This is a layman point of view. It makes for nice
picturization but the picture makes for some ugly viewing when seen by folks
with some technical knowledge.
?
Why do you suppose that the thickness matters when
it comes to larger currents?
?
The resistance depends on the thickness. So when
you increase the thickness, the resistance decreases.
?
As the resistance to the current increases, more
and more energy is used in overcoming this resistance. Finally, this energy
increaes the temperature of the fuse beyond the melting point of the fuse's
material and it melts.
?
Like Mark said, in high voltage scenarios, this may
result in ion storms and the like, but that is not relevant here.
?
Rama
?
?
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----- Original Message -----
Sent: Friday, October 26, 2001 11:03
AM
Subject: RE: [Electronics_101] Re: Fuses
vs. resistors
Mark,
?
An
ideal fuse doesn't have any resistance. A real fuse of course has some
resistance but it is negitable. The reason why the fuse wire blows is simple,
there is only certain amount of electrons that can travel in a wire with a
certain dimension. When you try to squeeze in to many electrons...BOOM, there
goes the wire.
?
Same
as in your house. The more current your appliance needs, the thicker should
the wire be from the fuse box to the appliance's outlet on the wall. If its to
thin, heat will be produced and a fire might be the worst result. The reason
for that is that a real wire won't easily melt.
?
That's at least kind of how I learned how a fuse
works.
?
Mounir
Jim,
? If a fuse has no
resistence, then how do you suppose that it builds
up heat enough to
melt?? For that matter, what are all those
scientists working on
superconductors doing? (A superconductor is a
wire with no resistence,
btw)
? - mark
--- In Electronics_101@y..., Jim Purcell
wrote:
> Mark,
>
> > A fuse and
a resistor are precisely the same thing.
>
> What gave you that
idea? It's true the there are devices
> called fuzed resistors, the
ones you describe below. But
> mostly resistors have a completely
different purpose.
> The word 'fuze' means 'melt'. Generally a fuze
has zero
> ohms resistance and will 'blow' when the rated
current
> is exceeded, thus interrupting the current path and
protecting
> circuitry or preventing the device from burning up and
causing
> a fire.
>
> > In fact, you'll
> >
frequently find small resistors--about 1 ohm or so--installed in
power
> > supplies.? They are used as fuses.
>
> To avoid confusion they are called fuse resistors, or
'fusistors'.
> [Please note, I am not trying to be combative here,
just debunking
> misinformation.
>
> > This last point
is important: if the fuse wire is subject to air
currents or
>
> is in contact with a heat sink, its temperature may not rise
sufficiently to
> > melt the wire.
>
> I have
never seen a heat sinked fuse. That doesn't mean they don't
exist.
> As old as I am there are still lots of things, even in
technology
that I haven't
>
> seen. I can't think of a
reason for one unless it was operating in
such a hot
> environment
that it might 'blow' prematurely.
>
> > The wire is sized
such that it'll melt when the power dissipated
in it
> >
reaches i*i*r, where i is the current at which the fuse must open
and r
is
> > the resistance of the fuse wire.
>
> All this
it true but since a fuse generally has very low resistance
we
usually
> just reference the current rating. And the 'power' required
to blow
the fuze has
> little to do with the power of the
protected device except that the
device
> determines how much
current flows. For all practical purposes,
power as no
>
importance in selecting fuzed.? Voltage does but only as it affects
the arcing
> that might occur after the fuze has blown, don't want
it to arc
over after doing
> it's job.
>
> > There
are a few more unknowns
>
> Which can be totally ignored by any
but EE and those who are anal
retentive. :-0
>
> > The
other task that a fuse must perform is generally less
recognized
by
> > electronics people.? It must switch off the
current.? This is not
such an
> > easy task when a
highly-inductive circuit is involved.
>
> That's why I
mentioned the voltage rating. While orparting normally
a fuze of
>
negligible voltage drop. I X 0 is zero.
>
>
Jim
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