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A positive temperature coefficient (PTC) resettable fuse protects electronic devices in the event that excessive current or heat is generated. The devices generally increase the level of resistance at a specific threshold. The PTC fuse responds with a physical change that inhibits excess current from traveling through the circuit. The type of response depends on whether the device is a ceramic fuse or a polymeric fuse. PTC fuses can be used in automotive, consumer electronic, or industrial applications.
Ceramic fuses were the first type of PTC fuse developed and consist of polycrystalline substances containing barium titanate and other compounds. The crystal grains in the fuse conduct electricity and allow current to pass through, which produces heat. When the current or temperature rises beyond the established threshold, the compounds form a barrier that prevents the granular compounds from conducting electricity. This limits the amount of current passing through the circuit. If enough heat is generated, this type of fuse might become ferroelectric and reverse the flow of current in the opposite direction.
A polymeric PTC fuse contains a combination of polymer crystals and conductive carbon granules. Under normal conditions, the carbon granules permit current flow. When current levels rise above the established level, the polymer crystals undergo a physical change and become non-crystallized. As this occurs, the polymer expands, interfering with connections between the carbon granules, thus limiting the amount of available current. Polymeric fuses generally sense temperature changes and react more quickly than their ceramic counterparts.
Once the electrical fault passes, or the current is discontinued, the PTC fuse resets as the amorphous substances cool and revert to the original crystalline state. Though the devices might be incorporated into any number of applications, the PTC fuse must have sufficient expansion room to work effectively. The fuses are available in a variety of styles and sizes. Each fuse has a hold or threshold rating measured in amperes. Polymeric fuses also have a maximum voltage rating.
In the United States, to receive the Underwriters Laboratories seal of approval, PTC fuses must pass certain tests. The fuse must remain functional after 6,000 trips or after 1,000 hours of enduring current that exceeds the threshold. Fuses designed for telecommunication equipment may have different standards. These devices are generally not meant for use in electronic applications in which the recommended amount of current is continuously exceeded.