In article , Arny Krueger
wrote:

[snip]

There's an old JAES paper by Greiner of the University of Wisconson that
charted nonlinear distoriton due to fuse thermal effects.

Ah! Thanks for that info. I'll do a search on the CDROM set of papers I
have and see if I can unearth a copy. May save me wondering or having to
re-do measurements that have already been done! I had the feeling that
surely someone *had* done this, but could not recall ever seeing it.

My assumption until quite recently was that no-one would now use fuses in
the o/p of a power amp as the effect would depend upon the speaker - a
factor outwith the control of the amplifier designer. :-/

Speaker voice coils do this as well.

Although I assume (?) that in the short and medium term the thermal time
constants will be longer due to the mass involved. That said, I suppose the
coils dissipate lots more power than the fuse! :-)
[snip]

Perhaps ironically, light bulbs have their own time/resistance/current
nonlinearity problems.

Indeed, In fact one of our 1st/2nd year experiments used to be to use an
incandescent lamp to do some measurements on Stephan's Law, and this used
the rise in bulb resistance to determine the temperature of the bulb as a
function of the applied power. The snag with doing this with fuses is their
tendency to 'evaporate' half-way through a measurement unless you are
careful. :-)

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html

"Jim Lesurf" wrote in message

In article , Arny Krueger
wrote:

[snip]

There's an old JAES paper by Greiner of the University of Wisconson
that charted nonlinear distoriton due to fuse thermal effects.

Ah! Thanks for that info. I'll do a search on the CDROM set of papers
I have and see if I can unearth a copy. May save me wondering or
having to re-do measurements that have already been done! I had the
feeling that surely someone *had* done this, but could not recall
ever seeing it.

Glad that you have the CDs. I have them online here, but I'm getting tired
of searching it.

My assumption until quite recently was that no-one would now use
fuses in the o/p of a power amp as the effect would depend upon the
speaker - a factor outwith the control of the amplifier designer. :-/

Fuses inside the feedback loop seem to be less problematical from the
standpoint of distortion.

The real problem is with fuses that are reasonable for protecting speakers,
as opposed to fuses that are appropriate for protecting the amplifier.

Speaker voice coils do this as well.

Although I assume (?) that in the short and medium term the thermal
time constants will be longer due to the mass involved. That said, I
suppose the coils dissipate lots more power than the fuse! :-)
[snip]

That's it. Part of the problem is that fuses are often made up of materials
that are intentionally chosen to be nonlinear, to assist the process of
protection.

Perhaps ironically, light bulbs have their own
time/resistance/current nonlinearity problems.

Indeed, In fact one of our 1st/2nd year experiments used to be to use
an incandescent lamp to do some measurements on Stephan's Law, and
this used the rise in bulb resistance to determine the temperature of
the bulb as a function of the applied power. The snag with doing this
with fuses is their tendency to 'evaporate' half-way through a
measurement unless you are careful. :-)

The trick is to do your measurements quickly.

These days I do most of my measurements by playing a test suite with one or
two channel of a sound card, and making the measurements with the record
side of said card, or something like that.

In article , Arny Krueger
wrote:
"Jim Lesurf" wrote in message



My assumption until quite recently was that no-one would now use fuses
in the o/p of a power amp as the effect would depend upon the speaker
- a factor outwith the control of the amplifier designer. :-/

Fuses inside the feedback loop seem to be less problematical from the
standpoint of distortion.

Yes. That seems like a much wiser location that outside the loop. I did
wonder about that when working on amps. But I suspected that having a pair
of fuses on the +/- rails also meant they 'shared' the current as a result
of the duty cycles on music, but would individually protect in the event of
a 'd.c.' problem and that the I2t behaviour made this useful. No real
evidence for this, though, just a feeling that made me chose the rails for
the fuses.


Perhaps ironically, light bulbs have their own
time/resistance/current nonlinearity problems.

Indeed, In fact one of our 1st/2nd year experiments used to be to use
an incandescent lamp to do some measurements on Stephan's Law, and
this used the rise in bulb resistance to determine the temperature of
the bulb as a function of the applied power. The snag with doing this
with fuses is their tendency to 'evaporate' half-way through a
measurement unless you are careful. :-)

The trick is to do your measurements quickly.

Indeed. :-) Alas, these day the only things I do 'quickly' are forget
what I was intending to do, or run out of breath. :-)

Did the rough fuse measurements by briefly touching two wires together to
connect the test circuit to the PSU I used. this meant I could do 1-2
second 'on' tests, but I decided not to push my luck beyond approaching
double the fuse rating. To do better I'd need to arrange an 'automated'
method of the kind you mentioned, but I decided that just a rough check
would be enough to confirm that the resistance *does* rise.

Not yet read the ref you mentioned, but intend to tomorrow. Also got hold
of a copy of our physics lab experiment that uses incandescent lamps to
experiment with Stephan's Law. These give info that relates the current,
resistance, etc. Will have a read through these things when I get a chance.

Slainte,

Jim

--
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Audio Misc http://www.st-and.demon.co.uk/AudioMisc/index.html
Armstrong Audio http://www.st-and.demon.co.uk/Audio/armstrong.html
Barbirolli Soc. http://www.st-and.demon.co.uk/JBSoc/JBSoc.html