Led Design Guide Page 29 LED Design Guide

2015 Littelfuse LED Lighting Design Guide www.littelfuse.com LED Lighting Design Guide Potential Problems with Alternative LED Lamp Overcurrent Protection Technologies Fuses are the "gold standard" for overcurrent protection, particularly in LED bulb and luminaire assemblies. In some applications, circuit designers have for various reasons turned to fusible resistors or fusible circuit board traces for overcurrent protection. These solutions may end up posing unsafe failure modes such as fire, as explained below. Fusible Resistors. These resistors are manufactured using high temperature alloys that have low thermal coefficients of resistance. This results in stable resistance across a wide temperature range, which is an important characteristic in resistors. In contrast, fuses that offer good overcurrent protection utilize a low temperature alloy with a higher thermal coefficient of resistance. This characteristic of fuses results in a sharp increase in resistance as the fuse element experiences an overcurrent situation, which is highly desirable for quickly bringing the fuse element to its melting temperature. Fusible resistors are effective at limiting current but waste energy and do not perform well on low-level overloads. In addition, because their resistance wires melt at 1300C to 1400C, this makes them a potential fire hazard ( Figure 17 ). Figure 17. Using fusible resistors for overload protection can pose a fire hazard. Potential fire hazard resulting from high temperature alloys that do not fuse well Material Typical Use Melting Temperature (C) Copper Fuse Element 1083 Silver 962 Nichrome Resistor Element 1400 Potential Problems with Alternative LED Lamp Overcurrent Protection Technologies 29

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