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High-power thermistor and inrush current

Inrush current

Unless inrush current protection is used, switching power supply circuits, AC motors, and lighting ballasts can generate extremely high peak inrush currents when turned on. Without protection, the only constraints on the amount of inrush current incurred are line impedance, input rectifier voltage drop, and capacitor equivalent series resistance.


High inrush currents will unnecessarily trip fuses and circuit breakers and then affect the electrical system. Assuming no surge current protection, it is necessary to use relays and fuses rated higher than any possible surge current. Considering the arc discharge of the contacts, the inrush current will also cause pit contacts on the switches and relays.

The surge current can be as high as 100 times the normal steady-state current, and generally lasts less than 1/2 of the normal 60 Hz cycle.

Surge current protection

It can provide inrush current protection through active circuits using power resistors, thyristors and triacs. Active circuits are generally valuable and difficult to plan. Another option for inrush current protection is NTC thermistor. Ametherm produces a special type of NTC thermistor, which is specially designed for surge current protection and is called a surge current restraint.

It is estimated that 90% of inrush current restraint uses inrush current restraint. The circuit using the surge current restraint is easy to plan and cheaper than the active circuit. The inrush current restraint is used in series with the line voltage. When connected, it has a high resistance to inrush current and is sensitively removed from the circuit, allowing the electrical system to operate normally.

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