Positive temperature coefficient
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Positive Temperature Coefficient (PTC) refers to materials that experience an increase in electrical resistance when their temperature is raised. Materials which have useful engineering applications usually show a relatively rapid increase with temperature, i.e. a higher coefficient. The higher the coefficient, the greater an increase in electrical resistance for a given temperature increase.
This effect can be used to create resettable fuses, often as part of a semiconductor. As the temperature rises, nearing danger levels, the resistance increases, so less current flows, thereby protecting the vulnerable equipment that receives the current flow.
Commonly used PTC materials include:
- High density polyethylene filled with a carefully controlled amount of graphite, so that the volume increase at the glass transition temperature causes the conducting particles to break contact (see Polyswitch fuses)
- Titanate ceramic materials, which work by grain boundary effects
PTC materials are sometimes used to make heating elements, as such elements act as their own thermostats, switching off the current when reaching their maximum temperature.
