature control applications.

Thermistors are known for their high accuracy and stability, even over long periods of use, and they don't require recalibration. This makes them a reliable choice for a wide range of applications.

Their mass producibility and cost-effectiveness also make them an affordable option for commercial products. Plus, they're immune to electric and magnetic noise disturbances, which can be a major issue with other types of sensors.

Here are some of the key applications for thermistor sensors:

Thermistors are a popular choice for temperature measurement due to their numerous advantages. They offer a compact size, making them ideal for measuring temperatures in tight spaces.

One of the key benefits of thermistors is their fast response times, which enable swift feedback for temperature control applications. This is particularly useful in applications where temperature control is critical.

Thermistors are known for their high accuracy and stability over long operational lifetimes, making them a reliable choice for many applications. In fact, they don't require recalibration, which can save time and resources.

For your interest: Temperature Control Using Thermistor

NTC and PTC thermistors are mass producible and cost-effective, making them an affordable option for commercial products. This is a significant advantage over other temperature sensors.

Here are some of the key advantages of thermistors:

The use of sigma-delta ADCs with thermistors simplifies the design process and reduces costs. These ADCs offer high resolution and flexibility in terms of filter type and output data rate.

Temperature sensors and thermistors are used in a wide range of applications, from industrial machinery to household appliances. They play a crucial role in ensuring the safe and efficient operation of these devices.

Temperature sensors are used in industrial machinery, medical devices, and household appliances. They provide critical temperature data that helps prevent overheating, which can lead to equipment failure or even fires.

One of the key applications of PTC thermistors is thermal protection of motors, transformers, and power electronics components. This helps prevent overheating and extends the lifespan of these components.

Temperature compensation is also an important application of thermistors. It's used in signal conditioning and measurement circuitry to ensure accurate readings. This is particularly important in medical devices, where accuracy is critical.

Here are some examples of appliances that use temperature sensors and thermistors:

Overcurrent and overtemperature protection is another key application of PTC thermistors. They can be used as self-resetting fuses to prevent damage to equipment in the event of an electrical surge.

Inrush current limiting is also an important application of PTC thermistors. It's used in switch-mode power supplies to prevent damage to components and extend their lifespan. This is particularly important in high-power applications, where electrical surges can be particularly damaging.