Inrush current limiter

Inrush current limiter

An inrush current limiter is a component used to limit inrush current. Negative temperature coefficient (NTC) thermistors and fixed resistors are often used for this. Less often, other components are also used.

The inrush current limiter is placed in series with the circuit whose inrush current needs to be limited.


A thermistor's resistance is initially high. When the equipment is turned on, the thermistor's resistance limits the initial current. Current flow heats the thermistor, and when hot its resistance is a lower value, allowing most of the input voltage to be appear on the following circuit. It is inherently impossible for 100% of supply voltage to appear on the following circuit, as the thermistor must continue to dissipate power to stay hot and therefore low resistance.

* Inrush current limiting thermistors are typically disk shaped, with two radial leads on one side.
* Their power handling is proportional to their size.
* They are rated according to their resistance at room temperature.

Fixed resistor

Fixed resistors are also widely used to limit inrush current. These are inherently less efficient, since the resistance never falls from the value required to limit the inrush current. Consequently they are generally chosen for lower power circuitry, where the additional ongoing power waste is minor. Inrush limiting resistors are much cheaper than thermistors. They are found in more or less all CFL lamps (light bulbs).


A typical application of inrush current limiters is in the input stage of non-Power Factor Corrected switching supplies, to reduce the initial surge of current from the line input to the filter capacitors.


* [ Inrush Current Limiters] Description - Application Notes and Inrush Energy Calculator
* [ RTI Electronics] makers of Sure-Gard Inrush Current Limiters
* [ EPCOS] ' [,property=Data__en.pdf;/PDF_InrushCurrentLimiting_AN2.pdf Application notes] on Inrush Current Limiters

See also

* Soft start

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