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.

True, but some people with technical knowledge like to carry reason to extremes. Maybe they would complain about saying that the sun goes down at night, since as far as we know it doesn't move, we do. A resistor is a resistor and a fuse is a fuse and a wire is a wire. All can conduct current but each is a different device.? All can open up in cace current exceeds their rating, but each is a different device..?Why do you suppose that the thickness matters when it comes to larger currents?

As long as we are splitting hairs, cross sectional area is what determines a conductors resistance, that and the ressistivity of the material it is made of. And of course, if the conductor is carrying RF frequencies, the resistance of the center of the conductor doesn't matter since the current all travels on the outer surface,? which factor is called 'skin effect'.?As the resistance (to the current) [that phrase is redundant] increases, more (and more) [this one too] energy is used ...

Like Mark said, in high voltage scenarios, this may result in ion storms and the like, but that is not relevant here.
We were talking ordinary electronics here, most of which today operates at considerably less than 120V. If working at the kV level one needs to be concerned about fuzes and conductors melting and arcing. Resistors can cause 'flameouts' however, even without high voltages. So some devices use special resistors designed not to cause any flame if the burn out.

I didn't mean for this to turn into a pissing contest, I do tend to? like to get things stated correctly, a trait the burned my dad to the anger point.

Jim

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

-----Original Message-----
From: herosformula@... [mailto:herosformula@...]
Sent: Friday, October 26, 2001 10:54 AM
To: Electronics_101@...
Subject: [Electronics_101] Re: Fuses vs. resistors

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 <jpurcell@w...> 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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-----Original Message-----
From: Mounir Shita [mailto:mshita@...]
Sent: Friday, October 26, 2001 2:04 PM
To: 'Electronics_101@...'
Subject: RE: [Electronics_101] Re: Fuses vs. resistors

-----Original Message-----
From: kenyonjones@m... [mailto:kenyonjones@m...]
Sent: Friday, October 26, 2001 12:19 PM
To: Electronics_101@y...
Subject: [Electronics_101] Light Activated Alarm


Hello Everyone,

I am new to the group and know just enough about electronics to be
dangerous. The circuit that I am trying to complete is 9V DC. I

placed a phototransistor just before a relay. The relay is 5V. A
piezo alarm is connected to the "Normally Open" lead on the relay.
For some reason, I cannot get enough power through the
phototransistor to activated the relay switch. When I test the
circuit, the alarm quietly sounds and becomes slightly louder when
more light is applied.

It is as though there is resistance with regards to the
phototransistor.

Please forgive me is my terminology is incorrect. I hope that

understands this. Any help would be greatly appreciated.

Sincerely,

Kenyon Jones




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