The fundamental structure of a traditional fuse is simple: there is a conductive element that is designed to melt in response to excessive current flow, and a housing encloses the element and provides a way to incorporate the fuse into a circuit. But there certainly are variations on the fuse theme, and these variations are relevant to designers because they influence board design and the fuse’s operational characteristics.
The “Harder to Replace” Category
Physically replacing a component is rarely convenient, and sometimes it is highly inconvenient or even impossible. Thus, fuses are not as popular as they were back when it was more difficult to implement an alternative means of overcurrent protection. And actually, there are some types of fuses that are even more inconvenient than others, namely, those that are soldered directly to a circuit board.
The harder-to-replace devices include surface-mount fuses and leaded fuses (as in they have wire leads, not leaded as in containing the element lead—I hope this doesn’t lead to any confusion). You don’t want to use these types of fuses in applications that are likely to experience overcurrent events. And if you ask me, you shouldn’t use leaded or SMT fuses except as an extra layer of protection in devices that will almost certainly never need them.
Leaded fuses include axial, radial, and through-hole styles. The following images provide examples of various types of harder-to-replace fuses.
Top left: Surface-mount and through-hole “FLAT PAK” fuses. Top right: Thin film chip fuses. Bottom left: Axial leaded fuses. Bottom right: Radial leaded fuses. All images taken from the Littelfuse website.
The “Easier to Replace” Category
This group includes any fuse that fits into a holder, such that replacement can be accomplished without a soldering iron. Theoretically all you should need is your fingers, though I have a vague memory of using pliers to remove a cartridge fuse from a pair of surprisingly tight clips.
The standard easier-to-replace fuse for typical electronic devices is the cartridge fuse. An example is shown below, along with a through-hole clip (when you’re laying out the PCB, make sure you double-check that the clip spacing is consistent with the length of your fuse).
Images courtesy of Digi-Key.
This is not the only type of easier-to-replace fuse, though. If you like to work on cars, you’re surely familiar with blade fuses:
Image courtesy of Digi-Key.
As far as I know, blade fuses are limited to automotive applications, but I see no reason why you couldn’t use them for any other type of project. Of course, you would need the right sort of holder, but insertion and extraction might be more convenient than with a cartridge fuse.
The Game Changer
In my opinion, replacing blown fuses is a drag. That’s why I consider resettable fuses (AKA PTC fuses) to be a major development in the world of circuit-protection components.
A purist might prefer to place resettable fuses in an entirely different category, since they lack a fundamental characteristic of traditional fuses, namely, a conductive element that melts when it is subjected to excessive current. However, PTC fuses serve an equivalent purpose and are implemented in a similar way so, in this article, they are treated as simply a different type of fuse.
The name “PTC fuse” comes from “positive temperature coefficient”. Resettable fuses are made from a material whose resistance increases as temperature increases. Actually, the resistance increases rapidly once the temperature reaches the trip point (as shown in the diagram below), and this is why PTC fuses provide functionality similar to that of traditional fuses: the resistance is low during normal operation, but if an overcurrent condition causes the temperature to exceed the trip point, the resistance becomes high enough to limit the current to levels that won’t cause damage.
Plot taken from this document published by Littelfuse.
The term “resettable fuse” is perhaps a bit generous; they don’t come with a reset button. Rather, you bring the device back to its low-resistance state by turning off the power and letting it cool down.
PTC or Traditional?
It goes without saying that PTC fuses are greatly preferred in systems that experience frequent overcurrent events (the meaning of “frequent” depends on your point of view—once a year could be considered frequent if replacing a fuse would be extremely inconvenient). In fact, you might wonder why anyone would choose a traditional fuse over a PTC fuse. Well, it turns out that PTC fuses come with some disadvantages:
- They have higher resistance during normal operation.
- A tripped PTC is not an open circuit; it’s simply a component with enough resistance to significantly limit current flow. This might be undesirable in some applications.
- A PTC might conceal the occurrence of overcurrent events. If you have to replace a fuse, you know what happened. If you’re using a PTC and something in the circuit changes and eliminates the overcurrent condition, the PTC could naturally cool down and return to its low-resistance state. Or someone in the lab might see that the system isn’t working normally and cycle power without investigating the actual cause of the failure.
- PTCs are more sensitive to changes in ambient temperature. This is conveyed by the following plot; the curve labeled “C” is for a PTC, and the curves labeled “A” and “B” are for traditional fuses.
Plot taken from this document published by Littelfuse.
Another piece of information conveyed by this plot is that the form factor of a traditional fuse can influence its susceptibility to ambient temperature variations. For example, thin film fuses (corresponding to curve A) are significantly more sensitive to ambient temperature than leaded and cartridge fuses (corresponding to curve B).
In this article, we reviewed types of traditional fuses and we also discussed the differences between traditional fuses and resettable fuses. Feel free to leave a comment if you have any thoughts on which fuse type is most appropriate in a certain design situation.
Featured image used courtesy of Pfnicholls [CC BY-SA 4.0].