Why Does the Resistance of Patch Fuses Become Large and Constant?
The operating principle of a fuse is that an overcurrent causes the thermal balance on the melt to be disrupted. When the temperature of the melt rises to the melting point of the metal material, the middle part of the melt changes from solid to liquid. Due to the surface tension and gravity of the metal material suspended in the tube, the liquid part of the melt pulls away from both ends and sags downwards. The voltage caused arcing causes the melt temperature to continue to rise, further arcing, and further distance pulling apart, Until the circuit is completely cut off. For SMD fuses, their operating principle is the same. However, due to different structural states, the surrounding of the metal melt is tightly surrounded by the polymer or ceramic materials of its substrate.
Even the melted metal cannot contract towards both ends and can only rely on diffusion, infiltration, or absorption towards the surrounding materials. If the overcurrent disappears during this process (such as the instantaneous pulse phenomenon), The process of diffusion or absorption is still in progress, which will result in an increase in resistance and incomplete melting of the melt. Let's take a look at the consequences of this phenomenon again: because the overcurrent has disappeared at this time and has not caused any adverse effects on the circuit, although the fuse has not been completely blown at this time, the capacity of the melt has weakened, and when subjected to overcurrent again, it will be blown quickly, ensuring the protective effect on the circuit; If the second overcurrent is still an instantaneous pulse, it will cause the resistance to increase again without fully melting, and the capacity of the melt will also weaken again.
In short, the phenomenon of SMD fuse (patch fuses) with increased resistance and incomplete blown does not affect its protective function for the circuit. As long as the overcurrent lasts for a long time, it will be completely blown. On the contrary, if there is no change after experiencing overcurrent, there may be a problem with the protective function of the fuse. Compared with tubular fuses, the melt of a slow break fuse is composed of two or more metal materials, which also undergoes a process of diffusion and infiltration between different materials when subjected to overcurrent. Therefore, it has the ability to withstand pulses and also has the opportunity to increase resistance.
Fig.1
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