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Jan 12 2018

Thermistor Introduction--​Temperature Sensitive Component

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Summary

Thermistors are a type of sensitive component, which are divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC) according to the temperature coefficient. The typical feature of thermistors is their sensitivity to temperature and exhibits different resistance values at different temperatures. The resistance of the positive temperature coefficient thermistor (PTC) is larger when the temperature is higher. As to the negative temperature coefficient thermistor (NTC), the higher the temperature is, the lower the resistance of it will be. Both of them are the same as semiconductor devices.This paper introudces some basic knowledge of thermistor,including characteristics of thermistors,features; working principle;categories;or thermistor application and etc.

Article coreThermistor IntroductionAttributeSensitive component
English nameThermistor

Category

Semiconductor/Electronic component
FeatureTemperature sensitiveTemperature coefficient classification PTC/NTC

Catalogs

Catalogs

I.What is Thermistors

PTC (Positive Temperature Coeffiient)

II.Characteristics of Thermistors

NTC (Negative Temperature CoeffiCient)

III.Feature of Thermistors

CTR(CritiCal Temperature Resistor)

IV.Thermistors Working principle

VI.Thermistor Application

V.Thermistor Categories





Introduction

I.What is Thermistors

Before you read this article, you can watch the following video first..

 This video is about some basic knowledge about thermistor.

Thermistors are a type of sensitive component, which are divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC) according to the temperature coefficient. The typical feature of thermistors is their sensitivity to temperature and exhibits different resistance values at different temperatures. The resistance of the positive temperature coefficient thermistor (PTC) is larger when the temperature is higher. As to the negative temperature coefficient thermistor (NTC), the higher the temperature is, the lower the resistance of it will be. Both of them are the same as semiconductor devices.

Name: Thermistor

Temperature coefficient classificationPTC/NTC

Feature: Temperature sensitive

AttributeSensitive component

Thermistor is a kind of thermal resistance, that is to say, the working principle of thermistor is that temperature causes resistance changes. But now the thermal resistance is generally industrialized, which basically refers to PT100, CU50 and other commonly used thermal resistance. The differences of them are: general thermal resistance usually refers to the metal thermal resistance (PT100), etc.. They are called thermistor because the temperature coefficient of the semiconductor thermal resistance is more than 10~100 times larger than that of the metal, the temperature change at 10-6 ℃ can be detected, and the resistance value can be chosen arbitrarily in the range of 0.1 ~ 100k Ω.

thermistor--Thermistor Introduction--Temperature Sensitive Component

However, the resistance curve of thermistor with temperature is non-linear, and the linearity of each of the same models is not the same, and the temperature measurement range is relatively small. So the industry generally uses metal thermal resistors, which is what we usually call thermal resistors. The thermistor is generally used in the circuit board, such as the usual mentioned that can be similar to a fuse. As the resistance value varies greatly with temperature, it can be used as a protector. Of course, it's just one thing. There are many other usage of thermistor. For example , the temperature compensation of the cold end of the thermocouple is compensated by the thermistor. In addition, because the relationship between the resistance value and the temperature is nonlinear, the consistency of the components is poor, and not as standard as a thermal resistance signal.

II.Characteristics of Thermistors

  • ① High sensitivity. The temperature coefficient of the resistance is more than 10~100 times larger than that of the metal, and the temperature change of 10-6℃ can be detected .

  • ② Wide operating temperature range. The room temperature device is suitable for -55℃ to 315℃, and the suitable temperature for high temperature devices is higher than 315℃ .(currently up to 2000 ℃), and the low temperature device is suitable for -273 ℃ ~ -55 ℃;

Characteristics of Thermistor--Thermistor Introduction--Temperature Sensitive Component

  • ③ Small size. The ability to measure the temperature of voids, cavities and blood vessels in organisms that cannot be measured by other thermometers;

  • ④ Easy to use. Resistance value can be arbitrarily selected between 0.1 and 100 kΩ;

  • ⑤ Easy to process into complex shapes, suitable for mass production;

  • ⑥ Good stability, Strong overload capacity

III.Feature of Thermistors

Thermistor, heat sensitive semiconductor resistor. The curve of resistance value with temperature is nonlinear.

The PPTC thermistor (polymer positive temperature coefficient) is made of a polymer material filled with carbon black particles. This material has a certain conductivity, which can pass the rated current. If the current that passing through the thermistor is too high, the heating power will be greater than the heat dissipation power, and the temperature of the thermistor will begin to rise. At the meanwhile, The polymer matrix in the thermistor begins to expand, which separates the carbon black particles and causes the resistance to rise, then effectively reduces the current in the circuit. At this moment, the circuit still has a very small current passing through which the current makes the thermistor maintain enough temperature to keep it in a high resistance state. When the problem is solved, the PPTC thermistor cools down quickly and reverts to the original low-resistance state, then works again like a new thermistor..

Curves of resistance with temperature--Thermistor Introduction--Temperature Sensitive Component

The resistance-temperature characteristic of the thermistor can be approximated by the following equation: R = R0exp {B (1 / T-1 / T0)}: R: resistance at temperature T (K), Ro: temperature T0, ( K), B: B value, * T (K) = t (ºC) + 273.15.

In fact, the thermistor B value is not constant, Its variation varies according to the composition of the material, with a maximum of up to 5K /℃. Therefore, the application of a larger temperature range 1, there will be some error between the measured value. Here, if the B value in formula 1 is used to calculate the function of temperature as shown in formula 2, the error between the measured value and the measured value can be reduced and considered to be approximately equal.

BT = CT2 + DT + E, in this equation, C, D, E are constants. In addition, the fluctuation of B value caused by different production conditions will cause the change of constant E, but the constant C and D will not change. Therefore, when discussing the fluctuation of B value, only the constant E needs to be considered. The constants C, D, E are calculated from the four points (temperature, resistance) data (T0, R0) for constants C, D, (T1, R1). (T2, R2) and (T3, R3), calculated by the formula 3 ~ 6. First of all, according to the pattern 3 T0 and T1, T2, T3 resistance values obtained B1, B2, B3, and then into the following various samples.

IV.Thermistors Working principle

Thermistor is a sensor resistance. The value of thermistor resistance will change with the temperature changes, which is different with general fixed resistance. The resistance value of metals increases with the increase of temperature, but the resistance value of semiconductors decreases sharply with the increase of temperature, and it is nonlinear. At the same temperature, the resistance of the thermistor is about 10 times that of the lead thermistor, so it can be said that the thermistor is especially sensitive to the change of temperature. This temperature characteristics of semiconductors is because the conductivity of the semiconductor is carrier (electron, hole) conductive. Because the number of carriers in semiconductors is much smaller than the number of free electrons in metals, the resistivity of semiconductors is great. With the increase of temperature, the number of carriers participating in the conduction in semiconductors will increase, so the conductivity of semiconductors increases and its resistivity decreases.

platinum thermistor--Thermistor Introduction--Temperature Sensitive Component

The thermistor is a kind of thermistor which makes use of the characteristic that the resistance value of semiconductor varies significantly with temperature. It is made of certain metal oxides in different formulations. In a certain temperature range, according to the measurement of the resistance of thermistor, we can know the temperature change of the measured medium.

When the thermistor is installed in the circuit, the action time of the thermistor decreases sharply with the increase of current when the ambient temperature is the same; the thermistor has shorter action time and smaller maintenance current and action current when the ambient temperature is relatively high. When the circuit is working normally, the temperature of the thermistor is close to room temperature and the resistance is very small, the current won’t be blocked in the circuit if the thermistors are in series. When the circuit is overcurrent due to the fault, the temperature of the thermistor rises because of the increase of heating power. When the temperature exceeds the switching temperature, the resistance will increase rapidly, and the current in the circuit will decrease to the safe value quickly.

Thermistor will be in the long-term inoperative state; when the temperature and current of the environment are in the C area, the thermal power of the thermistor is close to the heating power, so it may not act as well. When the ambient temperature is the same, the action time is shortened sharply with the increase of current, and the thermistor has shorter action time and smaller maintenance current and action current when the ambient temperature is relatively high.

Thermistor Introduction--Temperature Sensitive Component

  • (1) PTC effect

PTC effect is a kind of material which has the effect of PTC (posiTIvetemperaturecoefficient), that is, the positive temperature coefficient effect. Only indicates that the resistance of this material increases with the increase of temperature. For example, most metal materials have ptc effect. In these materials, the resistance increases linearly with the increase of temperature. This is known as the linear PTC effect.

12.NTC and PTC.PNG

  • (2) Non - linear PTC effect

The phase change material will show the phenomenon that the resistance increases sharply from the narrow temperature range to more than ten orders of magnitude, that is, the nonlinear PTC effect. A considerable number of types of conductive polymer will show this effect, such as polymer ptc thermistor. These conductive polymers are useful for making overcurrent protection devices .

  • (3) High polymer PTC thermistor is used for overcurrent protection, which is often called PPTC ( hereinafter referred to as thermistor ). Due to its unique positive temperature coefficient resistance characteristic, it is very suitable for use as over - current protection devices. The use method of the thermistor is the same as the ordinary fuse , and is used in series in the circuit .



Detail

 V.Thermistor Categories

  • PTC (Positive Temperature Coeffiient)

PTC thermistor--Thermistor Introduction--Temperature Sensitive Component

PTC refers to a sharp increase in resistance at a temperature, with a positive temperature coefficient of thermistor phenomenon or material, can be used exclusively as a constant temperature sensor. The material is based on BaTiO3 or SrTiO3 or PbTiO3 as the main component Sintered bodies in which a small amount of oxides such as Nb, Ta, Bi, Sb, Y, and La are doped and semiconducting by atomic valence control. At the same time, the oxides and other additives which increase the temperature coefficient of the positive resistance of Mn,Fe,Cu,Cr and other additives are added, and the common ceramic process is used to form them. Semiconducting of platinum titanate and its solid solution by sintering at high temperature, thus to obtain positive characteristics of the thermistor material whose temperature coefficient and the Curie temperature varies with the composition and the sintering conditions (especially the cooling temperature).

BaTiO belongs to the perovskite type structure and is a ferroelectric material while the pure barium titanate is an insulating material. After proper heat treatment, the resistivity of barium titanate increases by several orders of magnitude in the vicinity of Curie temperature by adding trace rare earth elements to the material, resulting in PTC effect. This effect is related to the ferroelectric property of BaTiO3 crystal and the phase transition of the material near Curie temperature. The barium titanate semiconducting porcelain is a polycrystalline material with intergranular interfaces between the grains. When the semi-conductive ceramic reaches a certain temperature or voltage , the crystal grain boundary changes , so that the resistance sharply changes. 

The PTC effect of barium titanate semiconductors originates in the grain boundaries (grain boundaries). For conducting electrons, the intergranular interface is equivalent to a barrier.When the temperature is low, due to the effect of the electric field in titanic acid, it is easy for electrons to cross the barrier, then the resistance value will be smaller. When the temperature reaches the Curie point temperature (i.e. the critical temperature), the internal electric field is destroyed, which cannot help the conducting electron to cross the barrier. This is equivalent to the rise of the barrier and the sudden increase of the resistance value, which will results in the PTC effect. The physical models of the PTC effect of barium titanate semiconductors include the barrier model of the sea surface, the barium vacancy model and the superposition barrier model of Daniels et al. The PTC effect is explained reasonably from different aspects. 

Experiments show that the resistance-temperature characteristic of PTC thermistor can be approximately expressed by the experimental formula in the range of operating temperature:

RT = RT0 expBp (T-T0)


PTC effect--Thermistor Introduction--Temperature Sensitive Component

RT, RT0 in the formula represent the resistance value when the temperature is T, T0, Bp is the material constant of this material. The PTC effect originates from the properties of the precipitated phases between the grain boundaries and the intergranular boundaries of ceramics, and varies significantly with the kinds of impurities, the concentration of impurities, the sintering conditions, etc. Recently, one of the practical thermistors is silicon thermistor using silicon wafer, which is a PTC thermistor with small size and high precision, which is composed of n-type silicon. The electron scattering caused by the impurities increases with the increase of temperature, and the resistance increases.

PTC thermistor in 1950, followed by the emergence of barium titanate in 1954 as the main material of the PTC thermistor. PTC thermistor can be used as temperature measurement and control in industry, temperature detection and adjustment in a certain part of automobile, and a large number of civil equipment, such as controlling the water temperature of instantaneous water heater, the temperature of air conditioner and cold storage, using itself heating for gas analysis and wind speed machine, and so on. Here's an example of heating, overheat protection of heaters, motors, transformers, high-power transistors and other electrical applications.

In addition to being used as heating element, PTC thermistor can also play the role of "switch". It has three functions: sensitive element, heater and switch, which is called "heat sensitive switch". When the current passes through the component, the temperature will rise, that is, the temperature of the heating body will rise. When the temperature exceeds the Curie point, the resistance will increase, which will limit the increase of current. So the decrease of current will lead to the decrease of the component temperature, and the decrease of resistance will also increase the current of the circuit and increase the temperature of the element. Therefore, it not only have the function of temperature maintaining at a specific range, but also play a role as switches. If taking used of this resistance temperature characteristic, and making it as heating source such as heater, electric iron, wardrobe, air conditioner and so on, it can also protect electric appliances from overheating.

  • NTC (Negative Temperature CoeffiCient)

NTC thermistor--Thermistor Introduction--Temperature Sensitive Component

NTC refers to a thermistor with a negative temperature coefficient and a material with a negative temperature coefficient decreased exponentially with the rise of temperature. The material is the use of manganese, silicon, copper, cobalt, iron, nickel, zinc and other two or more than two kinds of metal oxides for mixing and molding into semiconductor ceramics, which can be made into thermistor with negative temperature coefficient (NTC). The resistivity and material constant vary with the ratio of material composition, sintering atmosphere, sintering temperature and structure state. Now there are also non-oxide NTC thermistor materials such as silicon carbide, stannic selenide and tantalum nitride.

NTC thermosensitive semiconductors are mostly oxide ceramics of spinel structure or other structures, with a negative temperature coefficient, and rthe resistance values can be approximately expressed as:

Rt = RT * EXP (Bn * (1 / T-1 / T0)


NTC effect--Thermistor Introduction--Temperature Sensitive Component

RT, RT0 in the formula represent the resistance value when the temperature is T, T0, Bn is the material constant of this material. The resistivity of ceramic grain itself changes due to temperature change, which is determined by semiconductor properties.

The development of NTC thermistor has gone through a long period. In 1834, scientists found that silver sulphide has negative temperature coefficient first. In 1930, the scientists found that Cuprous oxide-copper oxide has performance of negative temperature coefficient, which is successfully used in the temperature compensation circuit of aeronautical instrument . Then, due to the continuous development of transistor technology, the research of the thermistor has made significant progress. In 1960,N1C thermistor was developed. NTC thermistors are widely used in temperature measurement, temperature control, temperature compensation and so on. Here is an example of application of temperature measurement.

Its measuring range is generally -10 ~ 300 ℃, but also can be -200 ~ 10 ℃. It can even be used in 300 ~ 1200 ℃ environment for temperature measurement. RT is a NTC thermistor. R2 and R3 are used as bridge balance resistors; R1 is the initial resistance; R4 is a full scale resistance, calibration head, also known as calibration resistance; R7, R8 and W are divider resistances, providing a stable DC power supply for the bridge; R6 is in series with the gauge head (microammeter), which can modify the calibration of the gauge head and limit the current flowing through the meter head. R5 is in parallel with the gauge head for protection. Using a thermal sensor RT as a temperature sensor at the unbalanced bridge arm (R1 / RTT). Since the resistance of the thermistor varies with the change of temperature, the indicator of the head connected to the diagonal line of the bridge changes accordingly. This is the working principle of the thermistor thermometer .

The precision of thermistor thermometer can reach 0.1 ℃, and the temperature sensing time can be less than 10 s. It is not only suitable for granary thermometer, but also for temperature measurements of food storage, medical science , scientific farming, ocean, deep wells, high altitudes and glaciers, etc.

  • CTR(CritiCal Temperature Resistor)

CTR(CritiCal Temperature resistor has the characteristic of negative resistance mutation. At a certain temperature, the resistance value decreases dramatically with the increase of temperature and has a large negative temperature coefficient. The material is a mixture of vanadium, barium, strontium, phosphorus, etc. It is a semi-glass-like semiconductor, also called glass thermistor. The sudden change temperature is changed with addition of oxides such as germanium, tungsten, molybdenum, etc. This is because the addition of different impurities makes the lattice spacing of vanadium oxide different. CTR can be used as temperature control alarm and other applications.

The theoretical research and application development of thermistor have made remarkable achievements. With the application of high, fine and sharp technology, the conductive mechanism and application of thermistor have been further explored. Further research on new materials with excellent properties will make rapid progress.

In the detection, the ohmic range of multimeter (nominal resistance value) is used to determine the gear (generally R × 1 block), which can be divided into two steps:

(1).First, conducting normal temperature detection (indoor temperature is close to 25 ℃). The actual resistance value of PTC thermistor was measured by using alligator clip instead of test pen and compared with the nominal resistance value. The difference between them is normal in ±2 Ω. If the difference between the actual resistance value and the normal one is too big, then it is shown that its performance is poor or damaged.


(2)Secondly, on the basis of normal room temperature test, the second step of heating temperature detection can be carried out. Put a heat source (such as an electric iron) near a thermistor and heating it. Then we just observe the value of the multimeter. If we see that the value changes with the increase of temperature, it indicates that the resistance value is gradually changing (the NTC resistance value of the negative temperature coefficient thermistor will become smaller and the PTC resistance value of the positive temperature coefficient thermistor will become larger). When the resistance value changes to a certain value, the display data will gradually stabilize, indicating that the thermistor is normal. If the resistance value does not change, it is indicates that the performance of the thermistor is getting worse and cannot continue to be used.

Notice!

The following points should be noted when testing:

(1) Rt is measured by the manufacturer at ambient temperature of 25℃, so when multimeter is used to measure Rt, it should also be carried out when the ambient temperature is close to 25℃, so as to ensure the credibility of the test.

(2) The measured power should not exceed the specified value so as to avoid the measurement error caused by the current thermal effect.

(3) Correct operation is important! Do not hold the thermistor with your hand in order to prevent the body temperature from affecting the test.

(4) Be careful not to bring the heat source too close or in direct contact with the PTC thermistor to prevent it from burning.



Analysis

VI.Thermistor Application

Thermistor can also be used as electronic component for temperature compensation of instrument circuit and thermocouple cold end etc. The automatic gain control can be realized by using the self heat characteristic of the NTC thermistor. The RC oscillator is composed of the amplitude stabilizing circuit, the delay circuit and the protection circuit. When the self-heating temperature is much larger than the ambient temperature, the resistance value is also related to the environment heat dissipation conditions. So in the flow meter, flowmeter, gas analyzer and thermal conductivity analysis, the characteristic of thermistor is often used to make special detection element. The special detecting element. PTC thermistor is mainly used for overheating protection of electrical equipment, contactless relay, constant temperature, automatic gain control, motor startup, time delay, automatic demagnetization of color TV, fire alarm and temperature compensation, etc.

PTC thermistors Lighting Switching Application Circuit--Thermistor Introduction--Temperature Sensitive Componentthermistor-temperature-circuit--Thermistor Introduction--Temperature Sensitive Component



 Book Recommendation

  • The 2018-2023 World Outlook for Thermistors Temperature Sensors

This study covers the world outlook for thermistors temperature sensors across more than 190 countries. For each year reported, estimates are given for the latent demand, or potential industry earnings (P.I.E.), for the country in question (in millions of U.S. dollars), the percent share the country is of the region, and of the globe. These comparative benchmarks allow the reader to quickly gauge a country vis-à-vis others. Using econometric models which project fundamental economic dynamics within each country and across countries, latent demand estimates are created. This report does not discuss the specific players in the market serving the latent demand, nor specific details at the product level. The study also does not consider short-term cyclicalities that might affect realized sales. The study, therefore, is strategic in nature, taking an aggregate and long-run view, irrespective of the players or products involved. This study does not report actual sales data (which are simply unavailable, in a comparable or consistent manner in virtually all of the countries of the world). This study gives, however, my estimates for the worldwide latent demand, or the P.I.E., for thermistors temperature sensors. It also shows how the P.I.E. is divided across the world's regional and national markets. For each country, I also show my estimates of how the P.I.E. grows over time (positive or negative growth). In order to make these estimates, a multi-stage methodology was employed that is often taught in courses on international strategic planning at graduate schools of business.

--Icon Group International (Author)

  • Thermistors

--F.J. Hyde(Author)



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1 comment

    • valentines day gifts list on 2018-1-22 12:35:18

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