The main characteristics of thermistor are:

(1) High sensitivity, the resistance temperature coefficient is more than 10 ~ 100 times larger than metal, can detect the temperature change of 10-6℃;

② Wide operating temperature range, normal temperature device is suitable for -55℃ ~ 315℃, high temperature device is suitable for higher than 315℃ (currently the highest can reach 2000℃), low temperature device is suitable for -273℃ ~ -55℃;

③ Small volume, can measure other thermometers can not measure the temperature of voids, cavities and blood vessels in the body;

④ Easy to use, resistance value can be selected between 0.1 ~ 100kΩ;

(5) Easy to process into complex shapes, can be mass production;

⑥ Good stability and overload ability

Thermistor will be inactive for a long time; When the ambient temperature and current are in zone c, the heat dissipation power of thermistor is close to the heat generating power, so it may or may not operate. When the ambient temperature is the same, the operation time of thermistor decreases sharply with the increase of current. Thermistor has shorter operation time and smaller maintenance current and operation current when the ambient temperature is relatively high.

1. ptc effect is a material with positive temperature coefficient (ptc) effect, that is, positive temperature coefficient effect, which only means that the resistance of the material increases with the increase of temperature. For example, most metal materials have ptc effect. In these materials, the ptc effect manifests as a linear increase in resistance with increasing temperature, which is commonly referred to as the linear ptc effect.

2, nonlinear ptc effect After the phase change of the material will show resistance along the narrow temperature range of several to a dozen orders of magnitude phenomenon, that is, nonlinear ptc effect, quite a variety of conductive polymers will show this effect, such as polymer ptc thermistor. These conducting polymers are useful for making overcurrent protection devices.

3, polymer ptc thermistor used for overcurrent protection polymer ptc thermistor is often called self-recovery fuse (hereinafter referred to as thermistor), because of its unique positive temperature coefficient resistance characteristics, so it is very suitable for use as overcurrent protection devices. Thermistors are used in series in the circuit like ordinary fuses.

When the circuit is working normally, the thermistor temperature is close to room temperature, the resistance is very small, series in the circuit will not hinder the current through; When the circuit due to the fault of the overcurrent, thermistor due to the increase in heating power resulting in a rise in temperature, when the temperature exceeds the switching temperature, the resistance will surge instantly, the current in the loop rapidly reduced to the safety value. Is a schematic diagram of current variation during thermistor protection of AC circuit. After the action of the thermistor, the current in the circuit is greatly reduced. t in the figure is the action time of the thermistor. Due to the good design of polymer ptc thermistor, it can adjust its sensitivity to temperature by changing its switching temperature (ts), so it can play the role of overtemperature protection and overcurrent protection at the same time, such as kt16-1700dl specification thermistor due to the operation temperature is very low, so it is suitable for lithium ion battery and nickel metal hydride battery overcurrent and overtemperature protection. Polymer ptc thermistor is a direct heat type, step type thermistor, its resistance change process is related to its own heating and heat dissipation, so its ihold current (ihold), action current (itrip) and action time are affected by environmental temperature. When the ambient temperature and current are in zone a, the heating power of thermistor is greater than the cooling power and will act; When the ambient temperature and current are in zone b, the heating power is less than the heat dissipation power. Polymer ptc thermistor can be used repeatedly because the resistance can be restored. FIG. 6 is a schematic diagram of resistance changes with time during the recovery process after thermistor action. The resistance can be restored to about 1.6 times the initial value in ten to dozens of seconds, at which time the maintenance current of the thermistor has been restored to the rated value and can be used again. The thermistor with smaller area and thickness recovers relatively quickly. But the thermistor with larger area and thickness recovers relatively slowly