What Is a Thermistor and How Does It Work

A thermistor is a type of temperature-sensing device that's incredibly useful in a wide range of applications.

Thermistors are made from a special kind of material that changes its electrical resistance in response to changes in temperature. This property makes them super useful for measuring temperature.

As the temperature increases, the electrical resistance of a thermistor decreases, and as the temperature decreases, the resistance increases. This means that thermistors can be used to detect even small changes in temperature.

Thermistors are often used in electronic devices, such as thermostats and temperature controllers, to help regulate temperature and prevent overheating or overcooling.

Discover more: How Do Thermistors Work

A thermistor is a type of temperature sensor that changes its electrical resistance in response to changes in temperature.

Thermistors are made from a special material that expands and contracts as the temperature rises or falls, causing its electrical resistance to change. This property allows thermistors to be used in a variety of applications, including temperature control systems and thermostats.

Thermistors are often used in applications where a high degree of accuracy is required, such as in laboratory equipment and medical devices.

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So, what is a thermistor? A thermistor is a type of temperature-sensing device.

It's essentially a resistor that changes its resistance value in response to temperature changes. Thermistors are super sensitive to temperature fluctuations.

They're often used in applications where precise temperature control is crucial, like in industrial processes or medical equipment. Thermistors are also used in everyday devices like thermostats and temperature sensors.

A thermistor's resistance value decreases as the temperature increases, and vice versa. This property makes them ideal for measuring temperature changes.

Thermistors are available in different types, including negative temperature coefficient (NTC) and positive temperature coefficient (PTC) thermistors. NTC thermistors decrease in resistance with increasing temperature, while PTC thermistors increase in resistance with increasing temperature.

In summary, thermistors are temperature-sensing devices that use resistance changes to measure temperature fluctuations.

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A thermistor is a type of resistor whose resistance is dependent on temperature.

Thermistors can be classified into two main types: positive temperature coefficient (PTC) and negative temperature coefficient (NTC).

Suggestion: Thermistor and Temperature

PTC thermistors increase in resistance as temperature increases.

NTC thermistors decrease in resistance as temperature increases.

Thermistors are commonly used in temperature measurement and control applications.

They can be found in everything from household thermostats to industrial temperature control systems.

Thermistors have several advantages over other temperature-sensing devices, including high accuracy and fast response time.

However, they can also be sensitive to environmental factors such as humidity and vibration.

In some cases, thermistors can be used in conjunction with other temperature-sensing devices to improve accuracy and reliability.

Thermistors come in two main types: NTC (Negative Temperature Coefficient) and PTC (Positive Temperature Coefficient). NTC thermistors decrease in resistance as temperature rises, while PTC thermistors increase in resistance as temperature rises.

The materials used to manufacture thermistors are typically metal oxides, such as chromium, manganese, cobalt, iron, and nickel. These oxides form a ceramic body with conductive metal terminals.

Thermistors can be produced in various shapes, including beads, disks, and cylindrical shapes, and are often encapsulated with an impermeable material like epoxy or glass. Some common shapes include bead thermistors for embedding into devices, and rod, disk, or cylindrical thermistors for optical surfaces.

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Here are some common configurations of NTC thermistors:

Thermistors are classified into two main types: NTC and PTC. NTC thermistors decrease in resistance as temperature rises, usually due to electrons being bumped up by thermal agitation from the valence band to the conduction band.

NTC thermistors are commonly used as temperature sensors or in series with a circuit as an inrush current limiter. They can achieve accuracies over wide temperature ranges, such as ±0.1 °C or ±0.2 °C from 0 °C to 70 °C, with excellent long-term stability.

PTC thermistors, on the other hand, increase in resistance as temperature rises, often due to increased thermal lattice agitations of impurities and imperfections. They are commonly installed in series with a circuit to protect against overcurrent conditions, acting as resettable fuses.

Some thermistors are made using powdered metal oxides, while others use ceramic or polymer materials. NTC thermistors have a typical operating temperature range of −55 °C to +150 °C, though some glass-body thermistors can withstand up to +300 °C.

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Here are the main types of thermistors:

There are also different configurations of NTC thermistors, including:

Thermistors are typically built using metal oxides, which are pressed into shapes like beads, disks, or cylinders and then encapsulated with a material like epoxy or glass.

These metal oxides are often from the iron group of metals, such as chromium, manganese, cobalt, iron, and nickel, which form a ceramic body with conductive metal terminals like silver, nickel, and tin.

NTC thermistors are made from oxides like chromium, manganese, cobalt, iron, and nickel, while PTC thermistors are usually prepared from barium, strontium, or lead titanates.

Some thermistors can be produced using a resonant acoustic mixing process, which reduces production time and eliminates the calcination step.

Here's a breakdown of the common materials used to make thermistors:

There are two main types of thermistors: NTC (negative temperature coefficient) and PTC (positive temperature coefficient).

NTC thermistors decrease in resistance as temperature increases, making them useful for temperature measurement and compensation. This decrease in resistance is due to the increase in charge carriers in the material as temperature rises.

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PTC thermistors, on the other hand, increase in resistance as temperature increases, making them useful for current limiting and protection.

Here are some examples of PTC thermistor applications:

Negative Temperature Coefficient is a type of thermistor that decreases in resistance as temperature increases.

Many NTC thermistors are made from a pressed disc, rod, plate, bead or cast chip of semiconducting material such as sintered metal oxides.

Raising the temperature of a semiconductor increases the number of active charge carriers by promoting them into the conduction band.

In certain materials like ferric oxide (Fe2O3) with titanium (Ti) doping an n-type semiconductor is formed and the charge carriers are electrons.

Materials such as nickel oxide (NiO) with lithium (Li) doping create a p-type semiconductor, where holes are the charge carriers.

The resistance of the material is linearly proportional to the temperature over small changes in temperature if the right semiconductor is used.

There are many different semiconducting thermistors, with a range from about 0.01 kelvin to 2,000 kelvins (−273.14 °C to 1,700 °C).

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PTC thermistors are a type of temperature-sensing device that changes resistance in response to temperature changes. They're commonly used in a variety of applications, including circuit protection, timers, and heaters.

In circuit protection, PTC thermistors work by increasing their resistance when heated, which limits the current flowing through the device. This creates a self-reinforcing effect that drives the resistance upwards, preventing damage to the circuit.

PTC thermistors are also used in timers, such as those found in CRT displays. They heat up when current flows through them, causing the degaussing coil to shut off after a short period. This ensures that the magnetic field produced by the coil decreases smoothly and continuously.

PTC thermistors can be used as heaters in the automotive industry, providing cabin heating or heating diesel fuel in cold conditions. They're also used in temperature-compensated voltage-controlled oscillators in synthesizers and in lithium battery protection circuits.

In some electronic circuits, PTC thermistors can prevent thermal runaway, where devices draw more power as they get hotter. They can also prevent current hogging, where devices connected in parallel draw uneven amounts of current.

PTC thermistors are available for immediate delivery through stocking distributors, making it easy to find the right product for your needs.

The Steinhart-Hart Equation is a widely used model for characterizing the performance of thermistors. It's a third-order approximation that provides a more faithful characterization of the resistance-temperature transfer function over a wider temperature range.

The equation is not dimensionally correct, meaning that a change in the units of resistance results in a different form of the equation. However, in practice, it gives good numerical results for resistances expressed in ohms or kiloohms.

The Steinhart-Hart equation is typically truncated after the cubed term, resulting in the standard equation: 1/T = A + B(lnR) + C(lnR)^3. This equation calculates with greater precision the actual resistance of a thermistor as a function of temperature.

The more narrow the temperature range, the more accurate the resistance calculation will be. Most thermistor manufacturers provide the A, B, and C coefficients for a typical temperature range.

The Steinhart-Hart equation is often used in conjunction with the B-parameter equation, which is essentially the Steinhart-Hart equation with specific values for the coefficients. The B-parameter equation can be used to convert the function of resistance vs. temperature of a thermistor into a linear function of lnR vs. 1/T.

Here are the typical values for the Steinhart-Hart parameters for a thermistor with a resistance of 3 kΩ at room temperature (25 °C = 298.15 K):

These values demonstrate the relationship between temperature and resistance for this particular thermistor.

Self-heating effects can introduce significant errors in temperature measurements, but this effect can also be exploited in certain applications. This is because the heat generated by the thermistor's electrical resistance can be transferred to the surrounding environment, allowing it to detect subtle changes in thermal conductivity.

The power dissipated in a thermistor is typically maintained at a very low level to ensure insignificant temperature measurement error due to self-heating. However, some applications depend on significant self-heating to raise the body temperature of the thermistor well above the ambient temperature.

In such cases, the thermistor can be used to detect even subtle changes in the thermal conductivity of the environment, making it useful for applications like liquid-level detection, liquid-flow measurement, and air-flow measurement. The dissipation constant, which measures the thermal connection of the thermistor to its surroundings, is typically given for the thermistor in still air and in well-stirred oil.

Here are some examples of thermistor applications that exploit self-heating:

Self-heating effects can introduce significant errors in temperature measurement if a correction is not made.

The electrical heating generated by a current flowing through a thermistor can raise the temperature of the thermistor above that of its environment, which can be a problem if the thermistor is used to measure the temperature of the environment.

This effect can be exploited, however, to make a sensitive air-flow device or serve as a timer for a relay.

The power dissipated in a thermistor is determined by the formula P = IV, where I is the current and V is the voltage drop across the thermistor.

At equilibrium, the rate of heat transfer from the thermistor to the surroundings is equal to the rate of heat generated by the electrical current.

The dissipation constant, K, is a measure of the thermal connection of the thermistor to its surroundings, and it is generally given for the thermistor in still air and in well-stirred oil.

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Typical values for a small glass-bead thermistor are 1.5 mW/°C in still air and 6.0 mW/°C in stirred oil.

To minimize temperature measurement error due to self-heating, the power dissipated in a thermistor is typically maintained at a very low level.

However, some thermistor applications depend on significant self-heating to raise the body temperature of the thermistor well above the ambient temperature, allowing it to detect subtle changes in the thermal conductivity of the environment.

Using PTC thermistors as an inrush current limiter is the optimal choice in certain situations, such as in high-power applications like the MM35-DIN Series for High Power Inrush Current Applications.

PTC thermistors offer fast response times, with some models responding in as little as ±1%.

One of the key benefits of PTC thermistors is their ability to limit inrush current, making them a popular choice for applications such as transformer inrush current protection.

PTC thermistors are available in various forms, including radial leaded, surface mount, SMD chip, epoxy, glass-encapsulated, and probe assemblies.

In contrast to NTC thermistors, PTC thermistors have a unique construction that involves non-conductive crystalline organic materials mixed with carbon black particles, making them conductive.

The benefits of using NTC thermistors include fast response times, accuracy, customizable packaging, noise immunity, and cost efficiency.

Here are some key benefits of NTC thermistors:

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 produced by resonant acoustic mixing of metal oxides, followed by a sintering process, which reduces production time and eliminates the calcination step entirely.

Some thermistors are manufactured using a polycrystalline ceramic material that contains barium titanate, which has been doped with rare earth materials to give it a positive temperature coefficient resistance.

Here are some common thermistor materials and their applications:

Silicon PTC thermistors, also known as "Silistor" thermistors, display a significant positive temperature coefficient resistance, making them suitable for temperature compensation and sensing applications.

The symbol for a thermistor is crucial to understand, and it's not as simple as it seems. The thermistor symbol used will depend on the type – note the subtle difference between NTC and PTC, indicated by the + or – before t°.

A thermistor is typically represented by a symbol that looks like a resistor, but with a lowercase 't' or a Greek letter beta (β) attached to it. This symbol is often used in circuit diagrams and schematics.

The symbol for a thermistor can be quite small, so it's easy to overlook. However, it's essential to pay attention to the symbol's orientation and the presence of a + or – sign, as this can indicate whether it's an NTC or PTC thermistor.

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Ceramic Switching PTC thermistors are made using a polycrystalline ceramic material that contains barium titanate, which has been doped with rare earth materials to give it positive temperature coefficient resistance.

This type of material is ideal for PTC thermistors because it exhibits a highly non-linear resistance-temperature curve.

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

These materials are often doped with rare earth materials to enhance their properties.

The manufacturing process for PTC thermistors can be more complex than other types of thermistors, but it allows for the creation of devices with unique properties.

Polymeric thermistors, also known as Resettable Fuses, are a type of thermistor that displays a nonlinear positive temperature coefficient effect. They're thermally activated devices that can be influenced by ambient temperature fluctuations.

In normal operating conditions, Polymeric PTC thermistors have minimal resistance and little impact on circuit performance. However, if the circuit system goes into a fault state, they respond by going into a high resistive or "tripping" state.

The benefits of using Polymeric PTC thermistors include being resettable, compact in size, and having minimal power loss. They also come in various configurations, such as radial leaded and surface mount.

Here are some key specifications to consider when working with Polymeric PTC thermistors:

Silicon is a stable material that offers both stability and a longer operational life, making it a great choice for thermistors that need to withstand high temperatures.

Silicon thermistors, also known as "Silistor" PTC thermistors, are linear devices with a significant positive temperature coefficient resistance.

One of the benefits of silicon thermistors is their high-temperature coefficient, which means they can detect temperature changes accurately.

They also come in multiple configurations, making them versatile and adaptable to different applications.

Silicon thermistors are highly reliable, which is a major advantage in industries where equipment failure can be costly.

Here are some key benefits of silicon thermistors:

Thermistors are incredibly versatile and can be used in a wide range of applications. They're perfect for temperature measurement, temperature compensation, and temperature control, making them a staple in various industries.

Thermistors are also known for their fast response time, with some models responding in as little as ±1%. This makes them ideal for applications where immediate feedback is required. They're also highly accurate, with an accuracy range of 0.05 to 0.20 ˚C.

Some of the key benefits of thermistors include their compact size, cost efficiency, and noise immunity. They're also customizable to meet different application requirements, making them a great choice for a variety of projects.

Here are some of the key applications for thermistors:

A circuit's function is to control the flow of electricity, and it does this by automatically adjusting the resistance of components like thermistors. This means that the circuit can adapt to changing conditions, like temperature.

In a circuit, components like thermistors work as variable resistors, which can change their resistance in response to external factors. This is useful in applications where temperature needs to be monitored or controlled.

A thermistor, for example, can be used in a circuit to sense temperature changes, making it a crucial component in many temperature-sensing applications.

Thermistors are incredibly versatile temperature-sensing devices that can be used in a wide range of applications. They're perfect for temperature measurement, compensation, and control.

Temperature measurement is one of the most common applications of thermistors. They can be used to monitor temperature changes in various environments, from food preparation equipment to chemical manufacturing processes.

Here are some specific applications of thermistors:

In addition to these applications, thermistors can also be used for inrush current limiting, which is especially useful in electronic devices. They can help protect against power surges and ensure a stable power supply.

Thermistors are also used in automated control systems, where they can activate cooling or heating via computer or electronic control of valves, pumps, inlets, flow rates, and so on.

In terms of packaging, thermistors come in a variety of forms, including radial-leaded, surface-mount, SMD chip, epoxy, glass-encapsulated, and probe assemblies.

Overall, thermistors are a reliable and accurate choice for temperature sensing applications, offering fast response times, compact sizes, and cost efficiency.

To ensure your thermistor works as expected, calibration is crucial. You'll need to place it alongside pre-calibrated temperature sensors in a controlled environment to adjust the temperature and note the thermistor's response over a range of temperatures.

The relationship between temperature and resistance is curved, not linear, so you'll need at least five points to plot the curve on your thermistor graph and predict the points in between. This means you can't rely on just two temperature/resistance pairings to get an accurate reading.

If you're calibrating a thermistor for a very temperature-sensitive application, make sure you choose one that's most accurate within that range. This is especially important in applications where the wrong type could set off a chain of events in motion, such as continually adding heat when it should be cooling.

Here are some key considerations to keep in mind when selecting a thermistor:

Calibration is a crucial step in ensuring your thermistor is accurate and reliable. You'll need to place it alongside pre-calibrated temperature sensors in a controlled environment, such as an enclosed, insulated, fluid-filled tank or gas-filled chamber.

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To determine the range of operating temperatures, you'll need to know where your thermistor will be used. This will help you choose the right type of thermistor and ensure it's accurate within that range. Knowing the range of operating temperatures will also help you decide how many calibration points you'll need.

To calibrate a thermistor, you'll need at least five temperature/resistance pairings to plot the curve on your thermistor graph. The more points you have, the better your curve will be. You'll also need to be aware of the temperature range where the temperature/resistance curve will be steeper, and less precise over small temperature changes.

Choosing the right thermistor is crucial, especially when it comes to temperature control. The wrong type can set off a chain of events that could have disastrous consequences.

Base resistance is a critical factor to consider. If you're installing a new application, you need to select the right base resistance based on your application requirements. If you're replacing a thermistor, make sure to match the current base resistance.

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Resistance vs. temperature curve is another important consideration. If you're installing a new application, determine the correct resistance vs. temperature curve relationship. If you're replacing a device, be sure to match the information from the existing thermistor.

Thermistor packaging should also be carefully selected. Make sure the packaging accommodates your application requirements.

Here are some special considerations to keep in mind:

A thermistor is a type of temperature-sensing device that's commonly used in a wide range of applications, from electronics to industrial processes.

The dissipation constant is a key parameter that affects a thermistor's performance, and it's expressed in milliwatts per degree C (mw/°C) at a specified ambient temperature.

A thermistor's material constant, also known as beta (β), is a measure of its resistance at one temperature compared to its resistance at a different temperature. It's expressed in degrees kelvin (°K) and can be calculated using a specific formula.

Maximum power rating is another important parameter that determines how much power a thermistor can dissipate for an extended period of time without losing its characteristics.

Here are some key terms related to thermistors:

A thermistor's operating temperature range is crucial to its performance, and it's essential to ensure that the device operates within its specified range to avoid any issues.

The heat capacity of a thermistor is the amount of heat required to increase its body temperature by one degree centigrade (1°C), and it's typically expressed in watt-second per cubic inch per degree C (watt-sec / in3/°C).

A PTC thermistor's switching temperature is the point at which its resistance begins to increase rapidly, and it's a critical parameter that affects the device's performance.

The switching time of a PTC thermistor is the amount of time it takes for the device to switch into its high resistance state, and it's an essential parameter to consider when designing a thermistor-based system.

Here are some key parameters related to PTC thermistors:

A polymeric PPTC thermistor's hold current is the maximum steady state current that can be passed through the device at 23°C without causing it to trip.

The maximum initial resistance of a PPTC thermistor is the maximum resistance of the device in its initial state at 23°C, and it's an essential parameter to consider when designing a PPTC-based system.