NTC thermistor in series in AC circuit functions like current insurance.
How does this insurance do in the circuit? When the circuit is powered on, there produces an instant surge current. Thermistors can suppress this current. After this, the current will keep stable, and then the NTC thermistor will decrease its resistance to an ignorable value, which will not influence the current.
During power-off at room temperature, the resistance of thermistors is relatively high, and at the moment of power-on, the resistance remains high, which is used to suppress the surge current. Then the temperature increases, and the resistance decreases to an ignorable value. If there is another surge current at this time, NTC thermistor cannot help and it needs a fuse. That is to say, NTC thermistor can only function at the moment of power-on. For the power supply that has a NTC thermistor inside, it cannot frequently power on and off. Restart only when the NTC thermistor cools down to its cold resistance. That means NTC cannot function when there is frequent power on and off.
Physical photo of NTC thermistor
