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Current-limit switches are virtually ubiquitous in system controls. They provide a safe means for regulating the current delivered to a load circuit. The switches allow the load current to increase to a programmed limit but no higher. Typically, the current limit is a function of the voltage across an external resistor, produced by the current from a fixed source internal to the switch IC. This voltage serves as the reference for an internal current-limiting amplifier.
Source : http://www.edn.com/article/CA47125.html