The characteristics and working principle of thermistor

Thermistor is a kind of sensitive element. The typical characteristic of thermistor is sensitivity to temperature. So what are the characteristics of the thermistor? How does it work?

Thermistor is a type of sensitive element, according to the temperature coefficient is divided into positive temperature coefficient thermistor (PTC) and negative temperature coefficient thermistor (NTC). The typical characteristic of thermistor is that it is sensitive to temperature and exhibits different resistance values at different temperatures. Positive temperature coefficient thermistor (PTC) has a higher resistance value when the temperature is higher, and negative temperature coefficient thermistor (NTC) has a lower resistance value when the temperature is higher. They are both semiconductor devices.

However, it should be noted that the thermistor is not a semiconductor device under tax heading 85.41 in the import and export process.

Thermistor is a sensitive component that has been developed early, with many types and more mature development. The thermistor is composed of semiconductor ceramic materials, and the thermistor is made of semiconductor materials, mostly with a negative temperature coefficient, that is, the resistance value decreases as the temperature increases. The main features of thermistor are high sensitivity; wide operating temperature range; small size; easy to use; easy to process into complex shapes and can be mass-produced; good stability and strong overload capacity.

Because the semiconductor thermistor has unique properties, it can be used not only as a measuring element, but also as a control element and circuit compensation element in application. Thermistors are widely used in various fields such as household appliances, electric power industry, communications, military science, aerospace, etc., and their development prospects are extremely broad.

The main features of thermistor are:

①The sensitivity is high, and its temperature coefficient of resistance is 10-100 times larger than that of metal, and it can detect temperature changes of 10-6℃;

②Wide operating temperature range, room 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℃;

③Small size, able to measure the temperature of voids, cavities and blood vessels in organisms that other thermometers cannot measure;

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

⑤Easy to process into complex shapes and mass production;

⑥Good stability and strong overload capacity.

working principle

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

1. The ptc effect is a material that has the ptc (positive temperature coefficient) effect, that is, the positive temperature coefficient effect, which only means that the resistance of this material will increase with the increase of temperature. For example, most metal materials have the 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. Non-linear ptc effect The material that undergoes a phase change will show a phenomenon that the resistance increases sharply from several to a dozen orders of magnitude within a narrow temperature range, that is, the non-linear ptc effect. Quite a few types of conductive polymers will exhibit this phenomenon. Effect, such as polymer ptc thermistor. These conductive polymers are very useful for making overcurrent protection devices.

3. Polymer ptc thermistors are used for overcurrent protection. Polymer ptc thermistors are often called self-recovery fuses (hereinafter referred to as thermistors). Because of their unique positive temperature coefficient resistance characteristics, they are extremely suitable Used as an overcurrent protection device. The use of thermistor is the same as an ordinary fuse, which is used in series in the circuit.

When the circuit is working normally, the temperature of the thermistor is close to room temperature, and the resistance is very small. It will not hinder the flow of current when connected in series in the circuit; and when the circuit has overcurrent due to a fault, the thermistor will increase in temperature due to the increase in heating power. When the temperature exceeds the switching temperature (ts, see Figure 1), the resistance will instantly increase, and the current in the loop will quickly decrease to a safe value. It is a schematic diagram of the current change during the protection of the AC circuit by the thermistor. After the thermistor is activated, the current in the circuit has been greatly reduced. In the figure, t is the operating time of the thermistor. Because the polymer ptc thermistor has good designability, its sensitivity to temperature can be adjusted by changing its own switching temperature (ts), so it can play both over-temperature protection and over-current protection at the same time, such as kt16 The -1700dl specification thermistor is suitable for over-current and over-temperature protection of Li-ion batteries and Ni-MH batteries due to its low operating temperature. The influence of ambient temperature on polymer ptc thermistor The polymer ptc thermistor is a direct heating, step type thermistor, its resistance change process is related to its own heating and heat dissipation, so its maintenance current (ihold ), operating current (itrip) and operating time are affected by ambient temperature. When the ambient temperature and current are in zone a, the heating power of the thermistor is greater than the heat dissipation power and will act; when the ambient temperature and current are in zone b, the heating power is less than the heat dissipation power, and the polymer ptc thermistor can be restored because of the resistance. Repeated use. Figure 6 is a schematic diagram of the resistance change with time during the recovery process after the thermistor has been activated. The resistance generally recovers to a level of about 1.6 times the initial value in ten seconds to several tens of seconds. At this 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; while the thermistor with larger area and thickness recovers relatively slowly.