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

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