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. It turns out, though, that this is not good design practice. 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. A fuse cannot protect anything that is connected in parallel with it.įuse Design Best Practices: Rated Current vs.
#Add a fuse 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. For example, you need 3 amps to trip a 0.5-amp fuse when the duration of the overcurrent condition is only 1 ms. 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.Īs you can see, transient amplitudes must be much higher than the rated current. The following plot shows the time-current characteristics for a group of surface-mount fuses made by Panasonic. 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. Consequently, the current rating of a fuse is a simplification of its real behavior. 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). Plot taken from this document published by Littelfuse. The following plot shows how ambient temperature affects the actual current rating-relative to the nominal 25☌ current rating-of three types of fuses.
The specified current rating of a fuse is relevant only to a specific ambient temperature (usually, or maybe always, 25☌), 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. 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 rest of this article will help you to understand some important details related to the behavior and use of fuses.Ī fuse is not tripped directly by current rather, the current creates heat, and heat trips the fuse. Though the fundamental operation of a fuse is not complicated, there are subtle points to keep in mind. 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.Heat raises the temperature of the conductor.Power is dissipated in the form of heat.Current flowing through a conductor’s nonzero resistance leads to power dissipation.Axial, Cartridge, Surface Mount, PTC.: The Many Types of FusesĪ fuse is a simple and highly effective way to protect a device from dangerous levels of current:.Thanks in advance to anyone that can answer my question.This article provides an overview of some subtle yet important aspects of fuse functionality and design. you would think one fuse would be for the existing circuit and one for the new piggy back circuit. My device (radar detector) would NOT power up unless both fuses where plugged into the add a fuse. I guess I thought the power potential at the fuses is whats controlled by the key ?Īnother problem Im curious about but not really an issue any more. The ground I made for the add a fuse is just a wire screwed straight into the chassis near the fuse box.
Im assuming many ground side contacts are made once the key is turned on including the rear outlet. BUT the power IS properly cut off to the rear outlet when the key is turned off. There IS constant power (when the key turned off) to my add a fuse outlet I made. QUESTION: where exactly can I tap the ground side of my add a fuse so the power will be cut off when the key is turned off?ĮXPLANATION : Im using the rear power outlet fuse location (under the steering wheel) for my add a fuse tap location. PROBLEM: I have constant power to my add a fuse and I want it to shut off with the key.
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