A fuse must be one of the simplest of all electronic components, right? It is after all just a piece of wire that melts when too much current flows through it opening the circuit. It’s protective function comes from it being the “weakest link” in the circuit that fails before anything else is damaged.
Fuses have been around since the earliest days of electronics and electrical distribution systems. Louis Breguet, a pioneer of telegraph systems in the mid 1800s, recommended the use of thinner than normal conductors to protect telegraph stations from damage during lightning strikes. Edison patented a fuse in 1890 for use with his early power distribution systems.
Though they may be simple in construction and relatively easy to understand, selecting the right fuse for your application is not necessarily a straightforward task. For the purposes of this discussion I am focussing on the type of fuses you might use in general electronics applications – fuses for electrical power systems are a wholly different ballgame (and one I’m not familiar with).
When choosing a fuse, we need to look at a few parameters. The first (and most obvious) is the operating current rating. This is the current at which a fuse will conduct continuously without tripping. Most manufacturers recommend selecting a fuse with an operating current such that the normal maximum current is 75% of the rating to avoid nuisance tripping.
You might be tempted to think that running the fuse at 100% of its rated current is OK, but a close reading of the data sheet should convince you otherwise. The data sheet for the Littlefuse 443LC range I intend to use states that they are rated to conduct 100% of its operating current rating for “4 hours minimum”. There is no guarantee they will do so indefinitely! If we assume for the sake of example that we have a maximum current of 1.5A we would therefore select a 2A fuse.
You will need to further re-rate the fuse if the ambient temperature in the vicinity of the fuse is higher or lower than 25˚C. Figure 1 shows the re-rating curve for the 443LC series. You can see here that at 80˚C the fuse should be further derated by 95%. We would therefore need a minimum 2.1A fuse under these circumstances, although we have to opt for a 2.5A fuse as this is the next available value above 2A.
This is the re-rating curve for the Littlefuse 443LC series fuses extracted from the manufacturer’s data sheet. The 75% standard derating should be adjusted by this factor depending on the maximum ambient temperature in the vicinity of the fuse.
You also have to choose a fuse with sufficient voltage rating to safely interrupt the current under fault conditions. The fuse rated voltage should be higher than the maximum voltage that could appear across the fuse when it is opening. There may be different voltage ratings for AC or DC operation. The 443LC series is rated for 280VAC.
Fuses also have an interrupt current rating. This is the maximum fault current that the fuse can interrupt. If your power source is capable of delivering more current the fuse may be unable to cope with the fault energy and the body may rupture catastrophically (this is a polite way to say it may explode). Our example fuse is rated to interrupt currents up to 50A. Fuses are available with interrupt ratings in the thousands of amps if you need them.
Small fuses like the 443LC series also have a meaningful level of resistance and therefore a voltage drop. The 2.5A fuse for example has a cold resistance of 35mΩ and a voltage drop at rated current of 135mV, suggesting a hot resistance of around 54mΩ. You may need to take this into account, depending on your application.
All this is fine, but we don’t yet have a sense of the circumstances under which the fuse will blow. The key to this is the time-current curves (TCC) published by the fuse manufacturer. Figure 2 shows the curves for the 443LC fuses.
The time-current curve (TCC) graph for the 44LC series fuses allows the user to estimate the time it will take for the fuse to blow for a given overload current. As the curves approach vertical the uncertainty in rupture time increases.
The horizontal axis shows the fuse current, and the vertical axis shows the average time it will take for a fuse to blow. For the sake of our example, let’s assume that our source is capable of delivering 20A briefly under short circuit conditions. The curve for the 2.5A fuse intersects the 20A line at about 60ms. This suggests the fuse will blow in about 60ms under short circuit conditions.
On the other hand, if there was an overload of say 5A, the fuse would take somewhere around 20 seconds to open. You can see that the graph gets more vertical at currents closer to the fuse’s rated current making it difficult to determine just how long it will take to blow.
These are by nature conservative graphs – they are made at 25˚C and with the fuse “cold”. If the fuse has been operating at or near its rated current at the time of the fault, or the ambient temperature of the fuse is higher, (or both) the fuse will likely blow faster than the TCC suggests.
You will also see fuses rated with a “nominal melting I2t” rating. This is another way of expressing the energy necessary to rupture the fuse. In our example, the 2.5A fuse has a nominal I2t rating of 18.13 A2s. You can use this to calculate the rupture time for a given current level just like the TCC. In this case the rupture times for 20A and 5A are 45ms and 725ms respectively.
You can see that for overloads close to the fuse’s rated current the rupture time is fairly uncertain. Fuses are better at protecting circuits from large fault currents than from marginal overloads – you might be better with some kind of electronic protection in these cases if this is a critical requirement.
Fuses also behave differently with short pulses of current. There are some calculations you can do to determine how the fuse will behave under these conditions, but they are beyond the scope of this short article. Fuses are simple devices – but they do require a little bit of work to select the optimum one for your application.
Bibliography
“Fuse (Electrical).” In Wikipedia, September 22, 2024. https://en.wikipedia.org/w/index.php?title=Fuse_(electrical)&oldid=1247100788.
“Nano 2 Fuses – Littelfuse.” Accessed October 19, 2024. https://www.littelfuse.com/products/fuses/surface-mount-fuses/nano-2-fuses.aspx.opamp_comparator_tutorial_appli-e.pdf.
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Andrew Levido (andrew.levido@gmail.com) earned a bachelor’s degree in Electrical Engineering in Sydney, Australia, in 1986. He worked for several years in R&D for power electronics and telecommunication companies before moving into management roles. Andrew has maintained a hands-on interest in electronics, particularly embedded systems, power electronics, and control theory in his free time. Over the years he has written a number of articles for various electronics publications and occasionally provides consulting services as time allows.
