dear sir/madam,
my self karishma i,m kindly requesting you to give this papers.I have read some points of this paper & a very interesting topic.[/size][/font]
A Resettable Fuse is a Positive Temperature Coefficient Polymer (PPTC) device that is a passive electronic component used to protect against overcurrent faults in electronic circuits. The device is also known as a polyfuse or polyswitch. They are similar in function to PTC thermistors in certain situations, but operate in mechanical changes rather than load bearer effects in semiconductors. These devices were first discovered and described by Gerald Pearson at Bell Labs in 1939 and are described in U.S. Patent No. 2,258,958.
A polymeric PTC device is comprised of a nonconducting crystalline organic polymer matrix which is charged with carbon black particles to make it conductive. While cool, the polymer is in a crystalline state, with the carbon forced into the regions between crystals, forming many conducting chains. Since it is conductive (the "initial resistance"), a current will flow. If too much current is passed through the device, the device will begin to warm up. As the device warms up, the polymer will expand, changing from a crystalline state to an amorphous state. The expansion separates the carbon particles and breaks the conducting pathways, causing the device to heat faster and expand more, further increasing the resistance. This increase in resistance substantially reduces the current in the circuit. A small current still flows through the device and is sufficient to maintain the temperature at a level that will keep it in the high resistance state. It can be said that the device has interlocking functionality. Retention current is the maximum current at which the device is not triggered. The trip current is the current at which the device is tripped.
When the current is removed, the heating due to the holding current will stop and the PPTC device will cool. As the device cools, it recovers its original crystalline structure and returns to a low resistance state where it can contain the current as specified for the device. This cooling typically takes a few seconds, although a triggered device will retain a slightly higher resistance for hours, slowly approaching the initial resistance value. Resetting will often not take place even if the fault alone has been eliminated with the power still flowing since the operating current may be above the PPTC retention current. The device can not return to its original resistance value; it is likely to stabilize at a significantly higher resistance (up to 4 times the initial value). It could take hours, days, weeks or even years for the device to return to a resistance value similar to its original value, if it does.