A resettable fuse is a PPTC (Polymeric Positive Temperature Coefficient) device that is a piece of passive electronic equipment utilized in defending against overcurrent faults in electrical circuits. It is also popularly known as polyswitch or a multifuse or a polyfuse. In certain situations, it is noted that this device is much similar to PTC thermistors but instead of charge carrier effects, they are operated on mechanical changes in semiconductors. A PPTC device consists of a voltage rating and a current rating. This electronic device was invented by Gerald Pearson in 1939 at Bell Labs. It was described in US patent #2,258,958.
A PPTC device is manufactured using a crystalline organic polymer matrix which is non-conductive in nature. To make it conductive it is added up with black carbon particle. Several conductive chains of carbons are formed between the crystals when it is cooled. Now as it is good at conducting electricity, the current will pass easily. Passage of a large amount of current through the device will make it heated up. The polymer will expand due to the heating up of the device. As a result, the polymer will change to the amorphous state from its crystalline form. Now as the heat is causing the expansion in the polymer, the carbon particles will start to get separated and the conductive pathways will start to break. As a consequence of this, the device will heat at a much faster rate, expand more and the resistance will be increased. Due to this increase in the resistance the circuital current will decrease. The device still consists of a small amount of current (leakage current) flowing through it which is enough to keep the temperature at a level such that the equipment is at a high resistance state. The range of leakage current is from a few hundred mA at lower voltages to less than a hundred mA at rated voltage. It can be said that latching functionality is one of the characteristic features of the device.
As soon as the power is removed from the device, the heating caused by the leakage current will cease and the resettable fuse (PPTC) will start to cool down. With the cooling of the device, the original crystalline structure is regained and it gets back to its low resistance state that can hold current. The cooling process of the device generally takes a couple of seconds.
Resettable fuses are most commonly used in computer power supplies (generally of PC 97 standard), nuclear applications as well as aerospace applications. This device is also used in protecting audio loudspeakers (tweeters). A circuit designed with a PPTC device connected in parallel to the light bulb or a resistor helps in limiting the current through the tweeter to a safer limit. Thus the tweeter will not get damaged if the amplifier is conveying more power that is endurable by it.
This was all we had about Resettable fuses. Brands that make them are TE Connectivity, Littelfuse and Bourns. Make sure to follow them by the links as provided. Thank you for reading up the blog.
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