Fundamentals of Resettable Functionality in PPTC Devices
Protecting an electronic circuit from 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 Polymeric Positive Temperature Coefficient (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 overcurrent condition is eliminated. This resettable functionality can help manufacturers reduce warranty, 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 term sometimes used in describing PPTC devices, they are in fact, not fuses, but nonlinear thermistors that limit current. Because all PPTC devices go into a high-resistance state under a fault condition, normal operation can still result in hazardous voltage being present in parts of the circuit. It is important that the circuit designer recognize critical differences between a fuse and the PPTC device.
Fuses are current interruption devices, and once a fuse “activates”, the electrical circuit is broken, and there is no longer current present in the circuit. This electrical interruption (or open circuit) is a permanent condition. However, once a PPTC device trips into a high resistance state, a small amount of current continues to flow 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 and the power is cycled, this heat source is eliminated. The device can then return to a low-resistance status and the circuit is restored to a normal operating condition.
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PPTC 、 resettable PPTC (Polymeric Positive Temperature Coefficient) devices |
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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2016/11/23 |
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1.1 MB |
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