31-01-2009, 12:40 AM
PPTC circuit-protection devices are used to protect against harmful over current surges and over-temperature faults. Like traditional fuses, these limit the flow of dangerously high current during fault conditions. The PPTC device, however, automatically resets after the fault is cleared and power to the circuit is removed. Understanding the precise nature of these devices can help designers develop more reliable equipment and reduce warranty, service and repair costs. Improvements in size, cost and performance have resulted in wide spread acceptance of polymeric positive temperature coefficient (PPTC) devices, but several product names and commonly used terms like ?resettable fuse? have resulted in some confusion regarding the precise nature of these devices.Protecting an electronic circuit front damage due to excessive current or heat is the primary function of many circuit protection technologies. In the past, this protection took the form of a fuse or fusible link, but in many of today?s applications resettable devices such as PPTC devices, ceramic PTC devices, bimetal breakers and thermostats are the preferred solution.These devices do not require replacement after a fault event, and allow the circuit to return to the normal operating condition after the power has been removed and/or the over-current condition is eliminated. This resettable functionality can help manufacturers reduce warrants, service and repair costs, however, proper application requires an understanding of how the device resets and the circuit conditions that must be met before reset will occur.Although resettable fuse? is a common ten used in describing PPTC devices, they are in fact, non-linear thermistors and not fused A PTC device is a non-linear thermistor that limits current. Because under a fault condition all PTC devices go into a high resistance state, normal operation can still result in hazardous voltage being present in parts of the circuit. It is important that the circuit designers recognize critical differences between the two devices.Fused are current-interruption devices, and once a fuse ?blows? the electrical circuit is broken, and there is no longer current flowing through the fuse. This electrical interruption (or open circuit) is a permanent condition.However, once a PPTC device trips, there is a small amount of current flowing through the device PPTC devices require a low-joule heating leakage current or external heat source in order to maintain their tripped condition. Once the fault condition is removed, this heat source is eliminated. The device can then return to a low-resistance status and the circuit is restored.