What is the difference between a NTC thermistor and a PTC thermistor?

Differential equation parameter in thermal physics

A temperature coefficient describes the relative change of a physical property that is associated with a given change in temperature. For a property R that changes when the temperature changes by dT, the temperature coefficient α is defined by the following equation:

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d R R = α d T {\displaystyle {\frac {dR}{R}}=\alpha \,dT}

Here α has the dimension of an inverse temperature and can be expressed e.g. in 1/K or K&#;1.

If the temperature coefficient itself does not vary too much with temperature and α Δ T &#; 1 {\displaystyle \alpha \Delta T\ll 1} , a linear approximation will be useful in estimating the value R of a property at a temperature T, given its value R0 at a reference temperature T0:

R ( T ) = R ( T 0 ) ( 1 + α Δ T ) , {\displaystyle R(T)=R(T_{0})(1+\alpha \Delta T),}

where ΔT is the difference between T and T0.

For strongly temperature-dependent α, this approximation is only useful for small temperature differences ΔT.

Temperature coefficients are specified for various applications, including electric and magnetic properties of materials as well as reactivity. The temperature coefficient of most of the reactions lies between 2 and 3.

Negative temperature coefficient

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Most ceramics exhibit negative temperature dependence of resistance behaviour. This effect is governed by an Arrhenius equation over a wide range of temperatures:

R = A e B T {\displaystyle R=Ae^{\frac {B}{T}}}

where R is resistance, A and B are constants, and T is absolute temperature (K).

The constant B is related to the energies required to form and move the charge carriers responsible for electrical conduction &#; hence, as the value of B increases, the material becomes insulating. Practical and commercial NTC resistors aim to combine modest resistance with a value of B that provides good sensitivity to temperature. Such is the importance of the B constant value, that it is possible to characterize NTC thermistors using the B parameter equation:

R = r &#; e B T = R 0 e &#; B T 0 e B T {\displaystyle R=r^{\infty }e^{\frac {B}{T}}=R_{0}e^{-{\frac {B}{T_{0}}}}e^{\frac {B}{T}}}

where R 0 {\displaystyle R_{0}} is resistance at temperature T 0 {\displaystyle T_{0}} .

Therefore, many materials that produce acceptable values of R 0 {\displaystyle R_{0}} include materials that have been alloyed or possess variable negative temperature coefficient (NTC), which occurs when a physical property (such as thermal conductivity or electrical resistivity) of a material lowers with increasing temperature, typically in a defined temperature range. For most materials, electrical resistivity will decrease with increasing temperature.

Materials with a negative temperature coefficient have been used in floor heating since . The negative temperature coefficient avoids excessive local heating beneath carpets, bean bag chairs, mattresses, etc., which can damage wooden floors, and may infrequently cause fires.

Reversible temperature coefficient

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Residual magnetic flux density or Br changes with temperature and it is one of the important characteristics of magnet performance. Some applications, such as inertial gyroscopes and traveling-wave tubes (TWTs), need to have constant field over a wide temperature range. The reversible temperature coefficient (RTC) of Br is defined as:

RTC = | Δ B r | | B r | Δ T × 100 % {\displaystyle {\text{RTC}}={\frac {|\Delta \mathbf {B} _{r}|}{|\mathbf {B} _{r}|\Delta T}}\times 100\%}

To address these requirements, temperature compensated magnets were developed in the late s.[1] For conventional SmCo magnets, Br decreases as temperature increases. Conversely, for GdCo magnets, Br increases as temperature increases within certain temperature ranges. By combining samarium and gadolinium in the alloy, the temperature coefficient can be reduced to nearly zero.

Electrical resistance

[edit] See also: Table of materials' resistivities

The temperature dependence of electrical resistance and thus of electronic devices (wires, resistors) has to be taken into account when constructing devices and circuits. The temperature dependence of conductors is to a great degree linear and can be described by the approximation below.

ρ &#; ( T ) = ρ 0 [ 1 + α 0 ( T &#; T 0 ) ] {\displaystyle \operatorname {\rho } (T)=\rho _{0}\left[1+\alpha _{0}\left(T-T_{0}\right)\right]}

where

α 0 = 1 ρ 0 [ δ ρ δ T ] T = T 0 {\displaystyle \alpha _{0}={\frac {1}{\rho _{0}}}\left[{\frac {\delta \rho }{\delta T}}\right]_{T=T_{0}}}

ρ 0 {\displaystyle \rho _{0}} just corresponds to the specific resistance temperature coefficient at a specified reference value (normally T = 0 °C)[2]

That of a semiconductor is however exponential:

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ρ &#; ( T ) = S α B T {\displaystyle \operatorname {\rho } (T)=S\alpha ^{\frac {B}{T}}}

where S {\displaystyle S} is defined as the cross sectional area and α {\displaystyle \alpha } and B {\displaystyle B} are coefficients determining the shape of the function and the value of resistivity at a given temperature.

For both, α {\displaystyle \alpha } is referred to as the temperature coefficient of resistance (TCR).[3]

This property is used in devices such as thermistors.

Positive temperature coefficient of resistance

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A positive temperature coefficient (PTC) refers to materials that experience an increase in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid increase with temperature, i.e. a higher coefficient. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase. A PTC material can be designed to reach a maximum temperature for a given input voltage, since at some point any further increase in temperature would be met with greater electrical resistance. Unlike linear resistance heating or NTC materials, PTC materials are inherently self-limiting. On the other hand, NTC material may also be inherently self-limiting if constant current power source is used.

Some materials even have exponentially increasing temperature coefficient. Example of such a material is PTC rubber.

Negative temperature coefficient of resistance

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A negative temperature coefficient (NTC) refers to materials that experience a decrease in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid decrease with temperature, i.e. a lower coefficient. The lower the coefficient, the greater a decrease in electrical resistance for a given temperature increase. NTC materials are used to create inrush current limiters (because they present higher initial resistance until the current limiter reaches quiescent temperature), temperature sensors and thermistors.

Negative temperature coefficient of resistance of a semiconductor

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An increase in the temperature of a semiconducting material results in an increase in charge-carrier concentration. This results in a higher number of charge carriers available for recombination, increasing the conductivity of the semiconductor. The increasing conductivity causes the resistivity of the semiconductor material to decrease with the rise in temperature, resulting in a negative temperature coefficient of resistance.

Temperature coefficient of elasticity

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The elastic modulus of elastic materials varies with temperature, typically decreasing with higher temperature.

Temperature coefficient of reactivity

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In nuclear engineering, the temperature coefficient of reactivity is a measure of the change in reactivity (resulting in a change in power), brought about by a change in temperature of the reactor components or the reactor coolant. This may be defined as

α T = &#; ρ &#; T {\displaystyle \alpha _{T}={\frac {\partial \rho }{\partial T}}}

Where ρ {\displaystyle \rho } is reactivity and T is temperature. The relationship shows that α T {\displaystyle \alpha _{T}} is the value of the partial differential of reactivity with respect to temperature and is referred to as the "temperature coefficient of reactivity". As a result, the temperature feedback provided by α T {\displaystyle \alpha _{T}} has an intuitive application to passive nuclear safety. A negative α T {\displaystyle \alpha _{T}} is broadly cited as important for reactor safety, but wide temperature variations across real reactors (as opposed to a theoretical homogeneous reactor) limit the usability of a single metric as a marker of reactor safety.[4]

In water moderated nuclear reactors, the bulk of reactivity changes with respect to temperature are brought about by changes in the temperature of the water. However each element of the core has a specific temperature coefficient of reactivity (e.g. the fuel or cladding). The mechanisms which drive fuel temperature coefficients of reactivity are different from water temperature coefficients. While water expands as temperature increases, causing longer neutron travel times during moderation, fuel material will not expand appreciably. Changes in reactivity in fuel due to temperature stem from a phenomenon known as doppler broadening, where resonance absorption of fast neutrons in fuel filler material prevents those neutrons from thermalizing (slowing down).[5]

Mathematical derivation of temperature coefficient approximation

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In its more general form, the temperature coefficient differential law is:

d R d T = α R {\displaystyle {\frac {dR}{dT}}=\alpha \,R}

Where is defined:

R 0 = R ( T 0 ) {\displaystyle R_{0}=R(T_{0})}

And α {\displaystyle \alpha } is independent of T {\displaystyle T} .

Integrating the temperature coefficient differential law:

&#; R 0 R ( T ) d R R = &#; T 0 T α d T &#; ln &#; ( R ) | R 0 R ( T ) = α ( T &#; T 0 ) &#; ln &#; ( R ( T ) R 0 ) = α ( T &#; T 0 ) &#; R ( T ) = R 0 e α ( T &#; T 0 ) {\displaystyle \int _{R_{0}}^{R(T)}{\frac {dR}{R}}=\int _{T_{0}}^{T}\alpha \,dT~\Rightarrow ~\ln(R){\Bigg \vert }_{R_{0}}^{R(T)}=\alpha (T-T_{0})~\Rightarrow ~\ln \left({\frac {R(T)}{R_{0}}}\right)=\alpha (T-T_{0})~\Rightarrow ~R(T)=R_{0}e^{\alpha (T-T_{0})}}

Applying the Taylor series approximation at the first order, in the proximity of T 0 {\displaystyle T_{0}} , leads to:

R ( T ) = R 0 ( 1 + α ( T &#; T 0 ) ) {\displaystyle R(T)=R_{0}(1+\alpha (T-T_{0}))}

Units

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The thermal coefficient of electrical circuit parts is sometimes specified as ppm/°C, or ppm/K. This specifies the fraction (expressed in parts per million) that its electrical characteristics will deviate when taken to a temperature above or below the operating temperature.

See also

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• Microbolometer (used to measure TCRs)

References

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Bibliography

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Thermistors are a type of semiconductor that react like a resistor sensitive to temperature - meaning they have greater resistance than conducting materials, but lower resistance than insulating materials. To establish a temperature measurement, the measured value of a thermistor's electrical resistance can be correlated to the temperature of the environment in which that thermistor has been situated.

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The term 'thermistor' is a portmanteau - it is derived from the term THERMally sensitive ResISTOR - and these devices are a very accurate and cost-effective option for temperature measurement.
The reasons that thermistors continue to be popular for measuring temperature is: - Their higher resistance change per degree of temperature provides greater resolution - High level of repeatability and stability (±0.1

What Are Thermistors Composed Of?

The relationship between a thermistor's temperature and its resistance is highly dependent upon the materials from which it is composed. Thermistor manufacturers typically determine this property with a high degree of accuracy - as this is the primary characteristic of interest to thermistor buyers.

Thermistors are made up of metallic oxides, binders, and stabilizers pressed into wafers and then cut to chip size, left in disc form, or made into another shape. The precise ratio of the composite materials governs their resistance/temperature "curve". Manufacturers typically control this ratio with great accuracy, as it determines how the thermistor will function.

Types of Thermistors

Thermistors are available in two types: those with Negative Temperature Coefficients (NTC Thermistors) and those with Positive Temperature Coefficients (PTC Thermistors). An NTC Thermistor's resistance decreases as its temperature increases. A PTC Thermistor's resistance increases as its temperature increases.
NTC Thermistors are more commonly used for temperature measurement while PTC Thermistors are primarily used for circuit protection.

Thermistors are composed of materials with known resistance. As the temperature increases, an NTC Thermistor&#;s resistance will increase in a non-linear fashion, following a particular &#;curve.&#; The shape of this resistance vs. temperature curve is determined by the properties of the materials that make up the thermistor. 

Thermistors are available with a variety of base resistances and resistance vs. temperature curves. Low-temperature applications (-55 to approx. 70°C) generally use lower resistance thermistors to 10,000Ω). Higher temperature applications generally use higher resistance thermistors (above 10,000Ω). Some materials provide better stability than others. Resistances are normally specified at 25°C (77°F). Thermistors are accurate to approximately ± 0.2°C within their specified temperature range. They&#;re generally durable, long-lasting, and inexpensive.

Thermistors with epoxy coatings are available for use at lower temperatures [typically -50 to 150°C (-58 to 316°F)]; thermistors are also available with glass coatings for use at higher temperatures [typically -50 to 300°C (-58 to 572°F)]. These coatings protect the thermistor and its connecting wires from humidity, corrosion, and mechanical stress.

Available Thermistor Configurations

Thermistors are available in several common configurations. The three most frequently employed are the hermetically sealed flexible thermistor (HSTH series), the bolt-on/washer type, and the self-adhesive surface-mount style.

HSTH Thermistors are completely sealed within PFA (plastic polymer) jackets to protect the sensing element from moisture and corrosion. They can be used to measure the temperature of an array of liquids ranging from oils and industrial chemicals to foods.

Thermistors with bolt- or washer-mounted sensors can be installed into standard-sized threaded holes or openings. Their small thermal mass enables them to respond to temperature changes rapidly. They&#;re used in many applications including household appliances, water tanks, pipes, and equipment casings.

Surface-mounted thermistors come with adhesive exteriors that can easily be stuck in place on flat or curved surfaces. They can be removed and re-applied and have several commercial and industrial applications.

Temperature Range, Accuracy and Stability

Thermistors are highly accurate (ranging from ± 0.05°C to ± 1.5°C), but only over a limited temperature range that is within about 50°C of a base temperature. The working temperature range for most thermistors is between 0°C and 100°C. Class A thermistors offer the greatest accuracy, while Class B thermistors can be used in scenarios where there&#;s less need for exact measurement. Once the manufacturing process is complete, thermistors are chemically stable and their accuracy does not change significantly with age.

Common Applications for Thermistors

Thermistors are often selected for applications where ruggedness, reliability, and stability are important. They&#;re well suited for use in environments with extreme conditions, or where electronic noise is present. They&#;re available in a variety of shapes: the ideal shape for a particular application depends on whether the thermistor will be surface-mounted or embedded in a system, and on the type of material being measured.  Thermistors are employed in a broad array of commercial and industrial applications to measure the temperature of surfaces, liquids, and ambient gasses. When sheathed in protective probes that can be reliably sanitized, they&#;re used in the food and beverage industries, in scientific laboratories, and in R&D. Heavy-duty probe-mounted thermistors are suitable for immersion in corrosive fluids, and can be used in industrial processes, while vinyl-tipped thermistor mounts are used outdoors or for biological applications. Thermistors are also available with metal or plastic cage-style element covers for air temperature measurement.

How Are Thermistors Wired?

Thermistors are very simple to wire. Most come with two-wire connectors. The same two wires that connect the thermistor to its excitation source can be used to measure the voltage across the thermistor. Related Products Learn more about the  HSTH- Hermetically Sealed Thermistor Sensors Order Now