![]() ![]() You have to consider the condition when you connect power right at the maximum voltage. What can be tricky about loads fed by ac wall power is that when you switch them on, the inrush might be less severe if you switched them when the input voltage just happens to be at zero volts. Incandescent bulbs, capacitive loads, and both linear and switching power supplies have a large inrush of current when you turn them on. Yet a fast-blow may be subject to nuisance failure, due to momentary overload. A fast-blow fuse will open quickly, before wires or traces or devices get too hot. Once you have established the package of the fuse, perhaps in conjunction with that effort you should decide on the speed of the fuse (Fig. A fuse can protect people from a shorted voltage to the case, as well as keep a product from catching fire. Underwriters Laboratories was started to help insurance companies reduce their fire insurance risk. The low-impedance source will provide plenty of current that will melt copper and start a fire (Fig. This could be a product that plugs in the wall, or is powered by a battery, or one that runs from the alternator in your car. After you get things working, you can then design in a fuse.Īnything powered by a low-impedance source needs a fuse. Then your misbehaving circuit will just get hot, instead of blowing up. You want to set the current less than what would melt a bond wire inside the transistor or IC. Rather than counting on a fuse to protect your transistors, you can power the circuit you’re developing with a laboratory power supply and set the current limit to an ampere or so. Another common failure is from electrolytic and tantalum capacitors, which can fail in a short-circuit. This can happen when shorts develop from abraded wires or magnet wire getting shorted from vibration and constriction due to ac magnetic fields. It would be even less suited to protect a laser diode, since those get ruined with a few nanoseconds of overcurrent.įuses are ideal to protect wires and printed-circuit-board (PCB) traces from melting and fire. ![]() Some anonymous wag came up with the aphorism, “A twenty-dollar transistor will always blow to protect a ten-cent fuse.” A fuse isn’t intended to protect a transistor. Fuses will also prevent lethal voltages from shocking users. Electronic systems have the same fire concerns and they need fuses as well (Fig. Electrical systems need them for the same reason. With the advent of electrical distribution in the 1800s, fuses became an essential device in preventing fires. Sometimes we wish there was no such component needed for our circuits. We have all been annoyed or exasperated by a blown fuse. It’s easy to trivialize the need for a fuse and how to select one. ![]()
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