It should be no surprise that circuit-protection devices for miniature and handheld products such as cell phones, laptop and palmtop computers, and cameras are using more self-resettable, polymeric (PTTC) devices. Traditional fuses that must be replaced when occasional faults open-circuit them are not convenient to access nor do they fit well in such small consumer products.
Unlike most one-shot fuses, polymer circuit protectors such as PolySwitch devices lack parts that can fail after some time. In addition, PolySwitch devices are smaller than competing technologies and are made in surface-mount device (SMD) packages that sit on pads as little as 1.2 by 0.6 mils. Under normal operating current, called hold current, the PPTC material maintains numerous low resistance paths from end to end, typically around 80 m as low as a fuse or ordinary conductor. However, during a fault current, the material heats and moves its polymer chains and carbon-black structure around to reduce the number of conductive paths. The increasing temperature raises the device's resistance and further reduces the current flow. As the current climbs to twice the hold-current value and the temperature reaches a threshold level, the PPTC device rapidly switches to a much higher resistance, blocking further current. The resistance becomes so large that effectively, it looks like an open circuit to normal current flow. The process takes less than a fraction of a second; the precise interval depends on device size. When the fault current diminishes, the PPTC cools and the resistance returns to its normal value.
Information for this article was contributed by Fred Rebarber, Raychem Circuit Protection, 308 Constitution Dr., Menlo Park, CA 94025, (650) 361-5114, Fax: (650) 361-7667, www.circuitprotection.com
Max time to trip
|Note: preliminary data; refer to Web site, www.circuitprotection.com|