NTC Thermistors

Thermistor is a kind of sensitive element, according to the temperature coefficient is divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC). Typical characteristics of thermistors are temperature sensitive, showing different resistance values at different temperatures. When the temperature is higher, the resistance value of the positive temperature coefficient thermistor (PTC) is higher, and the negative temperature coefficient thermistor (NTC) is lower when the temperature is higher.
But note that the thermistor does not belong to the semiconductor device under the heading 85.41 in import and export.

Characteristic
The main characteristic of thermistor is:
The sensitivity is higher, and the temperature coefficient of resistance is 10 ~ 100 times higher than that of the metal, and the temperature variation of 10-6 can be detected;
The working temperature range is wide, the room temperature device is suitable for -55 DEG C to 315 DEG C, the temperature of the high temperature device is higher than the temperature of 315 DEG C (at present, the maximum can reach up to 2000 DEG C), and the low temperature device is suitable for -273 DEG C to -55 DEG c;
The volume is small, and can measure the temperature, the temperature of the cavity, the cavity and the blood vessels in the body;
It is convenient to use, the resistance value can be selected arbitrarily in the 0.1 ~ 100k between;
Easy processing into complex shapes, and mass production;
The good stability, strong overload capacity.

Working principle
The thermistor is in long-term operation state; when the environmental temperature and current in the C area, power dissipation and heating power of the thermistor is close to, and may
Thermistor
Action may not act. Thermistor in the same temperature, action time is shortened sharply with the increase of current; maintain the current and operating current action time and smaller thermistor has shorter when the environment temperature is relatively high.
1, the PTC effect is a kind of material with PTC (positive temperature coefficient) effect, that is, the positive temperature coefficient effect, only refers to the resistance of the material will increase with the increase of temperature. Such as most metal materials have PTC effect. In these materials, the PTC effect is shown to increase linearly with increasing temperature, which is commonly referred to as the linear PTC effect.
2, after the transformation of material nonlinear PTC effect will show resistance along the narrow temperature range increased to ten several orders of magnitude of the phenomenon, namely nonlinear PTC effect, a variety of types of conductive polymer exhibit this effect, such as polymer PTC thermistor. These conductive polymers are useful for manufacturing overcurrent protection device.
3, polymer PTC thermistor for overcurrent protection polymer PTC thermistor and is often referred to as the self recovery fuse (referred to below as the thermistor), because of its unique characteristics of the positive temperature coefficient of resistance, which is extremely suitable for overcurrent protection devices. The thermistor is used as a common fuse in series.
When the circuit is in normal operation, the thermistor temperature and room temperature are similar, the resistance is small, in series in the circuit will not hinder current through the circuit; and when overcurrent occurs due to failure, because the thermistor heating power has led to an increase in temperature rise when the temperature exceeds the temperature switch (TS, see Figure 1), the resistance moment will increase and the loop current decreases rapidly to a safe value. As the change of the thermal resistance on the current process of AC circuit protection in schematic diagram. After the action of the thermistor, the current in the circuit is greatly reduced, and the T is the action time of the thermistor. The polymer PTC thermistor can be designed, by changing the temperature of the switch (TS) to adjust the sensitivity to temperature, which can be realized simultaneously over temperature protection and overcurrent protection two functions, such as the kt16 specification of 1700dl thermistor temperature is very low due to the action, over-current and over temperature it is suitable for the protection of lithium ion battery and Ni MH battery. The influence of environmental temperature on polymer PTC thermistor polymer PTC thermistor is a type of direct heating, step type thermistor, the resistance change process and its related heat and heat, so the holding current (Ihold), operating current (iTrip) and movement time influenced by environmental temperature. When the ambient temperature and current in the a zone, the thermistor heating power is greater than the cooling power and action; when the environmental temperature and current in B when the heating power is less than the cooling power, the resistance of polymer PTC thermistor can be restored, which can be used repeatedly. Fig. 6 schematic diagram of the change of resistance with time in the process of recovery after thermal resistance. Resistance in a few seconds to tens of seconds can be restored to the initial value of 1.6 times the level of resistance to maintain the current has been restored to the rated value, can be used again. Relatively small area and thickness of the thermistor recovery is relatively fast, and the larger the area and thickness of the thermistor recovery is relatively slow.

Basic characteristics
The resistance temperature characteristic of thermistor can be approximated by the following equation: R=R0exp{B (1/T-1/T0)}:R: T (K) when the temperature resistance and temperature of T0, Ro: (K) when the resistance value, B:B value, *T (K) =t (S. C) +273.15. In fact, the B value of the thermistor is not constant, the size of the change of material composition, the maximum can reach 5K/ degrees C. Therefore, there is a certain error between the measured value and the applied value in the larger temperature range of 1. In this case, if the B value in the formula 1 is calculated as a function of temperature by the formula 2, the error between the measured value and the measured value can be reduced.
BT=CT2+DT+E, in the formula, C, D, E is a constant. In addition, due to the different production conditions caused by fluctuations in the value of B will cause constant E changes, but the constant C, D unchanged. Therefore, when we discuss the fluctuation of B value, we should consider the constant E. Constant C, D, E calculation, constant C, D, E by 4 points (temperature, resistance value) data (T0, R0). (T1, R1). (T2, R2) and (T3, R3), through the formula of 3 to 6. First by 3 patterns according to T0 and T1, T2, T3 of the resistance value obtained B1, B2, B3, then according to the following pattern.
Resistance value calculation example: according to the resistance temperature characteristic table, for 25 C when the resistance value is 5 (k), B value deviation of (K) of the thermistor at 10 degrees C to 30 degrees C resistance value. Step (1) according to the resistance temperature characteristic table, the constant C, D, E. To=25+273.15T1=10+273.15T2=20+273.15T3=30+273.15 (2) by BT=CT2+DT+E+50, BT. (3) the equations of R=5exp {(BT1/T-1/298.15)}, R. *T:10+273.15 ~ 30+273.15.

technical parameter
The nominal values of Rc: generally refers to the environment temperature is 25 DEG C when the actual value of the thermistor resistance. The actual resistance of RT: at a certain temperature by
Measured resistance value.
Material constant: it is a description of the physical characteristics of thermistor material parameters, but also the thermal sensitivity index, the greater the value of B, indicating that the higher the sensitivity of thermistors. It should be noted that, in practice, is not a constant B value, but increased slightly with temperature increasing.
The resistance temperature coefficient alpha T: it says the temperature change resistance changes at 1 DEG C for landing rate, unit.
The time constant: thermistor is thermal inertia, time constant, is a description of the parameters of thermal inertia thermistors. It is defined as, in no power state, when the ambient temperature by a specific temperature to suddenly change another specific temperature, body temperature thermistor two specific temperature difference between the 63.2% time required. Decreasing the thermal inertia that the thermistor is small.
The rated power is PM in the specified conditions, the thermistor long-term continuous load power dissipation allowed. In actual use shall not exceed the rated power. If the ambient temperature of the thermistor is more than 25 DEG C, the load must be reduced accordingly.
The rated working current: IM nominal current thermistor provisions under work value.
The measurement of power Pc: at the ambient temperature, the electric power consumed by the thermistor body test current heating resistance changes caused by no more than 0.1% of the time.
Thermistor
To the maximum voltage for NTC thermistor, refers to the specified ambient temperature, the maximum DC voltage thermistor caused by thermal runaway allowed continuous application; for PTC thermistor, refers to the provisions of the environmental temperature and static in the air, allowing continuous applied to the thermistor thermistor and ensure the normal work the maximum DC voltage characteristics of PTC part. The maximum working temperature of Tmax in the specified conditions, the highest temperature thermistor allowed by long-term continuous work.
These resistance switch temperature tb:PTC thermistor value began to jump when the temperature.
The dissipation coefficient H: raise the temperature of 1 DEG C, the power dissipation of the thermistor, the unit is mW/ DEG C.

Thermistor material classification
Heat sensitive materials can be generally divided into three categories, such as semiconductor, metal and alloy.
semi-conductor
These materials include single crystal semiconductor, polycrystalline semiconductor, glass semiconductor, organic semiconductor and metal oxide. They all have a very large resistance temperature coefficient and high turtle resistance rate, the sensitivity of the sensor made by them is quite high. According to the temperature coefficient of resistance can also be divided into negative temperature coefficient of resistance material and positive temperature coefficient of resistance material. In a limited temperature range, the negative temperature coefficient of resistance material a up to -6*10-2/ DEG C, positive temperature coefficient of resistance material a can be as high as -60*10-2/ DEG C. Such as barium titanate ceramics is an ideal positive resistance temperature coefficient of semiconductor materials. The above two kinds of materials are widely used in temperature measurement, temperature control, temperature compensation, instantaneous switch circuit, overload protection, time delay and other aspects, such as by making sub thermistor, thermistor switch and thermistor thermometer, thermistor thermistor relay switch and delay fault etc..
This kind of material due to the exponential relationship between resistance and mobility, so the temperature range is narrow, the uniformity is poor. .
Metal thermistor material
These materials are widely used as thermal resistance temperature measurement, current limiter and automatic constant temperature heating element. Such as platinum resistance thermometer, nickel resistance thermometer, copper resistance thermometer. Platinum side temperature sensor in a variety of media (including corrosive medium), showing a high accuracy and high stability characteristics. However, due to the scarcity of platinum and the high cost, their application is limited. Copper temperature sensor is cheap, but the long-term use in corrosive medium, can cause the static characteristic and the resistance changed significantly. Recent reports, copper temperature sensor can be used in the air temperature range of -60~180 DEG C. However, in order to abroad at -60~180 DEG C long measuring temperature and 250 degrees in the short term to measure temperature, generally use a lot of nickel and nickel as the temperature sensor, is a kind of ideal material, because of their high sensitivity, good reproducibility and stability of [1].
Alloy thermistor material
Thermistor material. This kind of alloy has high resistivity, and the resistance value is more sensitive to the change of temperature. As temperature sensor thermistor alloy performance requirements are as follows: (1) large enough resistivity; (2) high temperature coefficient of resistance; (3) is close to the experimental linear expansion coefficient; (4) the small strain sensitivity coefficient; (5) in the working temperature range of heating and cooling, repeat the resistance temperature curve should be good.

Classification
PTC
PTC（Positive Temperature CoeffiCient）A thermistor with positive temperature coefficient at a certain temperature Or like material, can be specially used for constant temperature sensor. The material is sintered with BaTiO3 or SrTiO3 or PbTiO3 as the main component, including the incorporation of small amount of Nb, Ta, Bi, Sb, Y, La and other oxides valence to control semiconductor, often the semiconductor of BaTiO3 etc. referred to as the semi conducting material (body) porcelain; also added to increase its positive temperature coefficient of resistance of Mn, Fe, Cu, Cr and other oxides additives, the ceramic forming process, high temperature sintering and titanate platinum and its solid solution semiconductor, resulting in positive characteristic thermistor materials. The temperature coefficient and the Curie temperature with composition and sintering conditions (especially temperature) changes.
Barium titanate crystal belongs to perovskite structure, is a kind of pure ferroelectric material, barium titanate is a kind of insulating materials. Rare earth elements in barium titanate materials, proper heat treatment, near the Curie temperature, resistivity increase several orders of magnitude, PTC effect, and the effect of ferroelectric crystal and phase transition of BaTiO3 near the Curie temperature of the material. The barium titanate semiconducting ceramics is a polycrystalline material that exists between the grains. The grain interface between the semi conducting ceramics when it reaches a certain temperature or voltage, the crystal grain boundary changes, thus the resistance changed dramatically.
The cause of the PTC effect of barium titanate semiconductive ceramics on the grain boundary (grain boundary). The electronic conductivity, the grain interface is equivalent to a barrier. When the temperature is low, the electric field in the role of barium titanate, lead to electron over the barrier easily, the resistance is smaller. When the temperature rises to the Curie temperature (that is near the critical temperature), the internal field is destroyed, it can help the electronic conductivity over the barrier. This barrier is equivalent to increasing the resistance value of a sudden increase of PTC effect. The physical model of PTC effect of barium titanate semiconductive ceramics with barium vacancy model and superimposed barrier model at sea surface barrier model, Daniels et al. They were from different aspects of the PTC effect is explained.
The experimental results show that, in the operating temperature range, the resistance temperature characteristics of PTC thermistor can be approximately expressed by the experimental formula:
RT=RT0 expBp (T-T0)
Thermistor
Thermistor
In the formula RT, RT0 said the temperature is T, T0 resistance value, Bp is the material constant of the material
The PTC effect originated in the ceramic grain boundary and grain boundary between precipitates and impurity with the nature, type, concentration and sintering conditions caused significant changes. Recently, enter the thermistor in practical use of temperature sensitive element silicon wafer, which is PTC thermistors, small size and high precision, which is composed of N silicon, electron scattering due to the impurity increases with temperature, so as to increase the resistance.
PTC thermistor appeared in 1950, followed by the emergence of barium titanate in 1954 as the main material of PTC thermistor. The PTC thermistor used for temperature measurement and control in industry, also used in temperature detection in certain parts of the car and adjustment, are also widely used in civilian equipment, such as the control of instant boiling water heater temperature, air conditioning and refrigeration temperature, heating by itself gas analysis and wind machines and other aspects. Following a brief introduction to a case of heater, motor, transformer, power transistors and other electrical heating and overheating protection applications.
The PTC thermistor can be used as a heating element, but also play the role of "switch", the three functions of the heater and the switch has sensitive components, called "thermo switch". The current through the element caused by the temperature, the heating temperature rise, when more than the Curie temperature, increase the resistance to limit the current increase, so the current decline in component temperature decreased, and the decrease of the resistance value of the circuit current increases, component temperature, cycle, so it can keep the temperature in a specific range of functions, and play a role switch. This made by resistance temperature characteristics of heating source, as the heating element is applied to the heater, electric iron, drying cabinet and air conditioning, but also to the overheating protection of electrical.
NTC
NTC（Negative Temperature CoeffiCient）Refers to the temperature with the increase in resistance index decreased, with a negative temperature coefficient of heat
The phenomenon of resistance and material. The material is a semiconductor ceramic using manganese, silicon, copper, cobalt, iron, nickel, zinc and other two or more than two kinds of metal oxides and process of mixing, molding, sintering, can be made with negative temperature coefficient (NTC) thermistor resistivity and its. The material constants with the material composition, sintering atmosphere, sintering temperature and structure changed. Now is a non oxide NTC thermistor material with silicon carbide, tin selenide, tantalum nitride as the representative.
Most of the NTC thermal sensitive semiconducting ceramics are spinel structure or other structures, which have negative temperature coefficient:
Rt = RT *EXP (Bn* (1/T-1/T0))
In the formula, RT and RT0 are respectively the resistance value of the temperature T, T0, and the Bn is the material constant, and the crystal grain is changed by the temperature change, which is decided by the characteristics of the semiconductor
The development of NTC thermistor has experienced a long period of.1834 years, scientists first discovered silver sulfide with negative temperature coefficient characteristics of.1930 years, the scientists found that the properties of cuprous oxide - copper oxide has negative temperature coefficient, which is successfully used in the temperature compensation circuit of aviation instrument. Then, due to the continuous development of transistor technology the research of the thermistor has made significant progress in.1960 developed NTC thermistor.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 -10 ~ +300 C, can be -200 to +10 DEG C, and even can be used for +300 ~ +1200 DEG C for temperature measurement with.RT NTC thermistor; R2 and R3 is the bridge balance resistance; R1 as the starting resistance; R4 for the full scale resistance, also known as check meter, check electrical resistance; R7, R8 and W as the pressure resistance, to provide a stable DC power supply and.R6 meter bridge (meter) series, current meter scale and modified the role of.R5 through the limit header and header protection. In parallel, unbalanced bridge arm (R1, RT) connected to a thermistor element RT as a temperature sensing probe. Because the thermistor resistance varies with temperature, which connected to the bridge between the diagonal header indicating a corresponding change. This is the working principle of thermistor thermometer.
Thermistor thermometer precision can reach 0.1 degrees Celsius, the temperature time can be reduced to below 10s. It is not only suitable for the granary thermometer, but also can be used in temperature measurement of food storage, medicine and health, scientific farming, marine, deep and high altitude glaciers and other aspects.
CTR
Thermistor
The critical temperature of the thermistor CTR (CritiCal Temperature Resistor) has the characteristics of negative resistance mutations, in a certain temperature, the resistance value decreases sharply with the increase of temperature has a negative temperature coefficient of material is very large. A mixed sintering of barium and strontium, vanadium, phosphorus oxide, semiconductor semi glass, also known as CTR is a glass thermistor. Sudden changes in temperature with adding germanium, tungsten and molybdenum oxides and variable. This is due to the different doping, the lattice spacing of vanadium oxide caused by different. If in a reducing atmosphere in an appropriate five of two vanadium into two vanadium oxide, the resistance is larger blast temperature if further reduced to three; two vanadium, then blast disappeared. The temperature corresponding to produce resistance blast position in half glass semiconductor physical blast, resulting in semiconductor metal phase shift.CTR As a temperature alarm and other applications
Theoretical research and application development of the thermistor has made remarkable achievements. With the application of high precision technology, deeper exploration and application of conductive mechanism of the thermal resistance, and excellent performance of new materials research, will achieve rapid development.

Testing
During the detection, with a multimeter ohm files (as the nominal resistance value to determine the position, general R * 1 block), concrete can be divided into two steps: first room temperature detection (indoor temperature close to 25 degrees Celsius), instead of the PTC thermistor probes are respectively clamped with alligator clip two pin measure the actual resistance, comparison and with the nominal value, the difference between the two - 2 Omega is in normal. If the actual resistance and nominal resistance is too large, then its performance is bad or damaged. The second heating detection, based on the normal test at room temperature, then the second step test - heating detection, a heat source (such as electric iron) near the heating thermistor, observation indicates the number of million, as seen by the million shows the number of temperature change, which indicates a gradual change in the resistance value (negative temperature coefficient thermistor NTC resistance becomes smaller, the positive temperature coefficient thermistor resistance becomes large, PTC) when the resistance change to a certain value display data will gradually stabilized, indicating the thermistor resistance is normal, if no change, the performance deterioration, can not continue to use.
The test should note the following: (1) Rt is measured by the manufacturer at ambient temperature is 25 degrees centigrade, so the multimeter is used to measure the Rt, should also be carried out at ambient temperature close to 25 degrees, in order to ensure the credibility of the test. (2) the measured power shall not exceed the specified value, so as to avoid the measurement error caused by the thermal effect. (3) pay attention to the correct operation. When testing, do not use the hand to hold the thermistor body, in order to prevent the human body temperature has the influence to the test. (4) be careful not to make the heat source with the PTC thermistor too close or direct contact with the thermistor, to prevent its hot.

Application
The thermistor can be used as electronic circuit components for circuit temperature compensation and thermocouple cold end temperature compensation. The automatic gain control can be realized by using the self thermal characteristics of the NTC thermistor, which forms a RC oscillator amplitude stabilization circuit, a delay circuit and a protection circuit. The temperature is far greater than the ambient temperature when the resistance is also associated with the radiation condition of environment, so the flow meter, flow meter, gas analyzer, thermal analysis is using the characteristic thermistor, made of special detection device. The PTC thermistor is mainly used for overheat protection, electrical equipment, relay contact, constant temperature, automatic gain control, motor starting, time delay, color television, automatic degaussing fire alarm and temperature compensation etc..

Main disadvantage

1. The relationship between the resistance and temperature of the serious nonlinear;
2. The consistency of the components is poor and the interchangeability is poor;
3. The components are easy to aging and the stability is poor;
4. In addition to the special high temperature thermistor, thermistor is only suitable for the vast majority of 0 to 150 DEG C, must pay attention to the use of.

Problem
If you intend to use the thermistor temperature sensor in the whole temperature range, then the design of the device will be challenging. The thermistor usually a high impedance and resistive device, so when you need to convert the resistance of the thermistor is voltage, an interface of the device can simplify the problem. However, the more challenging interface problem is how to capture the nonlinear behavior of thermistors in a digital form by using linear ADC.
The term "thermistor" is derived from a generalization of the description of the heat sensitive resistance. The thermistor consists of two basic types, namely the positive temperature coefficient thermistor and the negative temperature coefficient thermistor. Negative temperature coefficient thermistor is very suitable for high precision temperature measurement. To determine the temperature around the thermistor, you can use the Steinhart-Hart formula: T=1/ (A0+A1 (lnRT) +A3 (lnRT3)) to achieve. Among them, T is the Kelvin; RT is the resistance of thermistor in temperature at T; and A0, A1 and A3 are constants provided by thermistor manufacturer.
The resistance of the thermistor will change with temperature change, and this change is nonlinear, the Steinhart-Hart formula shows that this point. In the measurement of the temperature, the need to drive a reference current through the thermistor to create an equivalent voltage, the equivalent voltage has a nonlinear response. You can use a reference table equipped with a microcontroller to try to compensate for the nonlinear response of the thermistor. Even if you can run this kind of algorithm on the microcontroller firmware, you still need a high precision converter for data capture at the extreme temperature.
Another way is that you can use the "hardware linearization" technique and a lower precision ADC before digitizing. (Figure 1) one technique is to connect a resistor RSER with a thermistor RTHERM and a reference voltage or power supply (see Figure 1). The PGA (programmable gain amplifier) is set to 1V/V, but in such a circuit, a 10 bit precision ADC can only sense a very limited temperature range (approximately + + 25 C).
Figure 1, please note that the high temperature zone cannot be resolved in figure 1. But if you increase the gain of the PGA in the temperature value, the output signal of PGA can be controlled in a certain range, within this range of ADC can provide reliable conversion, thus the temperature of the thermistor for identification.
Microcontroller firmware temperature sensing algorithm can read 10 bit accuracy of the ADC digital value, and it is transmitted to the PGA hysteresis software program. The PGA delay program checks the PGA gain setting and compares the ADC value to the value of the voltage node shown in figure 1. If the ADC output exceeds the value of the voltage node, the microcontroller sets the PGA gain to the next higher or lower gain setting. If necessary, the microcontroller will again get a new ADC value. Then the PGA gain and ADC values will be transmitted to a microcontroller piecewise linear interpolation program.
Data acquisition from a nonlinear thermistor is sometimes considered an impossible task". You can use a series resistor, a micro controller, a 10 bit ADC and a PGA with reasonable use, in order to solve the measurement problem of nonlinear  thermistor caused after more than 25 DEG C temperature.

Difference

Thermistor symbol is PTC,

The resistance varies with temperature, a negative temperature type positive temperature type,
With the change of pressure sensitive resistance changes 2,
High, medium and low voltage varistor:
The main products are MYN type, MY31 type and MYG type three models

Thermistor alloy
Thermistor alloys have been widely used for temperature monitoring and impact. For example, it has been widely used in environmental monitoring, long-term storage of food, biological engineering and advanced military engineering .
Generally, the thermistor has high resistivity and resistance temperature coefficient, so it can be made into a miniaturized and highly sensitive temperature sensor. Such as the foil strain chip temperature sensor is a kind of ideal structure temperature measurement element. In addition, the thermistor alloy has been applied in the air temperature sensor and the temperature sensor of the large aircraft. It can be seen that the superiority of thermistor alloy will become more and more obvious .