NTC works as follows.Mini PIR Sensor Vendor

　　At normal temperature, NTC thermistor has a higher resistance value (generally 5Ω or 10Ω), that is, the nominal zero-power resistance value. When 10Ω NTC is connected in series, the power-on surge current is: I=220×1.414/(1+10)=28(A), which is 10 times lower than 311A when NTC thermistor is not used, which effectively suppresses the surge effect of current.

　　After power on, due to the rapid heating and temperature rise of the NTC thermistor, its resistance value will rapidly drop to a very small level within milliseconds, generally only a few tenths of ohms to a few ohms. For resistance current-limiting resistors, this means that the power consumption on the resistor is reduced by tens to hundreds of times due to the decrease in resistance value, so this design is very suitable for products that have higher requirements for conversion efficiency and energy saving, such as Switch the power supply.

　　After the power is turned off, the resistance value of the NTC thermistor will gradually recover to the nominal zero-power resistance value as it cools, and the recovery time varies from tens of seconds to several minutes. The next time it starts up, the above process loops again.

　　Improved power supply design

　　The above circuit using NTC surge suppressor already has the characteristics of energy saving compared with the circuit using fixed resistance. For some special products, such as industrial products, sometimes customers will put forward the following requirements: 1. How to reduce the failure rate of NTC to improve its service life? 2. How to minimize the power consumption of NTC? 3. How to make the power supply circuit connected with NTC thermistor in series adapt to the application conditions of the cyclic switch?

　　For points 1 and 2, because the main function of the NTC thermistor is to suppress surges, the energy it consumes after the product starts normally is not needed. If there is a feasible way to change the NTC thermistor from normal This requirement can be met by cutting off in the working circuit.

　　For point 3, first analyze why products using NTC thermistors cannot be switched frequently. From the analysis of the working principle of the circuit, we can see that under normal working conditions, a certain current flows through the NTC thermistor, and this working current is sufficient to make the surface temperature of the NTC reach 100°C to 200°C. When the product is turned off, the NTC thermistor must fully recover from the high temperature and low resistance state to the normal temperature and high resistance state in order to achieve the same surge suppression effect as the last time. This recovery time is related to the dissipation coefficient and heat capacity of the NTC thermistor, and the cooling time constant is generally used as a reference in engineering. The so-called cooling time constant refers to the time (in seconds) required for the NTC thermistor to cool down to 63.2% of its temperature rise after self-heating in a specified medium. The cooling time constant is not the time required for the NTC thermistor to return to normal, but the larger the cooling time constant, the longer the required recovery time, and vice versa.

　　Under the guidance of the above ideas, an improved circuit was produced. At the moment when the product is powered on, the NTC thermistor suppresses the inrush current to an appropriate level, and then the product is powered on and works normally. At this time, the relay coil acts after power is received from the load circuit, cutting the NTC thermistor from the working circuit. . In this way, the NTC thermistor only works when the product is started, and is not connected to the circuit when the product is working normally. This not only prolongs the service life of the NTC thermistor, but also ensures that it has sufficient cooling time, which can be suitable for applications requiring frequent switching.

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