Definition of technical terms of thermistor
English name: thermistor English name: thermistor definition 1: thermistor. The curve of its resistance changing with temperature is nonlinear. Discipline: electric power (first-class discipline); Definition of two disciplines 2: Thermosensitive temperature detection element made of solid semiconductor material with high temperature coefficient of resistance. Discipline: mechanical engineering (first-class discipline); Instrumentation components (two disciplines); Mechanical components of instruments and meters-sensitive components (third-level discipline) This content is approved and published by the National Committee for the Examination and Approval of Scientific and Technical Terminology.
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Thermistor is a kind of sensitive element, which can be divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient (NTC) according to different temperature coefficients. The typical feature of thermistor is that it is sensitive to temperature and shows different resistance values at different temperatures. Positive temperature coefficient thermistor (PTC) has higher resistance at higher temperature, while negative temperature coefficient (NTC) has lower resistance at higher temperature, which belongs to semiconductor devices.
catalogue
brief introduction
trait
principle of operation
basic feature
technical parameter
Graded PTC thermistor
NTC thermistor
CTR thermistor
App application
Brief introduction of main shortcomings
trait
principle of operation
basic feature
technical parameter
Graded PTC thermistor
NTC thermistor
CTR thermistor
App application
Main disadvantages
Expand and edit this introduction.
Thermistor is a sensitive element with early development, various types and mature development. Thermistors are composed of semiconductor ceramic materials. thermal resistor
The principle used is that temperature causes resistance change. If the concentration of electrons and holes are n and p, and the mobility is μn and μp, respectively, the conductance of the semiconductor is σ=q(nμn+pμp). Because n, p, μn and μp are all functions of temperature t and conductance is a function of temperature, the temperature can be calculated from the measured conductance. And can make a resistance-temperature characteristic curve. This is the working principle of semiconductor thermistor. Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) and critical temperature thermistor (CTR).
Edit the characteristics of this paragraph
The main characteristics of thermistor are: ① high sensitivity, its temperature coefficient is more than 10 ~ 100 times higher than that of metal, and it can detect the temperature change of 10-6℃; ② Wide working temperature range. Normal temperature device is suitable for -55℃ ~ 3 15℃, high temperature device is suitable for higher than 3 15℃ (up to 2000℃ at present), and low temperature device is suitable for -273℃ ~ 55℃. (3) Small size, which can measure the temperature of crevices, cavities and blood vessels in organisms that other thermometers can't measure; (4) It is convenient to use, and the resistance value can be arbitrarily selected from 0. 1 to 100 kω; (5) it is easy to be processed into complex shapes and can be produced in large quantities; ⑥ Good stability and strong overload capacity.
Edit the working principle of this paragraph.
Thermistor will not work for a long time; When the ambient temperature and current are in zone C, the heat dissipation power of the thermistor is close to the heating power, so it may be a thermistor.
Action can be action or not. When the ambient temperature is the same, with the increase of current, the action time of thermistor is shortened sharply. When the ambient temperature is relatively high, the working time of thermistor is short, and the holding current and working current are small. 1, ptc effect is a material with PTC (positive temperature coefficient effect), that is, positive temperature coefficient effect, which only means that the resistance of this material will increase with the increase of temperature. For example, most metal materials have ptc effect. In these materials, ptc effect shows that the resistance increases linearly with the increase of temperature, which is commonly called linear ptc effect. 2. The nonlinear ptc effect materials undergoing phase change will show the phenomenon that the resistance increases by several to ten orders of magnitude in a narrow temperature range, that is, the nonlinear ptc effect, and quite a few types of conductive polymers will show this effect, such as polymer ptc thermistors. These conductive polymers are very useful for manufacturing overcurrent protection devices. 3. Polymer ptc Thermistors for Over-current Protection Polymer ptc Thermistors are often called self-healing fuses (hereinafter referred to as thermistors), which are extremely suitable for over-current protection devices because of their unique positive temperature coefficient resistance characteristics. Thermistors are connected in series in the circuit like ordinary fuses. thermal resistor
When the circuit works normally, the temperature of the thermistor is close to room temperature and the resistance is very small, so the series connection in the circuit will not hinder the passage of current; However, when the circuit has an overcurrent due to a fault, the temperature of the thermistor will increase due to the increase of heating power. When the temperature exceeds the switching temperature (ts, see figure 1), the resistance will suddenly increase and the current in the loop will quickly decrease to a safe value. This is a schematic diagram of the current change during the protection of AC circuit by thermistor. After the thermistor acts, the current in the circuit is greatly reduced, and T in the figure is the action time of the thermistor. Due to the good designability of polymer ptc thermistor, its sensitivity to temperature can be adjusted by changing its own switching temperature (ts), so it can play both over-temperature protection and over-current protection. For example, KT 16- 1700 DL thermistor is suitable for over-current and over-temperature protection of lithium-ion batteries and nickel-hydrogen batteries due to its low working temperature. Influence of ambient temperature on polymer ptc thermistor Polymer ptc thermistor is a kind of direct heating and step thermistor, and its resistance change process is related to its own heat generation and heat dissipation, so its holding current (ihold), action current (itrip) and action time are all affected by ambient temperature. When the ambient temperature and current are in area A, the heating power of the thermistor is greater than the cooling power and will work. When the ambient temperature and current are in zone B, the heating power is less than the cooling power, and the polymer ptc thermistor can be recovered and reused because of its resistance. Fig. 6 is a schematic diagram of the change of resistance with time during the recovery process after thermistor action. Generally, the resistance can be restored to the initial value of 1.6 times in ten seconds to dozens of seconds. At this point, the holding current of the thermistor has returned to the rated value and can be used again. Thermistors with smaller area and thickness recover relatively quickly; However, the thermistor with larger area and thickness has better recovery.
Edit the basic features of this paragraph.
Temperature characteristics
The resistance-temperature characteristic of thermistor can be approximately expressed by the following formula: r = r0exp {b (1/t-1/T0)}: r: resistance at temperature T(K), ro: resistance at temperature t0, (k), B:B value, * t (k). C)+273. 15. In fact, the b value of thermistor is not constant, and it changes with the material composition, and the maximum value can even reach 5 k/c. Therefore, when the formula 1 is applied in a large temperature range, there will be some error between it and the measured value. Here, if the value of b in the formula 1 is calculated as a function of temperature, as shown in Formula 2, the error between this value and the measured value can be reduced and can be considered to be approximately equal. BT=CT2+DT+E, where c, d and e are constants. In addition, the fluctuation of B value caused by different production conditions will cause the constant E to change, but the constants C and D remain unchanged. Therefore, when discussing the fluctuation of B value, we only need to consider the constant E, and the constants C, D and E can be calculated by four points (temperature, resistance) data (t0, r0). (t 1，r 1)。 (t2, r2) and (t3, r3), by Formula 3? 6 calculation. First, we calculate B 1, B2, B3 from the resistance values of T0 and T 1, T2, T3 in mode 3, and then substitute them into the following modes. Example of resistance value calculation: According to the resistance-temperature characteristic table, try to find the resistance value of thermistor with a resistance value of 5 (kω) at 25℃ and a deviation of 50(K) at 10℃~ 30℃. Step (1) Find the constants c, d and e according to the resistance-temperature characteristic table. To = 25+273.15t1=10+273.15t2 = 20+273.15t3 = 30+273.1. (3) Substitute the numerical value into r = 5exp {(bt1/t-1/298.15)} to find R. * t:10+273.15 ~ 30+.
Edit the technical parameters of this paragraph.
thermal resistor
① Nominal resistance value Rc: generally refers to the actual resistance value of thermistor when the ambient temperature is 25℃. ② Actual resistance value RT: the resistance value measured at a certain temperature. ③ Material constant: it is a parameter that describes the physical characteristics of thermistor materials and is also an index of thermal sensitivity. The greater the value of b, the higher the sensitivity of thermistor. It should be noted that in practical work, the value of b is not a constant, but increases slightly with the increase of temperature. ④ Temperature coefficient of resistance αT: indicates the change rate of resistance when the temperature changes by 65438 0℃, and the unit is%/℃. ⑤ Time constant τ: Thermistor has thermal inertia, and time constant is a parameter to describe thermal inertia of thermistor. It is defined as the time required for the thermistor temperature to change by 63.2% of the difference between two specific temperatures when the ambient temperature suddenly changes from one specific temperature to another in the state of no power consumption. The smaller the τ, the smaller the thermal inertia of the thermistor. ⑥ Rated power PM: the allowable dissipation power of thermistor under long-term continuous load under specified technical conditions. The rated power shall not be exceeded in actual use. If the ambient temperature of thermistor exceeds 25℃, its load must be reduced accordingly. ⑦ Rated working current IM: the nominal current value specified by thermistor in working state. ⑧ Measurement power Pc: The electric power consumed when the resistance change of thermistor caused by heating of test current does not exceed 0. 1% at the specified ambient temperature. thermal resistor
⑨ Maximum voltage: For NTC thermistors, it refers to the maximum DC voltage that can be continuously applied at the specified ambient temperature, which is not convenient for the thermistor to get out of control; For PTC thermistor, it refers to the maximum DC voltage that is allowed to be continuously applied to the thermistor at a specified ambient temperature and still air to ensure the thermistor to work normally in the PCT characteristic part. Attending the maximum working temperature Tmax: under the specified technical conditions, the maximum temperature allowed for long-term continuous operation of thermistors. ⑾ Switching temperature TB: the temperature at which the resistance of PCT thermistor starts to jump. ⑿ Dissipation coefficient h: When the temperature rises by 65438 0℃, the power dissipated by the thermistor is mW/℃.
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PTC thermistor
thermal resistor
PTC (positive temperature coefficient 1 coefficient) refers to the thermistor phenomenon or the material with positive temperature coefficient, which increases rapidly at a certain temperature and can be used as a constant temperature sensor. The material is a sintered body with BaTiO3, SrTiO3 or PBT-IO3 as the main components, in which a small amount of oxides such as Nb, Ta, Bi, Sb, Y and La are doped to control the valence of atoms and make it semi-conductive. At the same time, oxides of manganese, iron, copper and chromium and other additives are added to improve the temperature coefficient of its positive resistance. Platinum titanate and its solid solution are semiconducted by ordinary ceramic technology and high temperature sintering, thus obtaining thermistor materials with positive characteristics. Its temperature coefficient and Curie point temperature vary with composition and sintering conditions (especially cooling temperature). Barium titanate crystal belongs to perovskite structure and is a ferroelectric material. Pure barium titanate is an insulating material. Adding trace rare earth elements into barium titanate, after proper heat treatment, the resistivity increases by several orders of magnitude near Curie temperature, resulting in PTC effect, which is related to the ferroelectricity of BaTiO3 crystal and the phase transition of the material near Curie temperature. Barium titanate semiconductor ceramic is a polycrystalline material with grain boundaries. When the semiconductor porcelain reaches a certain temperature or voltage, the grain boundary of the crystal changes, so the resistance changes dramatically. The PTC effect of BaTiO _ 3 semiconductor ceramics is caused by grain boundaries. For conducting electrons, the grain boundary is equivalent to a potential barrier. When the temperature is low, due to the electric field in BaTiO _ 3, electrons easily cross the barrier and the resistance is small. When the temperature rises to near the Curie point temperature (critical temperature), the internal electric field is destroyed. It can't help conductive electrons cross the barrier. This is equivalent to the rise of the barrier and the sudden increase of the resistance value, resulting in PTC effect. Physical models of PTC effect of BaTiO _ 3 semiconductor ceramics include seascape surface barrier model, barium vacancy model of Daniels et al. and superposition barrier model, which reasonably explain PTC effect from different aspects. The experimental results show that the resistance-temperature characteristics of PTC thermistor can be approximately expressed by the experimental formula in the working temperature range: rt = rt0expbp (.
In the formula, RT and RT0 represent the resistance values at temperatures t and T0, and Bp is the material constant of the material. PTC effect originates from the nature of the precipitates at the grain boundaries and grain boundaries of ceramics, and it changes significantly with impurity types, concentrations and sintering conditions. Recently, among the thermistors that have been put into practical use, there is a silicon temperature sensor using a silicon wafer, which is a PTC thermistor with large size and high precision, and is composed of N-type silicon. The electron scattering caused by impurities increases with the increase of temperature, so the resistance increases. The PTC thermistor appears in 1950, and then the PTC thermistor with barium titanate as the main material appears in 1954. PTC thermistor can be used for temperature measurement and control in industry, and also for temperature detection and adjustment of a certain part of automobile. They are also widely used in civil equipment, such as controlling the water temperature of instantaneous water heaters, the temperature of air conditioners and cold storage, and using their own heating for gas analysis and wind power generation. This paper briefly introduces the application of heating and overheating protection for heaters, motors, transformers, high-power transistors and other electrical appliances. PTC thermistor can be used not only as a heating element, but also as a "switch", which has three functions: sensitive element, heater and switch. It is called a thermistor switch. When the current passes through the element, the temperature rises, that is, the temperature of the heating element rises. When the Curie point temperature is exceeded, the resistance increases, which limits the increase of current, so the decrease of current leads to the decrease of component temperature, and the decrease of resistance leads to the increase of circuit current and the increase of component temperature repeatedly. Therefore, it has the function of keeping the temperature in a specific range, and also plays the role of a switch. With this temperature resistance, it can be used as a heating source, such as a heater, an electric soldering iron, a clothes dryer, an air conditioner, etc. , and can also play an overheating protection role for electrical appliances.
NTC thermistor
NTC (negative temperature coefficient 1 coefficient) refers to thermistor phenomenon and materials with negative temperature coefficient, which decrease exponentially with the increase of temperature. This material is a semiconductor ceramic made of two or more metal oxides such as manganese, copper, silicon, cobalt, iron, nickel and zinc through full mixing, molding and sintering. Thermistors with negative temperature coefficient (NTC) can be manufactured. Resistivity and material constant vary with material ratio, sintering atmosphere, sintering temperature and structural state. Now there are non-oxide NTC thermistor materials represented by silicon carbide, tin selenide and tantalum nitride. NTC thermistor ceramics are mostly oxide ceramics with spinel structure or other structures, which have negative temperature coefficient. The resistance value can be approximately expressed as: in the formula, RT and RT0 are the resistance values at temperatures t and T0, respectively, and Bn is the material constant. The resistivity of ceramic particles varies with temperature, which is determined by the characteristics of semiconductors. thermistor
The development of NTC thermistor has gone through a long period. 1834, scientists first discovered that silver sulfide has a negative temperature coefficient. 1930, scientists found that cuprous oxide-copper oxide also has a negative temperature coefficient, and it was successfully applied to the temperature compensation circuit of aviation instruments. Subsequently, due to the continuous development of transistor technology, the research of thermistor has made great progress. N 1C thermistor was developed by 1960. . NTC thermistors are widely used in temperature measurement, temperature control and temperature compensation. This is an application example of temperature measurement. The measuring range is generally-10 ~+300℃ or -200 ~+650℃. R2 and R3 are bridge balanced resistors; R 1 is the initial resistance; R4 is a full-scale resistor, also known as a calibration resistor. R7, R8 and W are voltage dividing resistors, which provide a stable DC power supply for the bridge. R6 is connected in series with the ammeter (micro ammeter) to correct the ammeter scale and limit the current flowing through the ammeter. R5 is connected in parallel with the electric meter to protect the electric meter. The thermistor RT is connected to the unbalanced bridge arm (that is, R 1, RT) as a temperature sensing probe. Because the resistance of thermistor changes with temperature, the indicator of the instrument connected between the diagonal lines of the bridge also changes. This is how a thermistor thermometer works. The accuracy of thermistor thermometer can reach 0.65438 0℃, and the temperature sensing time can be as short as 65438±00s. It is not only suitable for granary thermometer, but also used for temperature measurement of grain storage, medical and health care, scientific breeding, ocean, deep well, high altitude and glacier.
CTR thermistor
thermal resistor
CTR (Critical Temperature Resistor) has a sudden change in negative resistance. At a certain temperature, the resistance drops sharply with the increase of temperature, which has a large negative temperature coefficient. The material is a mixed sintered body of oxides of vanadium, barium, strontium, phosphorus and other elements, and is a semi-glassy semiconductor. CTR is also called glass thermistor. With the addition of oxides such as germanium, tungsten and molybdenum, the temperature suddenly changes, which is caused by the different lattice spacing of vanadium oxide due to the doping of different impurities. If vanadium pentoxide becomes vanadium dioxide in a proper reducing atmosphere, the abrupt temperature of the resistor will increase. If it is further reduced to vanadium trioxide, the mutation disappears. The temperature at which the resistance mutation occurs corresponds to the position of the semi-glass semiconductor mutation, so the semiconductor-metal phase shift occurs. . CTR can be used as a temperature control alarm and other applications. Remarkable achievements have been made in the theoretical research and application development of thermistor. With the application of high precision and cutting-edge technology, the in-depth exploration of the conduction mechanism and application of thermistor and the in-depth study of new materials with excellent performance, it will surely become
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thermal resistor
Thermistors can also be used as electronic circuit components for temperature compensation of instrument circuits and cold ends of thermocouples. Using the self-heating characteristics of NTC thermistor, RC oscillator amplitude stabilization circuit, delay circuit and protection circuit can be formed, which can realize automatic gain control. When the self-heating temperature is much higher than the ambient temperature, the resistance value is also related to the heat dissipation conditions of the environment. Therefore, this characteristic of thermistor is often used in speedometer, flowmeter, gas analyzer and thermal conductivity analysis to make special detection elements. PTC thermistor is mainly used for overheat protection of electrical equipment, contactless relay, constant temperature, automatic gain control, motor start-up, delay, automatic degaussing of color TV, fire alarm and temperature compensation.
The main shortcoming of editing this paragraph.
thermal resistor
① The relationship between resistance and temperature is nonlinear; ② The consistency and interchangeability of parts are poor; ③ The components are easy to age and have poor stability; (4) Except for special high-temperature thermistors, most thermistors are only suitable for the range of 0 ~ 150℃, so be sure to pay attention when using them.