Just because an electronic device is simple, you shouldn’t relegate it to an afterthought in your embedded system design. Such is the case with fuses. Robert explores the fundamentals of this seemingly simple device. In this article, he dives into the history, key specifications and technology of fuses. He also steps you through an experiment to analyze the performance of fuses and shares his results.
By Robert Lacoste
Welcome back to the “Darker Side.” As electronic system designers, we’ve become used to dealing with some fantastic and ultra-complex pieces of silicon in our projects—microcontrollers running at hundreds of megahertz, multi-core processors with billions of transistors, wireless transceivers with Gbps of throughput, miniature power converters with close to 100% efficiency and so on. Ok, of course some small discrete parts are still required around those key building blocks, but we’re inclined to disdain such components in the design phase. That’s because they represent a very small portion of the overall bill of materials and have low perceived value.
All that said, if you are a regular reader of this column, you already know that’s a bad choice. Some electronic components seem very simple—passives in particular. But such devices may be the source of incredible trouble if you don’t understand the intimate details of their behavior. If you have any doubt, go and re-read my articles on capacitors—for example, Circuit Cellar 283 “Dielectric Absorption;” Circuit Cellar 317 “Decoupling Capacitors and RLC Networks;” Circuit Cellar 321 “All Ceramic Capacitors Aren’t Equal.”
This month, I will talk about another very simple part that isn’t as simple as it seems: The fuse.
WHAT’S A FUSE?
Of course, you’ve all seen a fuse before. Fuses are as old as electricity. According to Wikipedia, their first documented use was in 1864 for telegraph installations . The first patent on a fuse was registered by Thomas Edison (him again?) in 1890. Today, fuses are everywhere, and range from ultra-miniature, surface-mounted devices to massive units used in nuclear-powered generators. Let’s restrict the discussion to small fuses common in electronic devices, such as the ubiquitous 20 mm x 5 mm fuse cartridge, illustrated in Figure 1. The picture speaks for itself—a fuse is nothing more than a wire. It is designed to be a protection device, and open the circuit in case of overcurrent. The wire is designed to melt above a given current threshold and to open the circuit.
Let’s spend a few minutes on these words: “protection device.” What does this mean? What is protected by the fuse? The answer to this question is not as obvious as it seems, because a fuse serves two purposes. First, it helps to protect the components of the device itself—meaning the device after the fuse— from extensive damage in case of a fault. For example, a fuse at the input of a power supply could save sensitive parts from destruction if the power supply malfunctions. Second, a fuse isolates the device from the outer world when the device is faulty, and this helps to prevent greater damage to other equipment.
“Protection device” also means that a fuse should not be, by itself, a potential source of hazard. When the wire in a fuse is melting, it will be hot and liquid, and could start a fire without adequate precautions. That’s why a fuse wire is always hermetically sealed, like the glass tube in Figure 1. That’s a requirement. Fuses are regulated by standards, mainly IEC 60269  (for residential or large fuses) and IEC 60127 (for miniature fuses like my 20 mm × 5 mm example). Ok, Americans prefer UL248, which is a different standard—but the spirit is the same. In any case, these standards state that a fuse should not allow any external sign when a fault occurs. In other words, that means that everything should be contained within the fuse body. No smoke or other material is expelled. This is true as long as the fuse is used within its specifications. More on that in a minute.
The term “overcurrent” also needs some explanation. What is an overcurrent? Is it a current just above the nominal current? For how long? Or a short circuit with thousands of amperes? Let’s dig into more details …
At this point, I encourage you to look for the datasheet of any standard fuse, and to read it carefully. Of course, you will find that a fuse is first specified by its package type and rated current. The rated current, written on the fuse, is simply the maximum current that it can continuously conduct without any problem.
The second key characteristic of a fuse is its speed. How fast will it blow in case of trouble? As you might expect, this depends on several parameters, and the first is the current. The greater the current passing through the fuse, the faster the wire will melt and cut the link. What are the tolerated limits? For miniature fuses, two speed grades are available and specified by EIC60127-2: Quick acting (“F” type, for “fast”) and time-lag (“T” type). Typical values, their respective minimum and maximum breaking times, depending on the effective current are given in Table 1. A caution here: Standards are evolving, so always consult the latest official version of the standards for any precise information. Now, look again at Table 1. You will see, for example, that a quick-acting miniature fuse, when a current 275% higher than its rating is applied, must cut the wire in less than 2 seconds, but not less than 10 ms. These durations become respectively 10 seconds and 600 ms for a time-lag version. …
Read the full article in the August 349 issue of Circuit Cellar
(Full article word count: 2723 words; Figure count: 8 Figures.)
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