10A and 2A fuses in parallel=12A?

[iknowjohnny]

As the title asks, if i soldered 10 and 2 amp fuses together would it be the same as a 12A? I understand the logistics and all, just need something immediately.

[Zippy]

No guarantee that their resistivity and time constants are the same.

Short answer: No.

[iknowjohnny]

ok, thank you.

[Firestorm]

The current splits but the 2A fuse is still going to let go above 2A.

[Cantplay]

Fuze is a funny thing.

Put a 10A and 2A in series and use it to short your power supply. Both will blow.

That means that for a brief moment the 2A passed more than 10A current.

How many of us have measured volts with meter on ohms? When I opened it up and replaced the fuze it still didn't work.

My theory is that a fuze can pass nearly infinite current for very brief moment.

John

[Firestorm]

A short is always infinite current, theoretically. But there are no zero resistances in practice. So something burns.

[teemuk]

That means that for a brief moment the 2A passed more than 10A current.

Well, the current rating of fuses has always been more of a "nominal value".

It's not like the fuse will trip exactly at some specific current. How fast the fuse wire melts depends on both amplitude of current and TIME that current flows through the wire. The ratio of these two variables is not linear by far. For example, some fuses may pass 25% higher than rated current for an entire hour before failing but will fail in seconds (literally) with 100% higher than the rated current. Needless to mention, there are several different types of fuses with different characteristics concerning the time vs. current characteristics.

As is, the fuse rating is usually a compromise between fast + proper protection and tolerance to momentary current surges. Amps never draw steady current so proper fuse selection involves plenty of waveform arithmetics that acknowledges that. You simply can't pick a value like x amperages and trust that the fuse will always instantly trip at x, or that the circuit would not draw higher current than x under normal operating conditions. It simply doesn't work that way. A fuse that tolerates phenomenons like inrush surges and overall transient-nature of currents flowing in power supply isn't a poster example of good and quick protection. The fuses are usually to prevent catastrophic failures. More delicate and circuit-specific protections are implemented with more effective means.

[Structo]

I was thinking about Ohms and Kirchoffs.
If the fuses are in series, don't they both see the same current?

If they are slo blo, then they will take the current up to their rating for a short time before they blow, right?

I realize they are not resistors but doesn't the same series/ parallel rules apply?

[Tillydog]

I was thinking about Ohms and Kirchoffs.
If the fuses are in series, don't they both see the same current?

Yup

If they are slo blo, then they will take the current up to their rating for a short time before they blow, right?

Fuse ratings are just nominal and are a short-hand way to describe a more complicated behaviour. Slo-blo / standard / fast are just sub-sets of this. Have a look at the graph below. The nominal fuse ratings (amps) are along the top. Each curve shows how much current each fuse rating can take, and for how long.

[img:600:771]http://www.ce-mag.com/archive/05/01/ce0501p140b.jpg[/img]

(Graph and a reasonable explanation here: http://www.ce-mag.com/archive/05/01/018.html )

For example if you follow the curve for a 1 amp fuse, it can take 100 amps for about 1 mS (I don't know what type/speed rating of fuse this relates to, but the general behavior is typical).

Different styles of fuse and slo-blo, standard / fast fuses will have differently shaped curves, but along similar lines, so there's nothing to say that a fuse can't take quite a bit more than its rating for a few seconds at a time (whether slow or fast).

I realize they are not resistors but doesn't the same series/ parallel rules apply?

No, not when considering protection ratings. In series, the same current passes through both, so the behaviour will be dominated by the lowest rated or fastest fuse (which may not be the same one!). In parallel, there's nothing to say that the current will be apportioned to each fuse in proportion to its rating, so the answer could be anywhere between the rating of the smallest fuse and the combined rating of both.

You could calculate how the current is shared by looking at the *resistance* of each fuse, but this would be a 'cold' value and increases a lot as the current approaches the rating of the fuse, so it would be pretty meaningless in terms of trying to predict which would blow when.

As teemuk said, the rigorous design of fuse protection is very technical. Nominal ratings and rules of thumb are there to get us by for non-critical stuff like consumer electronics, amps, etc.

[briane]

while very interesting - 12A would be a pretty extreme rating for any amp.

so what are we running? 12 kt88's? even that I might fuse more in the 6-8 A range - just off the top of my head.

[romberg]

When a fuse "blows" it is not instantaneous. And depending on the voltage involved a spark may form across the gap that continues to conduct current even after the fuse-able material is mechanically disconnected. So, a 2A fuse in series with a 10A fuse may still conduct more than 10A for a period of time long enough to blow the 10A fuse. Crank up the voltage high enough and they may both conduct forever. It... is... ALIVE!

Mike

[Zippy]

When a fuse "blows" it is not instantaneous. And depending on the voltage involved a spark may form across the gap that continues to conduct current even after the fuse-able material is mechanically disconnected.

Yepper. I recall that most, if not all, of the fuses that I saw in the USN were a quenching style - filled with sand. An arc would melt the sand and form a barrier of glass between the ends of the popped element.

[vibratoking]

Sustaining an arc through a blown fuse with applied AC voltage is extremely unlikely since the voltage periodically makes zero crossings.

[Zippy]

Sustaining an arc through a blown fuse with applied AC voltage is extremely unlikely since the voltage periodically makes zero crossings.

And yet many high current breakers have active arc suppression systems (arc blowout coils).

[vibratoking]

I thought blowout coils were mainly used to mitigate premature degradation of the contacts...not necessarily to prevent a constant arc situation?