The main characteristics of thermistors are:

① The sensitivity is high, and the resistance temperature coefficient is more than 10 to 100 times larger than the metal, and the temperature change of 10-6℃ can be detected;

② Wide operating temperature range, normal temperature devices are suitable for -55℃ ~ 315℃, high temperature devices are suitable for temperatures higher than 315℃ (currently up to 2000℃), low temperature devices are suitable for -273℃ ~ -55℃;

③ Small in size, it can measure the temperature of voids, cavities and blood vessels in organisms that cannot be measured by other thermometers;

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

⑤ Easy to process into complex shapes, can be mass-produced;

⑥ Good stability, strong overload capacity

The thermistor will remain inactive for a long time; When the ambient temperature and current are in the c zone, the heat dissipation power of the 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. The thermistor has shorter operating time and smaller maintenance current and operating current when the ambient temperature is relatively high.

1, ptc effect is a material with ptc(positive temperature coefficient) effect, that is, positive temperature coefficient effect, only refers to the resistance of the material will increase with the increase of temperature. For example, most metal materials have ptc effect. In these materials, the ptc effect appears 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 more than a dozen orders of magnitude phenomenon, that is, the nonlinear ptc effect, quite a variety of types of conductive polymers will show this effect, such as polymer ptc thermistor. These conductive polymers are useful for making overcurrent protection devices.

3, polymer ptc thermistor 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 extremely suitable for use as overcurrent protection devices. The thermistor is used in series in the circuit like an ordinary fuse.

When the circuit is working normally, the temperature of the thermistor is close to room temperature, the resistance is very small, and the series in the circuit will not hinder the current through; And when the circuit due to the fault of the overcurrent, thermistor due to the increase in heating power caused by the temperature rise, when the temperature exceeds the switching temperature, the resistance will increase instantly, the current in the circuit rapidly reduced to a safe value. Is a schematic diagram of current change in the process of thermistor protecting AC circuit. After the thermistor action, the current in the circuit is greatly reduced, and 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 own switching temperature (ts), so it can play two roles of overtemperature protection and overcurrent protection at the same time, such as kt16-1700dl specification thermistor due to the low operating temperature, so it is suitable for lithium-ion batteries and nickel-metal hydride batteries overcurrent and overtemperature protection. Effect of ambient temperature on polymer ptc thermistor 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 maintenance 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 the thermistor is greater than the cooling power and will operate; When the ambient temperature and current are in the b zone, the heating power is less than the heat dissipation power, and the polymer ptc thermistor can be reused many times because the resistance can be recovered. Figure 6 shows the resistance change over time during the recovery process after the thermistor action. The resistance can generally be restored to the level of about 1.6 times the initial value in ten seconds to tens 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 small area and thickness recovers relatively quickly. The thermistor with larger area and thickness recovers relatively slowly