Measuring Temperature with Thermistors: A Detailed Explanation

Thermistors are incredibly versatile temperature-measuring devices. They can be used to measure temperatures in a wide range of applications, from industrial processes to medical equipment.

One of the key advantages of thermistors is their high accuracy, which is typically around ±0.5°C. This makes them a reliable choice for applications where precise temperature control is crucial.

In addition to their accuracy, thermistors are also highly sensitive to temperature changes, which allows them to detect even small changes in temperature. This makes them ideal for applications where temperature stability is critical.

Thermistors can be used to measure temperatures in a variety of environments, including high-temperature applications up to 300°C.

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A thermistor is a highly sensitive temperature-sensing device that's widely used in various applications due to its precision and reliability.

Thermistors are built into complex applications such as laser stabilization detectors, optical blocks, and charge-coupled devices, with a 10 kΩ thermistor being the standard one built into laser packages.

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Thermistors are a type of resistor whose resistance changes with the change in temperature, making them a great tool for measuring temperature.

They are inexpensive and accurate in measuring temperature, but do not operate well in extreme conditions such as very high or very low temperatures.

A thermistor's resistance is affected by temperature changes, but its temperature coefficient is low, making the changes in its electrical properties not that significant.

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Thermistors are incredibly useful for measuring temperature, and understanding how they work is essential for getting accurate readings.

A thermistor is made of semiconductor material whose resistance greatly depends on the surrounding temperature. This means that the resistance of a thermistor will change as the temperature changes.

There are two main types of thermistors: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors are the most commonly used and are great for temperature sensing and control applications.

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NTC thermistors have a resistance that decreases as the temperature increases. This is the opposite of PTC thermistors, which have a resistance that increases as the temperature increases.

The type of material used in the thermistor will dictate how much the resistance changes with temperature. This means that different thermistors will have different temperature-resistance curves.

For an NTC thermistor, the resistance decreases as the temperature increases. This is because the material used in the thermistor is sensitive to temperature changes.

The change in resistance can be calibrated and measured to calculate the exact temperature of the environment in a circuit. This requires maximum surface contact with the component or equipment, which can be achieved with special thermal paste.

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

Thermistors come in two main types: NTC and PTC. NTC thermistors have a negative temperature coefficient, meaning their resistance decreases as temperature increases. This property makes them ideal for measuring temperature in various applications.

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The two primary types of thermistors are Negative Temperature Coefficient (NTC) thermistors and Positive Temperature Coefficient (PTC) thermistors. NTC thermistors have a negative TCR value, indicating a decrease in resistance with increasing temperature. PTC thermistors, on the other hand, have a direct relationship between device resistance and temperature, with resistance increasing above a certain point.

A PTC thermistor's resistance to temperature change is very non-linear, initially decreasing with increasing temperature before hitting a minimum resistance value and then sharply increasing with further temperature increases.

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Thermistors come in two main types: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC) thermistors.

NTC thermistors decrease their resistance as temperature increases, making them ideal for temperature measurement and control. They have a non-linear relationship between resistance and temperature, with a negative slope when plotted on a graph.

PTC thermistors, on the other hand, increase their resistance as temperature increases. They are often used for circuit protection as they can limit current in response to increased temperatures.

There are two primary types of PTC thermistors: thermally sensitive silicon resistors (silistors) and switching PTC thermistors. Silistors exhibit a more linear increase in resistance with temperature change over their rated operating range.

The two primary types of thermistors are NTC and PTC thermistors. Their electrical properties are determined by their temperature coefficient, which can be either positive or negative.

Here's a breakdown of the two types of thermistors:

Identifying your I-type thermistor can be a bit tricky, but there's a way to figure it out. The type of thermistor can usually be identified by referring to its datasheet or part number.

If you don't have access to these, measuring the resistance at different temperatures can provide some clues about the type of thermistor you're dealing with.

Thermistors are temperature-measuring devices that come in two main types: NTC and PTC thermistors. An NTC thermistor decreases its resistance as the temperature increases, while a PTC thermistor increases its resistance as the temperature increases.

Testing a thermistor usually involves measuring its resistance at different temperatures and comparing the results to the expected resistance-temperature curve or characteristics given by the manufacturer.

The change in resistance of a thermistor needs to be converted to temperature to produce measurable data. This can be done using the Steinhart-Hart equation, which calculates the actual resistance of a thermistor as a function of temperature with extreme accuracy.

A thermistor's temperature coefficient is a measure of how much its resistance changes with a change in temperature. It is usually expressed in %/°C for NTC thermistors and ohms/°C for PTC thermistors.

Here are some key performance specifications and terms for thermistors:

A thermistor's characteristic is its ability to change its resistance in response to temperature changes. This is a fundamental property of thermistors, making them useful for temperature measurement and control.

One key characteristic of thermistors is their non-linear resistance-temperature relationship. This means that the resistance of a thermistor does not change in a straight line with temperature, but rather in a curved or exponential fashion.

The Steinhart-Hart equation is a mathematical model that describes this non-linear relationship between resistance and temperature. It's a powerful tool for accurately calculating temperature from measured resistance values.

Thermistors can be categorized into two main types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors decrease their resistance as temperature increases, while PTC thermistors increase their resistance as temperature increases.

Here's a summary of the key characteristics of thermistors:

Thermistors are widely used in various applications, including temperature measurement, control, and sensing. Their unique characteristics make them an ideal choice for many industries and applications.

Drift refers to the gradual change or shift in a thermistor's resistance value over time that isn't caused by changes in temperature.

This type of degradation can affect measurement accuracy.

A thermistor can measure negative temperatures, but only if the negative temperature is within its operational range.

It's crucial to check the specifications of the thermistor for its temperature range to ensure it can handle the temperature you're working with.

A thermistor's ability to measure negative temperatures is limited by its design and construction.

In some cases, a thermistor can accurately measure temperatures as low as -200°C, but this is highly dependent on the specific thermistor being used.

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Thermistors can be used to measure temperature by passing a known current through them and measuring the voltage drop across them. This change in resistance corresponds to a specific temperature.

To measure temperature with a thermistor, you'll need to use a multimeter to measure the resistance at different temperatures and compare the results to the expected resistance-temperature curve or characteristics given by the manufacturer.

Thermistors can measure a wide range of temperatures, including below-freezing temperatures, as long as the specific range is within the operational range of the thermistor. It's essential to check the specifications of the thermistor for its temperature range.

Here are some common applications of thermistors in temperature measurement:

The response time of thermistors is typically faster compared to other temperature sensors, making them ideal for applications that require rapid temperature monitoring.

The change in resistance of a thermistor is indeed measurable. This change can be converted to temperature, producing measurable data.

A thermistor's resistance varies with temperature, making it useful for detecting temperature changes. Unlike a standard resistor, which has a constant resistance value, a thermistor's resistance changes with temperature.

To measure the change in resistance, you can use a multimeter to measure the resistance of the thermistor at different temperatures. Compare this value to the expected resistance at that temperature based on the thermistor's datasheet or resistance-temperature chart.

The change in resistance is directly related to the temperature change. For example, an NTC thermistor's resistance decreases as the temperature increases. This means that as the temperature rises, the resistance of the thermistor will decrease, and vice versa.

Here's a simple way to understand this relationship:

In this example, as the temperature increases from 25°C to 50°C, the resistance of the thermistor decreases from 1000Ω to 500Ω. Similarly, as the temperature increases from 50°C to 75°C, the resistance decreases from 500Ω to 250Ω.

This measurable change in resistance makes thermistors a popular choice for temperature measurement applications.

Bead thermistors are manufactured in the shape of a bead, making them the smallest in size.

This compact design allows them to have the quickest response time.

They are made of a ceramic body with a wire connected directly to it, which offers better stability.

Their structure enables them to operate at very high temperatures.

To protect them from mechanical damage, bead thermistors are encapsulated in glass.

Unfortunately, their small size also means they have low current handling capabilities.

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Disc and chip thermistors are two common styles used for temperature measurement. They have a larger metal surface than bead type thermistors.

The body of a disc or chip thermistor is shaped in a disc or chip form, which allows it to have a larger metal surface. This design makes them suitable for high-density mounting.

Due to their larger surface, disc and chip thermistors have a slower response time compared to bead type thermistors. However, they also have higher current handling capabilities.

Chip thermistors are small, surface-mount devices designed for temperature sensing and compensation in various electronic applications. Their small size makes them ideal for high-density mounting.

Disc and chip thermistors are often used in medical devices like digital thermometers to measure body temperature accurately and quickly.

Cylindrical thermistors are a type of thermistor that's pressed into a cylindrical shape, making them larger in size compared to others. They're robust and reliable.

Their larger size might make them seem less portable, but it actually contributes to their durability and stability.

Thermistors are widely used in everyday household devices, ensuring optimal performance and safety. Digital thermometers, refrigerators, and ovens all rely on thermistors to monitor and control internal temperatures.

Thermistors are also used in protection applications, such as in lithium-ion battery packs, where they help prevent catastrophic failures by shutting off charging if the cells get too hot.

Some common uses of thermistors include:

Metalized Surface thermistors offer a unique design that sets them apart from other types. They feature metalized surface contacts instead of sintered lead wire, which provides a more direct connection.

These thermistors often have radial or axial metal contacts that allow for easy surface-mounting on a circuit board.

This design makes them ideal for applications where space is limited and a compact solution is required.

Their metalized surface contacts are also more resistant to wear and tear, making them a reliable choice for use in harsh environments.

Their compact size and robust construction make Metalized Surface thermistors a popular choice for use in a variety of applications.

Glass encapsulated thermistors are a type of thermistor that offers improved stability and protection from mechanical damage. They can operate at temperatures above 150° C.

Glass encapsulated thermistors are used in industries for accurate measurement. They are particularly useful in applications where high temperatures are involved.

These thermistors have a wide operating range, from 25° to 85° C. This makes them suitable for a variety of applications.

Glass encapsulated thermistors can be used to measure water temperature, but only if they are appropriately housed to protect them from direct contact with the water.

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Precision interchangeable sensors, like thermistors, offer a high level of precision and reliability in various applications.

They are the most precise thermistor manufactured based on a specific characteristics curve. This precision is essential in applications where small temperature variations can have a significant impact.

A notable example of precision interchangeable thermistors is their ability to offer interchangeability over the range of 0°C to 70°C. This means they can be easily swapped out for a new one without requiring individual calibration.

They are fast and have long-term stability with high-temperature accuracy. This makes them ideal for applications where temperature measurements need to be taken quickly and accurately.

Here's a comparison of the precision interchangeable thermistors with other thermistors:

As you can see, precision interchangeable thermistors offer a higher level of precision compared to standard thermistors. This makes them a popular choice for applications where accuracy is crucial.

Wireless sensors exist, and they can be integrated with thermistors to transmit temperature data wirelessly. This is particularly useful in applications where physical connections are not feasible.

You can find thermistors in everyday household devices, such as digital thermometers, refrigerators, and ovens, which monitor and control internal temperatures for efficiency and safety.

In some cases, thermistors are used in 3D printers for heated beds and hot ends, requiring calibration of filament temperature settings for optimal performance.

The cost of thermistors is often offset by the extra engineering effort to obtain an accurate temperature measurement, making them suitable for protection applications where a general idea of temperature is acceptable.

Here are some examples of applications where thermistors are used in conjunction with wireless sensors:

To implement a thermistor, you need to determine the base thermistor resistance, bias current, and setpoint temperature of the load on the temperature controller. This information will help you set up the thermistor correctly for accurate temperature measurement.

A thermistor should be placed close to the device that requires temperature control, either embedded or attached, to guarantee accuracy. The closer the thermistor is to the device, the more quickly it will react to temperature changes and the more accurate it will be.

To test a thermistor, you can measure its resistance at a known temperature and compare it to the expected resistance at that temperature as per the manufacturer's data sheet. This is a simple and effective way to determine if a thermistor is working correctly.

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Self-heating in a thermistor occurs when current passes through it, generating heat that can cause the thermistor's temperature to rise.

This phenomenon is crucial to account for in certain applications, as it can lead to inaccuracies in temperature measurements.

The heat generated by the thermistor can be prevented by reducing the current through it, which is a good strategy if you want to measure temperatures above ambient.

As the temperature goes up, the resistance of the NTC thermistor goes down, meaning that the majority of power will be dropped across the reference resistor since it has the larger voltage drop.

This helps prevent self-heating and allows for more accurate temperature measurements.

For example, if you're using a simple voltage divider with a thermistor as the top or bottom leg, reducing the current through the divider can help prevent self-heating.

By optimising the implementation, you can provide the widest possible range of voltage to measure the temperature more accurately.

In one implementation, the author used a simple divider with an upper divider that matches the resistance of the thermistor at 25 °C, which should expect half the input voltage.

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The voltage limits of the thermistor input of the temperature controller are crucial to get right.

The manufacturer will specify the voltage limits of the thermistor feedback to a temperature controller. This means you should choose a thermistor and bias current combination that produces a voltage within these limits.

Ideally, this voltage should be in the middle of the allowed range to ensure the most accurate readings.

The temperature controller feedback input needs to be in voltage, which comes from the thermistor resistance. This is then converted to temperature, but more on that later.

To avoid any issues, it's essential to choose a thermistor and bias current combination that fits within the specified voltage limits.

Adding a parallel resistor to your voltage divider can help linearize a section of the temperature range, making it easier to interpret the data. This is especially useful if you don't have the facilities to precisely collect the necessary data to determine the values for an algorithm.

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You can use a resistor in parallel to the NTC thermistor to achieve this. For example, you can add a resistor that matches the resistance of the thermistor at the centre point of the temperature range you are trying to measure. This can be useful if you're building a temperature sensor and need to optimize the resistance and topology to provide the widest possible range of voltage.

The linear section of the temperature range will need a voltage reading, which can be interpreted as differential temperature directly. By using a parallel resistor, you can create a more linear output, making it easier to read the temperature data.

It's worth noting that the parallel resistor should be placed close to the thermistor to prevent self-heating. However, in most cases, the difference in heat transfer between the two components is negligible.

You can test NTC thermistor boards yourself using open-source sensor test cards available on GitHub. These designs will save you time and effort in evaluating NTC thermistors.

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The project files for these boards can be found in the same GitHub repository, which also includes all the sensor cards developed during this series. You might even get a sneak peek at upcoming projects by checking the repository.

To test a thermistor, you'll need to measure its resistance at a known temperature and compare it to the expected resistance based on the manufacturer's data sheet. This is a straightforward process that will give you a clear understanding of your thermistor's performance.

If you're wondering whether a thermistor can be used to control a system's temperature, the answer is yes. By connecting a thermistor to a control circuit, it can regulate the temperature of a system, as commonly seen in thermostats and HVAC systems.

To verify if a thermistor is working correctly, measure its resistance with a multimeter at a known temperature and compare it to the expected resistance based on the thermistor's datasheet or resistance-temperature chart. This is the most common way to test a thermistor's functionality.

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Replacing a faulty thermistor is a relatively straightforward process that requires some basic electronics knowledge and caution.

First, ensure the device is powered off and disconnected from any power source to avoid any electrical shocks or damage.

A faulty thermistor can cause a circuit to shut down if the system interprets the readings as a dangerous over-temperature condition, so it's essential to replace it as soon as possible.

To replace a faulty thermistor, identify the faulty component, typically marked on the circuit board or wiring diagram.

Desolder or disconnect the old thermistor, then install the new one, ensuring it is properly connected and secured.

Properly connecting and securing the new thermistor is crucial to ensure accurate temperature readings and prevent any further issues.

Using thermistors to measure temperature can be a reliable and accurate method, but it's essential to take some precautions to ensure safe and proper operation.

First and foremost, thermistors are sensitive to mechanical stress, so they should be handled gently to avoid damage.

Proper safety measures should be observed when using thermistors, including ensuring the thermistor is not exposed to temperatures or voltages beyond its specification.

Overheating a thermistor can cause permanent changes in resistance and damage the device, so it's crucial to avoid operating it beyond its specified temperature range.

Thermistors can be sensitive to mechanical stress, so be gentle when handling them to prevent damage.

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A faulty thermistor can provide inaccurate temperature readings, which can result in improper functioning of the circuit or system it's part of. This can cause the circuit to shut down if the system interprets the readings as a dangerous over-temperature condition.

To troubleshoot a thermistor with inconsistent readings, start by checking the connections to ensure they are secure and free of corrosion. Loose or corroded connections can cause erratic readings from a thermistor.

Temperature fluctuations can cause changes in the resistance of a thermistor, altering its readings. For accurate results, the thermistor should ideally be in a stable temperature environment.

Self-heating effects can cause a thermistor to provide inaccurate temperature readings. This is because the thermistor itself can generate heat, which can alter its resistance and skew its readings.

Drift over time is another common problem that can occur when using thermistors. This means that the thermistor's resistance can change over time, even if the temperature remains constant.

Mechanical stress can cause changes in resistance, leading to inaccurate readings. This can happen if the thermistor is subjected to physical stress, such as being bent or flexed.

Non-linear characteristics can also cause inaccuracies in thermistor readings. This means that the thermistor's resistance doesn't change in a linear or predictable way, making it harder to get accurate temperature readings.

Loose or corroded connections can cause erratic readings from a thermistor. This is because the connection between the thermistor and the circuit can be disrupted, leading to inaccurate readings.

Physical damage to the thermistor itself can also cause erratic readings. This can happen if the thermistor is dropped or subjected to physical stress.

Electromagnetic interference can also cause erratic readings from a thermistor. This is because the thermistor can be affected by external electromagnetic fields, leading to inaccurate readings.

In a high-humidity environment, moisture can accumulate on the thermistor, altering its resistance and thus the temperature reading.

First, check the connections to ensure they are secure and free of corrosion. This is a common issue that can cause inconsistent readings.

The thermistor's location is also crucial, as it needs to be installed in a spot where it can accurately measure temperature without interference from other heat sources.

If the problem persists, the thermistor may need to be replaced.

Thermistors are a common component in many household appliances, helping to ensure optimal performance and safety. They're used in digital thermometers to measure body temperature quickly and accurately.

In refrigerators and ovens, thermistors monitor and control internal temperatures, preventing overheating and energy waste. This is especially important in ovens, where precise temperature control can make all the difference in cooking.

NTC thermistors are the type of thermistor typically used in household appliances due to their high sensitivity and accuracy over common temperature ranges. They're a reliable choice for many applications.

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Refrigerators often use thermistors to accurately monitor and control the internal temperature. This helps to keep food fresh and safe to eat.

Thermistors are also used to regulate water temperature in household appliances, preventing overheating or energy waste. This is especially useful in hot water dispensers and other appliances that rely on precise temperature control.

Here are some examples of household appliances that use thermistors:

Thermistors are typically factory-calibrated, but additional calibration can be performed by comparing the thermistor's output with the output of a reference temperature sensor under controlled conditions.

The quality of the thermistor itself is a crucial factor in its accuracy, as a well-made thermistor will provide more reliable readings.

Factory calibration is usually sufficient, but if the thermistor is faulty, it's best to replace it rather than trying to correct the readings.

A thermistor's resistance-temperature relationship is a key factor in its accuracy, and any deviations from this relationship can affect the readings.

Testing a thermistor involves measuring its resistance at different temperatures and comparing the results to the expected resistance-temperature curve or characteristics given by the manufacturer.

The stability of the operating environment can also impact the accuracy of a thermistor, as temperature fluctuations can affect its readings.

A thermistor's design and the quality of its construction can also affect its accuracy, so it's essential to choose a high-quality thermistor for your temperature measurement needs.

The precision of the calibration is also important, as any errors in the calibration process can affect the accuracy of the thermistor's readings.

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Thermistors are used in various applications in cars, such as monitoring engine temperature, cabin temperature, or coolant temperature, to ensure the vehicle operates within the optimal temperature range.

Thermistors play a crucial role in monitoring the temperature of a vehicle's engine, cabin, and coolant.

In addition to automotive applications, thermistors are used in electronics to measure temperature.

Thermistors are often used in electronic devices to detect overheating, which can cause damage or malfunction.

They are used to monitor the temperature of sensitive components, such as microprocessors and power supplies.

This helps prevent overheating and ensures the device operates within a safe temperature range.

Thermistors are also used in industrial settings to measure temperature in various processes and equipment.

Their accuracy and reliability make them a popular choice for temperature measurement in many applications.

In industrial settings, thermistors play a crucial role in precise temperature management and safety.

Thermistors are used in heating and cooling circuits, providing protection by shutting down systems during unsafe temperature levels. This prevents damage to equipment and ensures a safe working environment.

In manufacturing processes, thermistors are used in critical applications like plastic moulding or chemical synthesis, where precise temperature control is essential.

These processes require tight temperature tolerances to produce high-quality products, and thermistors help achieve this level of precision.

Here are some examples of industries that rely on thermistors for temperature management: