Improved NTC thermistor does not always switch problems

In the working principle of the circuit, it can be known that there is a product using NTC thermistor. Under normal working conditions, there is a certain current flowing through the NTC thermistor. This working current is enough to make the surface temperature of the NTC reach 100 ° C ~ 200 ° C. . When the product is turned off, the NTC thermistor needs to completely recover from the high temperature and low resistance state to the normal temperature and high resistance state to achieve the same surge suppression effect as the previous one.

This recovery time is related to the dissipation coefficient and heat capacity of the NTC thermistor, and the ice-cooling time constant is generally used as a reference in engineering. The so-called ice-cooling time constant refers to the time (in seconds) required for the NTC thermistor to cool to 63.2% of its temperature rise after heating from the specified medium. The ice-cold time constant is not the time required for the NTC thermistor to return to normal, but the greater the ice-cold time constant, the longer the recovery time required, and vice versa.

To improve this problem, the NTC thermistor can suppress the inrush current to a suitable level after the product is powered on. After that, the product is powered normally. At this time, the relay coil is energized from the load circuit and the NTC thermistor is removed. Cut off in the working circuit. In this way, the NTC thermistor only works when the product is started, and does not access the circuit when the product is working properly. This not only extends the service life of the NTC thermistor, but also ensures that it has sufficient ice cooling time, which can be applied to applications requiring frequent switching.

For applications that require frequent switching, a relay bypass circuit must be added to the circuit to ensure that the NTC thermistor is completely cold and return to its original state. When selecting the thermistor, it is necessary to consider the temperature of the working environment and the operating current applied to the NTC thermistor to select the resistance of the resistor.