Understanding Thermistors and Their Applications

Thermistors are a type of temperature sensor that's incredibly versatile and widely used.

They're made from a mixture of metal oxides, which changes their electrical resistance in response to temperature changes. This property makes them perfect for measuring temperature in a variety of applications.

One of the key benefits of thermistors is their high accuracy, with some models able to detect temperature changes as small as 0.1°C. This level of precision is essential in applications like medical equipment and laboratory settings.

In addition to their accuracy, thermistors are also relatively inexpensive compared to other temperature sensors, making them a cost-effective solution for many industries.

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A thermistor is a resistance thermometer, or a resistor whose resistance is dependent on temperature.

It's made of metallic oxides, pressed into a bead, disk, or cylindrical shape and then encapsulated with an impermeable material such as epoxy or glass.

There are two main types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC).

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With an NTC thermistor, when the temperature increases, resistance decreases, and when it decreases, resistance increases.

A PTC thermistor works the opposite way: when temperature increases, resistance increases, and when it decreases, resistance decreases.

This type of thermistor is generally used as a fuse.

Thermistors are easy to use, inexpensive, sturdy, and respond predictably to changes in temperature.

They're ideal when very precise temperatures are required, and they're commonly used in digital thermometers, cars, and household appliances.

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Thermistors come in two main types: NTC and PTC. NTC thermistors have a negative temperature coefficient, meaning their resistance decreases as temperature rises. They're commonly used as temperature sensors or in series with a circuit as an inrush current limiter.

PTC thermistors, on the other hand, have a positive temperature coefficient, meaning their resistance increases as temperature rises. They're often used to protect against overcurrent conditions, as resettable fuses.

NTC thermistors are generally produced using powdered metal oxides and can achieve accuracies over wide temperature ranges, such as ±0.1 °C or ±0.2 °C from 0 °C to 70 °C.

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The typical operating temperature range of a thermistor is −55 °C to +150 °C, though some glass-body thermistors can operate up to +300 °C.

Here are some common applications for NTC thermistors:

Thermistors can be produced in various styles, including axial-leaded glass-encapsulated, glass-coated chips, epoxy-coated, and surface-mount. They can also be packaged in different enclosures, such as stainless steel, aluminum, or plastic, to suit specific applications.

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Thermistors are typically built by using metal oxides, which are 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 ceramic body is composed of conductive metals such as silver, nickel, and tin, which make up the terminals. This combination of materials allows thermistors to function effectively.

PTC thermistors, on the other hand, are usually prepared from barium, strontium, or lead titanates. This difference in composition affects their performance and application.

Thermistors can also be produced using a resonant acoustic mixing process, which reduces production time and eliminates the calcination step. This process is a more efficient method of manufacturing thermistors.

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The Steinhart-Hart equation is a widely used model for characterizing thermistor temperatures. It's a third-order approximation that's more accurate than the linear model over wider temperature ranges.

The equation is a cubic equation in ln⁡ ⁡ R{\displaystyle \ln R}, and it's not dimensionally correct, meaning the units of R can affect the equation's form. But in practice, it gives good numerical results for resistances expressed in ohms or kiloohms.

The Steinhart-Hart equation can be solved to give resistance as a function of temperature, and the error is generally less than 0.02 °C over a 200 °C range. This makes it a reliable tool for thermistor temperature measurements.

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The equation is often truncated after the cubed term, which is why the standard Steinhart-Hart equation used is 1/T = A + B(lnR) + C(lnR)3. This simplification still provides accurate results for most thermistor applications.

The Steinhart-Hart equation is used to calculate the actual resistance of a thermistor as a function of temperature, and it's more accurate for narrower temperature ranges. Most thermistor manufacturers provide the A, B, and C coefficients for a typical temperature range.

The B-parameter equation is essentially the Steinhart-Hart equation with specific values for the coefficients, and it's used to characterize NTC thermistors. It can be solved to give the temperature as a function of resistance, and it's useful for estimating the B parameter.

The Steinhart-Hart equation is a powerful tool for thermistor temperature measurements, and it's widely used in practical devices. With the right coefficients, it can provide accurate results over a wide range of temperatures.

Thermistors are highly sensitive devices that can detect even the smallest changes in temperature. They are ideal for measuring single-point temperatures and are often used in applications where accuracy is crucial.

A thermistor's sensitivity is determined by the degree of response to a change in temperature, making it a valuable asset in various industries. Thermistors can be categorized into two types: positive-temperature-coefficient (PTC) and negative-temperature-coefficient (NTC) thermistors.

PTC thermistors have a positive temperature coefficient, meaning their resistance increases with increasing temperature. This type of thermistor is often used in applications where a high resistance is required at low temperatures.

NTC thermistors, on the other hand, have a negative temperature coefficient, meaning their resistance decreases with increasing temperature. This type of thermistor is often used in applications where a low resistance is required at high temperatures.

Here are some key characteristics of thermistors:

A thermistor doesn't actually "read" temperature like a thermometer does. Instead, its resistance changes with temperature.

The resistance of a thermistor changes with temperature, and how much it changes depends on the type of material used. This non-linear relationship is what makes thermistors unique.

The change in resistance is not a straight line, unlike other sensors. The graph of resistance vs. temperature is not linear, and the location of the line and how much it changes is determined by the construction of the thermistor.

Thermistors can measure a wide range of temperatures, from -260°C to 850°C. However, their accuracy and sensitivity vary depending on the specific application.

Thermistors are durable, long-lasting, and highly sensitive, making them a popular choice for temperature measurement. They're also relatively small and inexpensive compared to other temperature sensors.

Here are some key characteristics of thermistors:

Self-heating effects can be a significant issue in thermistor measurements, introducing errors if not corrected.

The electrical power input to a thermistor is given by the formula P = IV, where I is the current and V is the voltage drop across the thermistor.

This power is converted to heat, which is then transferred to the surrounding environment at a rate described by Newton's law of cooling.

The rate of transfer is well described by the equation dQ/dt = K(T(R) - T0), where T(R) is the temperature of the thermistor as a function of its resistance R, T0 is the temperature of the surroundings, and K is the dissipation constant.

At equilibrium, the two rates must be equal, meaning the thermistor's temperature is the same as the surroundings.

If the voltage across the thermistor is held fixed, the equilibrium equation can be solved for the ambient temperature as a function of the measured resistance of the thermistor.

A typical value for the dissipation constant of a small glass-bead thermistor is 1.5 mW/°C in still air.

The power dissipated in a thermistor is typically kept low to avoid significant temperature measurement errors due to self-heating.

However, some applications, such as liquid-level detection and air-flow measurement, rely on significant self-heating to raise the thermistor's body temperature above the ambient temperature.

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Thermistors can experience a significant drift over time, with epoxy coated NTC thermistors showing a drift of about 0.2 °C per year.

This type of drift is largely due to the materials used in the thermistor's construction and packaging.

Hermetically sealed NTC thermistors, on the other hand, experience a much smaller drift of about 0.02 °C per year, making them a more stable option.

Chemical changes in the sensor, such as chemical oxidation, can also cause a significant drift in thermocouples, with a typical drift of about 1-2 °C per year.

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NTC thermistors are highly accurate through incremental changes within their operating range. Small temperature changes reflect accurately due to large changes in resistance per °C. Thermocouples have lower accuracy and require a conversion of millivolts to temperature when used for temperature control and compensation. This means that NTC thermistors are a great choice for applications where precise temperature measurements are crucial.

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NTC thermistors operate within a wide range of temperatures, making them ideal for a variety of applications. They perform well in an operating range between -50 to 250 °C.

Thermocouples, on the other hand, operate within an even wider temperature range. They can handle temperatures from -200 °C to 1750 °C.

This versatility makes both NTC thermistors and thermocouples suitable for use in different environments.

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Thermistors can operate in a wide range of applications, from life safety to industrial settings. They're commonly found in fire detectors and thermometers due to their accuracy and stability.

NTC thermistors are particularly well-suited for life safety applications because they're accurate and stable. This is because they're less prone to calibration drift over time compared to thermocouples.

In industrial settings, thermocouples are often used due to their durability and cheaper production costs. However, their accuracy and stability can be affected by calibration drift.

Thermistors can be used in various ways, including temperature measurement and control, and copper coil compensation. They're also used in applications that depend on their self-heating effect, such as voltage or power level control.

Here are some examples of how thermistors can be used:

Overall, thermistors are a versatile and reliable choice for a wide range of applications.

Thermistor design and selection can be a bit overwhelming, but understanding the basics can make all the difference. Thermistors are available in various configurations, including surface-mount, leaded, and radial-leaded types, each with its own advantages and disadvantages.

The choice of thermistor configuration depends on the specific application, with surface-mount thermistors being ideal for high-density circuit designs. Leaded thermistors, on the other hand, are often used in applications where lead length is not a concern.

When selecting a thermistor, it's essential to consider the operating temperature range, which can be as low as -50°C or as high as 250°C, depending on the type.

Resistance and Bias Current Selection is a crucial aspect of thermistor design. A thermistor's resistance decreases as temperature increases, but it's essential to select the right bias current to ensure accurate temperature measurement.

The bias current is the current flowing through the thermistor when it's not being used for measurement. This current is typically in the range of 1-10 μA.

A higher bias current can lead to a higher temperature coefficient, which is the rate at which the thermistor's resistance changes with temperature. This can improve the thermistor's accuracy but also increase power consumption.

The bias current should be chosen based on the thermistor's power rating, which is usually specified by the manufacturer. For example, a thermistor with a power rating of 1 mW might require a bias current of 1 μA or less.

In some cases, a thermistor may have a built-in bias current limiting circuit to prevent damage from excessive current. This is particularly important for thermistors used in high-temperature applications.

Surface Mount Devices (SMD) are a popular choice for temperature measurement and control in various applications.

They are suitable for standard soldering techniques, making them easy to integrate into existing designs.

Thermometrics Surface Mount Devices (SMD) Series of NTC Thermistors are available in a range of sizes, including 0402, 0603, 0805 and 1206.

These compact devices are ideal for applications where space is limited.

They are intended for temperature measurement, control, and compensation, making them a versatile solution for many industries.

Radial Lead thermistors are a great option for PCB and probe mountings, offering high sensitivity to temperature changes.

They are available in various types, including RL10, RL14, RL20, RL30, and RL35/40/45, all of which have a similar operating range of -58°F to 302°F (-50°C to 150°C).

These thermistors are suitable for a wide range of applications due to their high sensitivity and wide operating range.

The bare lead wires of Radial Lead thermistors make them easy to mount on PCBs and probes.

Thermometrics Type SA Interchangeable Radial Lead NTC Thermistors are also available, offering the same high sensitivity and wide operating range as the other types.

All of these Radial Lead thermistors are designed for high-performance temperature measurement.

Inrush current is a critical consideration in thermistor design and selection. It's the surge of current that flows into a device when it's first powered on, and it can be a major contributor to component stress and failure.

Inrush current can be particularly problematic in high-power applications, which is why some thermistors are specifically designed to handle high inrush currents. For example, the MM35-DIN Series is designed for high power inrush current applications.

Transformer inrush current protection is another area where thermistors can play a key role. By limiting the inrush current to a transformer, thermistors can help prevent damage and ensure safe operation.

In some cases, thermistors can be used as inrush current limiters, such as PTC thermistors. These thermistors can help regulate the flow of current during startup and prevent inrush current-related problems.

Here are some common applications where inrush current is a concern:

Inrush current protection is essential in these applications, and thermistors can be a reliable and effective solution. By choosing the right thermistor for the job, designers can help ensure safe and reliable operation of their devices.

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

Thermistors can be manufactured from a variety of oxides, including chromium, manganese, cobalt, iron, and nickel, which form a ceramic body with conductive metal terminals.

PTC thermistors are usually prepared from barium, strontium, or lead titanates.

Thermistors can also be produced by resonant acoustic mixing of the previously mentioned oxides, followed by a sintering process.

Thermistors come in a variety of shapes, including disk, chip, bead, and rod, and can be surface mounted or embedded in a system.

A bead thermistor is ideal for embedding into a device, while a rod, disk, or cylindrical head are best for optical surfaces.

Thermistor chips are normally mounted on a printed circuit board (PCB).

Thermistors can be encapsulated in epoxy resin, glass, baked-on phenolic, or painted.

Thermistors can be quite sensitive, with a temperature coefficient of resistance that can be as high as 4% per degree Celsius. This sensitivity is a double-edged sword, as it allows thermistors to detect small changes in temperature but also makes them prone to errors.

Their performance is also affected by the type of thermistor, with negative temperature coefficient (NTC) thermistors being more common and suitable for most applications. NTC thermistors have a resistance that decreases as the temperature increases.

Thermistors can be affected by factors like temperature, humidity, and vibration, which can all impact their accuracy and reliability.

Cryogenic thermistors are designed to handle extremely low temperatures, with some probes capable of withstanding temperatures as low as -196°C/-320.8°F, the boiling point of Liquid Nitrogen.

They're used in various applications, including liquid level detection in cryogenic liquids.

The RL Series of NTC Type Cryogenic Thermistor Probes can operate in a range of 25°C/77°F to -196°C/-320.8°F, making them suitable for a variety of cryogenic applications.

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In harsh environments, thermistors like the Thermometrics Type CR1 can withstand the test. They're designed with a high-performance acid and moisture-resistant coating.

These thermistors consist of Tin (Sn) coated Alloy 52 leads, making them ideal for high-volume assembly.

Thermometrics Type CR1 thermistors are specifically suited for applications that require their durability and resistance to harsh conditions.