Polymer PTC thermistor for overcurrent protection

1. PTC effect

To say that a material has the PTC (Positive Temperature Coefficient) effect, that is, the positive temperature coefficient effect, only means that the resistance of the 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 the so-called linear PTC effect.

2. Nonlinear PTC effect

The material that undergoes a phase change will exhibit a phenomenon in which the resistance increases sharply within a narrow temperature range from several to a dozen orders of magnitude, that is, the nonlinear PTC effect, as shown in Figure 1. Many types of conductive polymers exhibit this effect, such as polymer PTC thermistors. These conductive polymers are very useful for making overcurrent protection devices.

3. Polymer PTC thermistor is 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 (ie PTC characteristics, as shown in Figure 1), they are extremely suitable for use as Overcurrent protection device. The use of thermistor is the same as an ordinary fuse, which is used in series in the circuit, as shown in Figure 2.

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. When the circuit has a large current due to a fault, the temperature of the thermistor will rise due to the increase of heating power. When the temperature exceeds the switching temperature (Ts, see Figure 3), 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 in the thermistor's protection of the AC circuit. After the thermistor is activated, the current in the circuit is greatly reduced. In the figure, t is the thermistor's operating time. 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. PTC thermistor is suitable for over-current and over-temperature protection of Li-ion batteries and Ni-MH batteries due to its low operating temperature.

4. The influence of ambient temperature on polymer PTC thermistor

The polymer PTC thermistor is a direct-heating, step-type thermistor, and its resistance change process is related to its own heating and heat dissipation, so its holding current (Ihold), operating current (Itrip) and operating time are affected by Influence of ambient temperature. Figure 4 is a schematic diagram of the relationship between thermistor's typical holding current, operating current and 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 thermistor will remain inactive for a long time; when the ambient temperature When the sum current is in the C zone, the heat dissipation power of the thermistor is close to the heating power, so it may or may not operate. Figure 5 is a schematic diagram of the relationship between the thermistor's operating time and current and ambient temperature. When the ambient temperature is the same, the operating time of the thermistor decreases 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.

5. Recovery characteristics of polymer PTC thermistor after operation The polymer PTC thermistor can be used repeatedly because of its recoverable resistance. 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 within ten seconds to several tens of seconds. At this time, the maintenance current of the thermistor has returned to the rated value and can be used again. Generally speaking, a thermistor with a smaller area and thickness recovers relatively quickly; a thermistor with a larger area and thickness recovers relatively slowly.

6. Features of polymer PTC thermistor

   Polymer PTC thermistor is a kind of conductive polymer material with positive temperature coefficient characteristics. The most significant difference between it and traditional fuses is that the former can be used repeatedly. Both of these products can provide over-current protection, but the same polymer PTC thermistor can provide this protection multiple times, and after the fuse provides over-current protection, it must be replaced with another one.

   The main difference between the polymer PTC thermistor and the bimetal circuit breaker is that the former is always in the off state and will not reset before the accident is eliminated, but the bimetal circuit breaker can reset itself when the accident still exists. This may cause electromagnetic waves and sparks during resetting. At the same time, reconnecting the circuit when the circuit is in a fault condition may damage the equipment, which is unsafe. The polymer PTC thermistor can maintain high resistance until the fault is eliminated.

   The difference between the polymer PTC thermistor and the ceramic PTC thermistor lies in the initial resistance of the element, the action time (reaction time to accident events) and the difference in size. Compared with the ceramic PTC thermistor, the polymer PTC thermistor with the same holding current has a smaller size, lower resistance and faster response.