A fuse is a simple and highly effective way to protect a device from dangerous levels of current:
- Current flowing through a conductor’s nonzero resistance leads to power dissipation.
- Power is dissipated in the form of heat.
- Heat raises the temperature of the conductor.
- If the combination of current amplitude and duration is sufficient to raise the temperature above the fuse’s melting point, the fuse becomes an open circuit and current flow ceases.
Though the fundamental operation of a fuse is not complicated, there are subtle points to keep in mind. The rest of this article will help you to understand some important details related to the behavior and use of fuses.
Heat, Not Current
A fuse is not tripped directly by current; rather, the current creates heat, and heat trips the fuse. This is actually a rather important distinction because it means that fuse operation is influenced by ambient temperature and by the temporal characteristics of the current.
The specified current rating of a fuse is relevant only to a specific ambient temperature (usually, or maybe always, 25°C), and consequently you need to adjust your fuse selection if you’re designing a device that will operate outdoors in, say, Antarctica or Death Valley. The following plot shows how ambient temperature affects the actual current rating—relative to the nominal 25°C current rating—of three types of fuses.
Plot taken from this document published by Littelfuse.
Regarding the temporal characteristics of the current passing through the fuse, we all know that the effect of heat accumulates over time (momentarily touching a hot skillet is nothing compared to picking it up and realizing that it’s hot when you’re halfway between the stove and the dining table). Consequently, the current rating of a fuse is a simplification of its real behavior. We can’t expect a fuse to respond to high-amplitude transients because the short duration of the higher power dissipation doesn’t increase the temperature enough to cause tripping.
The following plot shows the time–current characteristics for a group of surface-mount fuses made by Panasonic. The rated current is on top, and the curve represents the amount of time required to trip the fuse in relation to the amount of current flowing through the fuse.
Plot taken from this datasheet.
As you can see, transient amplitudes must be much higher than the rated current. For example, you need 3 amps to trip a 0.5-amp fuse when the duration of the overcurrent condition is only 1 ms.
Connect Them In Series!
I’m not going to dwell on this point because it’s so straightforward, but it’s worth mentioning just in case you’re up late designing a schematic and in your exhausted state you don’t notice that you placed the fuse in such a way that it is, for example, in series with only one of two voltage regulators. A fuse cannot protect anything that is connected in parallel with it.
Rated Current vs. Operating Current
It would be perfectly reasonable to assume that a fuse rated for 6 amps could be used in a circuit that might need 5 amps of steady-state current. It turns out, though, that this is not good design practice. The current rating of a fuse is not a high-precision specification, and furthermore (as discussed above) the actual tripping current is influenced by ambient temperature. Consequently, to avoid “nuisance tripping,” you should have a fairly generous gap between your expected steady-state current and your fuse’s rated current. This document from Littelfuse suggests a “rerating” of 25% (for operation at room temperature); thus, a fuse with a rating of 10 amps would be used only if the circuit’s steady-state current will stay below 7.5 amps.
You Have to Be Patient
Let’s say your circuit includes a delicate component that will certainly be damaged if it is subjected to currents higher than 1 amp. The circuit should never draw more than 500 mA under normal conditions, so you include a fuse with a rating of 900 mA. This is high enough to prevent nuisance tripping and low enough to ensure that the delicate component never sees 1 amp. Right?
Well, no. Consider the following spec for the Panasonic fuses mentioned earlier in the article:
Image taken from this datasheet.
We’ve already discussed the fact that heat takes time to accumulate, and in this case it takes a long time: you’ll have to wait at least four hours for the fuse to trip when the current is equal to the rating, and even at twice the rated current the delay is at least five seconds. The bottom line is that the delicate component might be toast long before the fuse trips. You’ll have to rethink your fuse selection or—and this is probably a more practical solution in a situation such as the one described above—implement a different method of dealing with overcurrent conditions.
Don’t Forget About Voltage
Fuses are designed to have very low resistance so that they don’t unduly interfere with the circuits that they are protecting. This low resistance means that the voltage drop across the fuse will be very small. Why, then, do fuses have a voltage rating?
It’s true that fuses see small voltage during normal operation, but the voltage rating is not relevant to normal operation. Rather, the voltage rating tells you what the fuse can endure after it has tripped. A blown fuse is an open circuit, and if the voltage across this open circuit is enough to cause arcing, the fuse can’t be relied upon.
It’s a good idea to keep an eye on voltage ratings if you’re using tiny surface-mount fuses, such as the one shown below (note how thin the actual fusing element is). The rating for an 0603 fuse, for example, could be 32 V or even 24 V.
Diagram taken from this datasheet.
We’ve covered some interesting details about how fuses work and how to effectively incorporate them into our designs. In a future article we’ll explore the different types of fuses.