The resettable fuse is an over-current electronic protection component. When it is misused or under some specific conditions, it may cause spontaneous combustion. How to avoid it, and how to cause it? Let's analyze the resettable fuse structure and the cause of the spontaneous combustion of the resettable fuse.
The melting part of the resettable fuse is mainly made of polyethylene mixed with carbon black, in which carbon black is conductive, and polyethylene itself is not conducive. When the mixed carbon black reaches a certain proportion, carbon black particles conduct electricity through mutual polyethylene contact. We temporarily call this mixture fuse melt. Polyethylene (PE) is a kind of thermoplastic resin that is prepared by polymerization of ethylene. It has a remarkable characteristic that the thermal expansion coefficient is relatively large, and the volume will expand after heating.
When the current flowing through the melt is too large, polyethylene will expand due to heating. At this time, the distance between carbon black particles in polyethylene is opened, the resistance increases and the calorific value rises sharply. When all carbon black particles can not form a path, the conductive path is cut off, and the current cannot pass through, which acts as the circuit breaker or the fuse. Due to no electric current flowing through the melt, the factors leading to the mixture heating disappeared, the polyethylene began to cool down, the volume began to shrink, the resistance gradually decreased, and the power was gradually restored. This is how the self resettable fuse works.
Please refer to the working principle of SMD resettable fuse:
From the above principle, we can see that the polyethylene will have a thermal expansion phenomenon when the fuse passes through the current. Whether the circuit is disconnected depends on whether the thermal expansion can pull the carbon black particles' distance. If polyethylene is heated for a long time, the molecular structure or properties will change, affecting the formation of carbon black particle distance. In fact, the most common problem is that the uneven distribution of carbon black particles is easy to cause local high temperature, and the increase of fluidity in the high-temperature region will lead to the accumulation of carbon black, which will further promote the non-uniformity of carbon black. This vicious cycle to a certain time, the fuse will not open, then the result is burned.
Generally, people who design circuits are clear about the working principle of resettable fuse to know the hidden danger of fuse and take countermeasures. Of course, some circuit designs are careless and easy to forget to adopt corresponding countermeasures. What are the countermeasures? It is more appropriate to connect a conventional fuse with the resettable fuse in series. When the resettable fuse fails, the conventional fuse will blow to avoid more damage caused by the resettable fuse's fire.
