Fuses and Circuit Breakers are a necessary safety device for all circuits. They come in many shapes, sizes, and colors.
Poly Fuses
A Poly Fuse, properly known as a Polymeric Positive Temperature Coefficient Device, will break a circuit at a certain temperature threshold. Think of it like a resistor – as more power flows through, the hotter it gets. At some known point, the device will reach critical temperature. Once the device cools down, the circuit is stored.
These are built into some (not all) processors and/or embedded system boards, used to protect GPIO pins and/or VCC from accidental shorts.
Fusible Links
The term “Fusible Link” can be used ambiguously to mean many things. In power systems it typically is a segment of wire smaller than the rest of the circuit, meant to be sacrificial during failure and somewhat easy to access for replacement. These are typically found in environments where a “very slow blow” condition is warranted, where a traditional circuit breaker or fuse is not practical.
A general example is the alternator and starter of a vehicle. Today’s modern alternators can supply 200+ amps at 13.8 volts (2700+ watts), and a starter can likewise draw hundreds of amps at 10.5-12 volts (1600+ watts). Placing a traditional fuse in a circuit that high will severely choke the circuit down to poor performance, so instead, a wire of 10 or 12 AWG might be intentionally spliced in a circuit of 0, 2, or 4 AWG . During spike loads, nothing will happen, however if a dead short were to occur, that smaller section of wire will quickly overheat, melt, and break the circuit in a controlled manner.
Circuit Breakers
Circuit Breakers are similar to a PolyFuse – they interrupt a circuit when it is overloaded, however they do not automatically reset when the offending condition is resolved. Rather, the user must manually move the lever to “OFF”, and then back to “ON”. Likewise, they can also be manually turned off at any time, so they also can function as a main power switch.
Traditional Fuses
ATO and Mini ATO
ATO is the most common fuse style, followed by Mini ATO. These types of fuses have two legs sticking out in-line, which you insert into a socket.
ATO fuses are good for circuits up to 30 or so amps. After that point, the 1/4″ wide contacts become a weak point in the circuit.
Likewise, Mini ATO fuses are good for circuits up to 10-15 or so amps. After that point, the 1/8″ wide contacts are a weak point.
Small fuses in the ATO form can easily fail due to vibration – the filament is very small and barely attached to the legs. This is partially by Mini ATO came to existence, by shortening the path and thus being able to use a slightly larger filament.
Violet: 3 amps
Tan: 5 amps
Brown: 7.5 amps
Red: 10 amps
Blue: 15 amps
Yellow: 20 amps
Clear/White-ish: 25 amps
Green: 30 amps
Orange: 40 amps**
J-Case and M-Case
J- and M-Case fuses have become popular in automotive systems thanks to their significant size reduction. Unlike ATO’s male legs, the M/J-case fuse has the female socket, and the circuit/host has the 2 legs that are parallel to each other rather than inline.
J- and M-Case fuses have the same issues as ATO regarding the 1/4″ connections being a weak point over 30 amps and the small filament being prone to vibration failure in low rated fuses.
M and J cases look very similar, but are not interchangeable. J-Case is closer to square, while the M-Case is more rectangular and has the visual slots in the casing.
MAXI
MAXI fuses are a step up from ATO fuses, designed for circuits from 40 to 100 amps. These fuses are typically a “slow blow” design, with the fused element being fundamentally a continuation of the 5/16″ wide legs. MAXI fuse holders will frequently have set screws to lock the fuse in place, to prevent it from falling out due to vibration. Because of the high rating and this vibration imperviousness, MAXI fuses are frequently chosen for robotics/rover type projects.
ANL
ANL is one of the largest fuse classifications you will likely see, ranging from 35 to 750 amps – designed for VERY specific circuits.
Fuse Diagram/Datasheets
https://www.littelfuse.com/~/media/automotive/aftermarket/fuse_selection_chart.pdf