1. Field of the Invention
The present application relates to a thermistor, and more particularly to an over-current protection device.
2. Description of Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Because the resistance of conductive composite materials having a positive temperature coefficient (PTC) characteristic is very sensitive to temperature variation, it can be used as the material for current sensing devices, and has been widely applied to over-current protection devices or circuit devices. The resistance of the PTC conductive composite material remains extremely low at normal temperature, so that the circuit or cell can operate normally. However, when an over-current or an over-temperature event occurs in the circuit or cell, the resistance instantaneously increases to a high resistance state (e.g., at least 102Ω), so as to suppress over-current and protect the cell or the circuit device.
PTC conductive composite material may include one or more crystalline polymers and conductive filler. The conductive filler is evenly distributed in the crystalline polymer. The polymer may be usually polyolefin such as polyethylene, while the conductive filler may be carbon black, conductive ceramic and metallic powder.
The conductive ceramic filler stacks to form conductive paths. Whenever over-current or over-temperature occurs, the stack changes and the number of the conductive paths decreases because the crystalline polymer of the composite material will be heated and then cooled to be re-crystallized. As a result, the resistance is difficult to return to its initial resistance value, i.e., the trip jump is high, when the conductive composite material undergoes repetitive over-current or over-temperature events.