Polyfuse Seminar Report: Abstract
A fuse is a one-time overcurrent protection device employing fusible link that melts after the current exceeds a certain level for a certain length of time. Typically, a wire or chemical compound breaks the circuit when the current exceeds the rated value. Polyfuse is a new standard for circuit protection.It is resettable. Technically, Polyfuses are not fuses but polymeric positive temperature coefficient thermistors (PPTC). Re-settable fuses provide overcurrent protection and automatic restoration.
Polyfuse: Introduction
Polyfuses is a new standard for circuit protection. It is re-settable by itself. Many manufacturers also call it Polyswitch or Multifuse. Polyfuses are not fuses but Polymeric Positive temperature Coefficient Thermistors (PPTC). We can use several circuit protection schemes in power supplies to provide protection against fault condition and the resultant over current and over temperature damage. Current can be accomplished by using resistors, fuses, switches, circuit breakers or positive temperature coefficient devices. Resistors are rarely an acceptable solution because the high power resistors required are expensive.One shot fuses can be used but they might fatigue and they must be replaced after a fault event. Another good solution available is the resettable Ceramic Positive Temperature Coefficient (CPTC) device. This technology is not widely used because of its high resistance and power dissipation characteristics. These devices are also relatively large and vulnerable to cracking as result of shock and vibration. The preferred solution is the PPTC device, which has a very low resistance in normal operation and high resistance when exposed to a fault. Electrical shorts and electrically overloaded circuits can cause over current and over-temperature damage.Like traditional fuses, PPTC devices limit the flow of dangerously high current during the fault condition. Unlike traditional fuses, PPTC devices reset after the fault is cleared and the power to the circuit is removed. Because a PPTC device does not usually have to be replaced after it trips and because it is small enough to be mounted directly into a motor or on a circuit board, it can be located inside electronic modules, junction boxes and power distribution centres.
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