What are thermistors and how do they limit inrush current?

Inrush currents are peak currents that occur when power is first applied to a load and is due to low impedance in the power mains transformers or the filter capacitors of switching power supplies (e.g., D.C. Drive controls or phase-controlled SCR converters). 1 These peak currents can damage the power system or a D.C. motor’s windings if they are not limited. 2 There a variety of ways to limit inrush current. One of those ways is by using an electronic device called a thermistor, which varies its resistance as a function of temperature. 3 There are two types of thermistors: Negative Temperature Coefficient (NTC) and Positive Temperature Coefficient (PTC). NTC thermistors decrease their resistance as temperature rises. Initially, NTC thermistors have a high resistance. When power is turned on, they limit inrush currents in the form of power dissipation or heat. As the NTC thermistor heats up, its resistance decreases to the point that most of the power mains voltage is applied to the load; in other words, the NTC thermistor’s voltage drop does not effect normal circuit function. 4 Positive temperature coefficient thermistors increase their resistance as temperature rises. However, they typically are not used to limit inrush currents; rather, they are used as motor overload protection or as a switch to energize the start winding of a permanent split-capacitor motor. 5

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