Posted on 05 April 2019

Protecting Appliances Using Inrush Current Limiters (ICL)








High inrush currents occur when certain types of electrical equipment such as switched mode power supplies, motors, transformers, halogen lamps, fluorescent lamps, and amplifiers are turned on. Such high currents are caused by the extremely low impedance of smoothing capacitorss or coils, which nearly produce short circuits at the moment when they are switched on. The inrush current must be limited in order to reduce the peak circuit current, protect other components, and avoid tripping fuses in error. Such currents can damage individual components or entire subassemblies. An inrush current limiter (ICL), comprising either a thermistor or a fixed resistor, is therefore connected in series with the circuit.

Inrush current limiter circuit principle

Figure 1: Circuit principle for an ICL

Negative temperature coefficient (NTC) thermistors specifically developed for this application limit current at turn-on thanks to their relatively high cold resistance. As a result of the current load, the thermistor heats up so that its resistance drops. The ICL thus provides protection against undesirably high inrush currents while ensuring low power consumption during continuous operation.

ICL-NTC operating principle

Figure 2. Operating principle of ICL-NTCs


For more information, please read the following articles:

Overvoltage Protection with Varistors

Protection Against Overcurrents with Fuses

PSpice Simulation Model


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