The principle and application of thermal resistance working


Introduction to thermistor resistance

Thermal resistance is a resistance made of special materials, and its resistance value changes with temperature changes. Depending on the resistance of the resistance value, the thermal resistance is divided into two categories, one is called the positive temperature coefficient thermistor (PTC). The resistance value increases with the increase of temperature; Resistance (NTC), the resistance value decreases with the increase in temperature.

Negative temperature coefficient thermistor (NTC)

Positive temperature coefficient thermistor (PTC)

Thermal resistance working principle


PTC is generally based on titanic acid, and a small amount of rare earth elements are added to titanic acid, sintering by high temperature. Titanate is a polycrystal material. There is a crystal particles interface between the internal crystals and the crystal. When the temperature is low, due to the effect of the internal electric field, the conductive electrons can easily cross the particle interface. At this time, its resistance value It will be relatively small. When the temperature rises, the internal electric field will be destroyed, and the conductive electrons will be difficult to cross the particle interface. At this time, the resistance value will rise.

NTC is generally made of metal oxide materials such as cobalt oxide and nickel oxide. There are fewer electrons and holes inside such metal oxides, and their resistance values ​​will be higher. When the temperature rises, the number of internal electrons and holes will increase, and the resistance value will be reduced.

The advantages and disadvantages of thermistor



High sensitivity, the temperature coefficient of thermistor is more than 10-100 times larger than the metal, which can detect the temperature changes of 10-6 ° C; the working temperature range is wide, the normal temperature devices are suitable for -55 ° C ~ 315 ° C, the temperature of the high temperature device is suitable for temperature Higher than 315 ° C (currently up to 2000 ° C), low temperature devices are suitable for -273 ° C ~ -55 ° C; small volume, which can measure the temperature that other thermometers cannot measure space.



The disadvantages of thermal resistance are mainly the relationship between resistance and temperature is not linearly serious; and the consistency of components is poor and poor interchangeability; once damaged, it is difficult to find a replaceable product. Not only that, the components of thermal resistance are easy to aging and the stability is relatively poor; in addition to the special high -temperature thermistor, most thermistor is only suitable for the range of 0 ~ 150 ° C.

Application of thermistor

The main application of thermistor is used as a temperature detection element. The temperature detection usually uses the thermistor of the negative temperature coefficient, which is NTC. Such as commonly used household appliances, such as rice cookers, induction cookers, etc., which can be used with a thermistor.


Super temperature alarm circuit


The figure above is an ultra -temperature alarm circuit composed of thermistor as a temperature detection element.

Its working principle is: when the environment or the detected temperature is low, the resistance value of the thermistor RT is high. At this time, the input terminal of the logic door IC-1 is high. The level, at this time the base of the triode VT1 is a low level, the VT1 cut off, the relay is not action; after the IC-2 reverse, the output is high level, the LED does not light; The oscillation circuit composed of -3 and IC-4 does not work, and the speaker has no sound output.

When the temperature rises, the resistance value of the RT decreases. When the temperature rises to a certain value, the input end of the IC-1 becomes a low level, and the high level is output after the opposite. Flat, transistor drive, relay coils are electrical, control the load after control; low level output after IC2 reverse, LED light; IC-3 input terminal is high level, oscillator work, and speakers emit sound alarm.

There are also the following applications in addition to the temperature detection element as a temperature detection element.


The application of NTC in switching power supply

The switching power supply is the working process of the AC-DC-AC-DC. After the AC power is rectified, it will be filtered through a capacitor. At the moment of power, because it is necessary to charge the capacitor, the instant current will be much larger than the normal work. In the previous rectification part, the insurance pipe has caused a certain impact. NTC is usually connected in the input end of the switching power supply. Due to the high resistance value of NTC at room temperature, the current limit will be played at the moment of power. The value will be reduced, and it will be equivalent to a wire at this time to ensure the normal operation of the power supply.

PTC anti -trading resistance

The old -fashioned appearance (CRT) TV shows the image by changing the movement direction of the electron beam through the electromagnetic field. Affected by the environmental magnetic field or geomagnetic field, the electron beam will be deviated, which will affect the displayed image and color. When the TV is turned on, it is necessary to perform demagnetic treatment of the visible tube. The demagnetic coil is surrounded by the imaging tube, and the passing of power frequency AC can achieve the purpose of demagnetization, and the process of demagnetization only needs to be called at the moment of power.

Entering PTC in a demagnetic circuit can achieve the purpose of automatic power off after demagnetization. Due to the small PTC resistance value at room temperature, the AC power will be powered by the demagnetization resistance to the anti -trading coil through the anti -trading resistance. If the current flows, the temperature of the anti -demagnetic resistance will increase. As the temperature rises, the resistance value will increase, and the entire resistance will increase, which will cause the entire entire The current of the demagnetic circuit is reduced to stop working to stop working.

PTC heating board

Traditional electricity heating is generally used as a thermal material, while its life span is short and the earthquake resistance is weak. Many household appliances, such as thermal heating heater and air -conditioning, use PTC as heating materials. The service life of the PTC is much longer than that of tungsten wire, and as the heating temperature rises, the resistance value of the PTC will also increase, and its working current will slowly decrease. When the temperature drops, the resistance value decreases, the current rises, the current rises It can play a certain automatic constant temperature.

Negative temperature coefficient thermistor (NTC)

Positive temperature coefficient thermistor (PTC)