Fuseology Selection Guide Page 8 Fuseology Selection Guide

Fuseology Selection Guide www.littelfuse.com Overcurrent circuit protection can be accomplished with the use of either a traditional fuse or PTC (positive temperature coefficient) device. PTCs are typically used in a wide variety of telecom, computer, consumer electronics, battery and medical electronics product applications where overcurrent events are common and automatic resettability desired. Littelfuse offers PTCs with the following general forms and features, and come in a variety of sizes and capacities: Surface Mount Devices: Radial Leaded Series: Battery Strap Devices: If your application requirements fall outside of our product range, in certain instances we can offer customized solutions. Please contact Littelfuse for more information. Traditional Fuses Vs. PTCs Fuses and PTCs are both overcurrent protection devices, though each offer their own unique operating characteristics and benefits. Understanding the differences between the two technologies should make the choice in selection easier, depending on the application. The most obvious difference is that PTCs are automatically resettable whereas traditional Fuses need to be replaced after they they are tripped. Whereas a fuse will completely stop the flow of current (which may be desired in critical applications) after most similar overcurrent event, PTCs continue to enable the equiment to function, except in extreme cases. Because they reset automatically, many circuit designers choose PTCs in instances where overcurrent events are expected to occur often, and where maintaining low warranty and service costs, constant system uptime, and/ or user transparency are at a premium. They are also often chosen in circuits that are difficult to access or in remote locations, were fuse replacement would be difficult. There are several other operating characteristics to be considered that distinguish PTCs and fuses, and it is also best to test and verify device performance before use within the end application. Littelfuse PTC Characteristics Both Polymeric (Positive Temperature Coefficient) PTC and traditional Fuse devices react to heat generated by the excessive current flow in a circuit. A fuse melts open, interrupting the current flow whereas a PTC limits current flow as it rises in temperature, changing from low to high resistance state. In both cases this condition is called "tripping." The graph at right shows the typical response of a PTC to temperature. Littelfuse Polymer PTCs are made chiefly of high density polyethylene mixed with graphite. During an overcurrent event, a Polymer PTC will heat and expand, which in turn causes the conducting particles to break contact and stop the current. The general procedure for resetting the device after an overload has occurred is to remove power and allow the device to cool down. Log resistance (ohms) Temperature (C) Trip Point PTC Characteristics and Terms To view a short online tutorial on using the Littelfuse iDesign tool, visit: http://vimeo.com/90118164 Littelfuse iDesign TM Online Fuse Design and Selection Tool 8

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