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The levels of miniaturization typically demanded by these end user groups place a premium on every square millimeter of board space. Designers need to minimize the real estate devoted to secondary over-current protection. Conventional wire fuses in SMD packages display a number of strong features: they are robust, have high breaking capacity, are available in ratings up to 10 A, and the technology also supports fast acting or time delay type fuse operation. They can address a wide range of applications, including over-current protection of power lines. On the other hand, package sizes are not likely to reduce below the industrystandard 2410 SMT outline. In consumer applications where low rated currents and breaking capacities are required, Chip Fuses are emerging to satisfy designers’ demands for the next level of component miniaturization.Instead, designers can use surface mount devices (SMDs) without a performance compromise. SMD- Mount Fuses employ diverse technologies to provide thermal-based fusing along with the full range of necessary fuse characteristics, such as fast acting and slow blow.
Though the fundamental operation of an Axial Lead & Cartridge Fuse is not complicated, there are subtle points to keep in mind. The rest of this article will help you to understand some important details related to the behavior and use of fuses.
How a Fuse Is Tripped: Heat, Not CurrentRegarding the temporal characteristics of the current passing through the Power Fuse, we all know that the effect of heat accumulates over time (momentarily touching a hot skillet is nothing compared to picking it up and realizing that it’s hot when you’re halfway between the stove and the dining table). Consequently, the current rating of a fuse is a simplification of its real behavior. We can’t expect a fuse to respond to high-amplitude transients because the short duration of the higher power dissipation doesn’t increase the temperature enough to cause tripping.It would be perfectly reasonable to assume that a fuse rated for 6 amps could be used in a circuit that might need 5 amps of steady-state current. It turns out, though, that this is not good design practice.
The current rating of a Resettable Fuse is not a high-precision specification, and furthermore (as discussed above) the actual tripping current is influenced by ambient temperature. Consequently, to avoid “nuisance tripping,” you should have a fairly generous gap between your expected steady-state current and your fuse’s rated current.
This document from Littelfuse suggests a “rerating” of 25% (for operation at room temperature); thus, a fuse with a rating of 10 amps would be used only if the circuit’s steady-state current will stay below 7.5 amps.