NTC resistors in series are mainly used as" current safeties" in ac circuits.
In order to avoid inrush currents in electronic circuits at the moment of switching on, a power NTC thermistor can be connected in series in the power supply circuit to effectively suppress the inrush currents at switching on, and after the inrush current suppression is completed, the resistance value of the power NTC thermistor will drop to a very small level due to the continuous action of the current through it, which consumes negligible power and does not affect the normal operating current. Therefore, the use of a power NTC thermistor in the power circuit is the easiest and most effective measure to suppress surges at power-on and to protect electronic equipment from damage.
A more ideal NTC application circuit is shown in the diagram below, where the NTC is connected in parallel with a relay contact, and the relay sucks on and short-circuits the NTC after power-up.
The resistance of the NTC is high when it is not energised, and is still high once the power is applied, limiting the inrush current.
After the power supply has been in normal operation for a period of time, frequent switching on and off will cause damage to the power supply, as the temperature of the NTC rises and the resistance value drops, the inrush suppression ability is already very limited, the NTC is used to suppress the power surge power supply equipment, can not be frequently switched on and off. It is necessary to wait for the NTC to cool down and return to its cold resistance before switching on again. Otherwise, the point of installing an NTC is defeated. For low power supplies with low current NTCs do not heat up much and are therefore useful.
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