PTC is the abbreviation of Positive Temperature Coefficient, which means positive temperature coefficient. It generally refers to semiconductor materials or components with large positive temperature coefficient.Usually we refer to PTC as positive temperature coefficient thermistor, or PTC thermistor for short.PTC thermistor is a typical semiconductor resistance with temperature sensitivity. When the temperature exceeds a certain temperature (Curie temperature), its resistance increases step by step with the increase of temperature.
It is a kind of thermistor. The resistance value of positive temperature coefficient thermistor increases step by step with the temperature of PTC thermistor. The higher the temperature, the greater the resistance value.
②Operating temperature rangeWide, normal temperaturedeviceIt is applicable to - 55 ℃~315 ℃, high temperature devices are applicable to temperatures higher than 315 ℃ (up to 2000 ℃ at present), and low temperature devices are applicable to - 273 ℃~- 55 ℃;
③ It is small in size and can measure the temperature of gaps, cavities and blood vessels in organisms that cannot be measured by other thermometers;
④ It is easy to use, and the resistance value can be arbitrarily selected from 0.1 to 100k Ω;
⑤ It is easy to process into complex shapes and can be produced in large quantities;
⑥ Good stability and strong overload capacity.
working principle
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The thermistor will not operate for a long timestate；When the ambient temperature andelectric currentHeat dissipation of thermistor when in zone cpowerIt is close to the heating power, so it may or may not act.Thermistor atambient temperatureWhen the same,Action timeIt shortens sharply with the increase of current;When the ambient temperature is relatively high, the thermistor has shorter operation time and smaller holding currentAction current。
1. The ptc effect is that a material has the ptc (positive temperature coefficient) effect, that is, the positive temperature coefficient effect. It 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, the ptc effect shows that the resistance increases linearly with temperature, which is commonly referred to as the linear ptc effect.
2. Nonlinear ptc effect Materials undergoing phase change will show a phenomenon that the resistance increases rapidly by several to ten orders of magnitude along a narrow temperature range, that is, nonlinear ptc effect. Quite a few types of conductive polymerization will show this effect, such as polymer ptc thermistors.These conductive polymers are useful for manufacturingOvercurrentIt is very useful for protective devices.
3. Polymer ptc thermistors for overcurrent protectionFuse(hereinafter referred to as thermistor), because of its unique positive temperature coefficient resistance characteristics, it is extremely suitable for overcurrent protectiondevice。Thermistors are used in the same way as ordinary onesFuseSimilarly, it is used in series in the circuit.
When the circuit works normally, the temperature of the thermistor is close to the room temperature, the resistance is very small, and the series connection in the circuit will not hinder the current flow;When the circuit is due tofaultAnd appearedOvercurrentWhen the temperature exceedsswitchWhen the temperature is (ts), the resistance will suddenly increase sharply, and the current in the circuit will rapidly reduce to a safe value. This is to protect the AC circuit from thermistorprocessChange of current in.After the thermistor acts, the current in the circuit decreases significantly, and t is the action time of the thermistor.Due to the good designability of polymer ptc thermistors, their sensitivity to temperature can be adjusted by changing their switching temperature (ts)degreeTherefore, it can simultaneously play two roles of over temperature protection and over current protection, such as kt16-1700dlSpecificationsThermistors are suitable for overcurrent and over temperature protection of lithium ion batteries and nickel metal hydride batteries due to their low operating temperature.The influence of ambient temperature on polymer ptc thermistor is a direct heating, step type thermistor. Its resistance change process is related to its own heating and cooling conditions, so its maintenance current (ihold)Action current(itrip) andAction timeAffected by ambient temperature.When the ambient temperature and current are in zone a, the thermistor will act when its heating power is greater than its cooling power;When the ambient temperature and current are in the b zone, the heating power is less than the cooling power, and the polymer ptc thermistor can be reused because the resistance can be recovered.After thermistor action, recoverprocessMedium resistance changes with time.In general, the resistance can be restored to the level of 1.6 times of the initial value in tens of seconds to tens of seconds. At this time, the holding current of the thermistor has been restored to the rated value and can be used again.Thermistors with smaller area and thickness recover relatively quickly;The thermistors with larger area and thickness recover slowly.
Basic characteristics
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temperature characteristic
The resistance temperature characteristic of thermistor can be approximately expressed by the following formula: R=R0exp{B (1/T-1/T0)}: R: resistance value at temperature T (K), Ro: resistance value at temperature T0, (K), B: B value, * T (K)=t (º C)+273.15.In fact, the B value of the thermistor is not constant, and its variation varies with the composition of the material, up to 5K/° C.Therefore, when equation 1 is applied in a large temperature range, there will be a certain difference between the measured value anderror。Here, if the value of B in equation 1 is calculated as a function of temperature as shown in equation 2, theerror, can be considered as approximately equal.
BT=CT2+DT+E，In the above formula, 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.Calculation of constants C, D and E, which can be calculated from the data (T0, R0), (T1, R1), (T2, R2), and (T3, R3) of four points (temperature and resistance values) through equations 3-6.First, calculate B1, B2 and B3 according to the resistance values of T0, T1, T2 and T3 in style 3, and then substitute them into the following styles.
Example of resistance value calculation: try to calculate the resistance value of 5 (k Ω) at 25 ° C and the resistance value of the thermistor with a deviation of 50 (K) at 10 ° C~30 ° C according to the resistance temperature characteristic table.Step (1) Calculate the constants C, D and E according to the resistance temperature characteristic table. To=25+273.15T1=10+273.15T2=20+273.15T3=30+273.15 (2) Substitute BT=CT2+DT+E+50 to calculate BT.(3) Substitute the numerical value into R=5exp {(BT1/T-1/298.15)} to find R*T:10+273.15～30+273.15。
1. High sensitivity, its resistance temperature coefficient is 10~100 times larger than that of metal, and can detect the temperature change of 10-6 ℃;
PTC thermistor
2. Wide operating temperature range, normal temperature devices are applicable to - 55 ℃~315 ℃, high temperature devices are applicable to temperatures higher than 315 ℃ (up to 2000 ℃ at present), and low temperature devices are applicable to - 273 ℃~55 ℃;
3. It is small in size and can measure the temperature of gaps, cavities and blood vessels in organisms that cannot be measured by other thermometers;
4. It is easy to use, and the resistance value can be arbitrarily selected from 0.1 to 100k Ω;
5. It is easy to process into complex shapes and can be produced in large quantities;
6. Good stability, strong overload capacity
design principle
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Thermistors are sensitive components that have been developed early, with many kinds and mature development.Thermistors are composed of semiconductor ceramic materials, and the principle of using is that temperature causes resistance changes.When the temperature is lower than Tc, the negative charge at the grain boundary is partially offset by the polarization charge, which greatly reduces the barrier height, and the grain boundary is in a low resistance state;When it is higher than Tc, the spontaneous polarization disappears, the negative charge at the grain boundary can not get the polarization charge barrier is high, and the grain boundary is in a high resistance state.The overall resistance of the material rises sharply.
If the concentration of electrons and holes is n and p respectively, and the mobility is μ n and μ p respectively, the conductivity of the semiconductor is:
σ=q（nμn+pμp）。
Because n, p, μ n and μ p are all temperature dependent functionsconductanceIt is a function of temperature, so the temperature can be calculated by measuring the conductance, and the resistance temperature characteristic curve can be made, which is the working principle of semiconductor thermistors.
Thermistors include positive temperature coefficient (PTC) and negative temperature coefficient (NTC) thermistors, andcritical temperature resistor （CTR）。
PTC thermistors with different reactions can also be connected in series to implement temperature protection at different points, so that the most economical and excellent protection can be achieved at different temperature stages for mobile phone batteries, electronics, electrical appliances and other parts.
main features
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Fig. 1 PTC thermistor
1. High sensitivity, its resistance temperature coefficient is 10~100 times larger than that of metal, and can detect the temperature change of 10-6 ℃;2. Wide operating temperature range, normal temperature devices are applicable to - 55 ℃~315 ℃, high temperature devices are applicable to temperatures higher than 315 ℃ (up to 2000 ℃ at present), and low temperature devices are applicable to - 273 ℃~55 ℃;
3. It is small in size and can measure the temperature of gaps, cavities and blood vessels in organisms that cannot be measured by other thermometers;
4. It is easy to use, and the resistance value can be arbitrarily selected from 0.1 to 100k Ω;
5. It is easy to process into complex shapes and can be produced in large quantities;
6. Good stability, strong overload capacity
characteristic curve
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As shown in Figure 1, PTC thermistor is a typical temperature sensitive semiconductor resistor. When the temperature exceeds a certain temperature (Curie temperature), its resistance value increases step by step with the increase of temperature.[1]
Its disadvantages are: the power is extremely unstable, and the power decay radiance is very large, which is very easy to cause a large power decline for half a year or several months, resulting in water and heat stagnation[2]。
Scope of application
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It is used in battery, security, medical, scientific research, industrial motor, aerospace and other electronic and electrical temperature control related fields.
