"Don Pearce" wrote in message
...
On Mon, 29 Nov 2004 13:55:30 +0000 (GMT), Jim Lesurf
wrote:

Following on from some discussions in other threads I couldn't find any
existing data on the resistance-current properties of fuses, so did a
quick
measurement as a simple experiment.

The results are shown on the graph at

http://www.st-and.demon.co.uk/temp/fuseplot.gif

Note that this is fairly rought data and I only chose one 0.5A fuse for
test purposes. Other individual types and values of fuse can be expected
to
differ in detail. However the results do show the tendency for the fuse
resistance to rise with current. Only used an AVO and a cheap DVM so the
lower current values are subject to random reading errors producing a
scatter of points.

For these measurements I only applied the current for a few seconds for
each reading. To get to higher currents I would probably need to do
pulsed
measurements. These would also be needed to look at the details of the
time-dependent behaviour when the current level varies.

Although I fitted a parabola for illustrative purposes, I doubt that is
the
correct function for making reliable predictions, particarly for fuses of
values that differ a lot from 0.5A. So the results are perhaps
'interesting' rather than particularly significant.

Slainte,

Jim

Jim, for the purposes of seeing how a fuse might affect linearity,
some sort of frequency response of resistance change vs frequency
would be needed. If a fuse has a very low thermal inertia, it might
change its resistance significantly during a cycle, particularly at
low frequencies. This would cause distortion if it was in series with
a speaker. At higher frequencies which it could not track, it would
simply cause amplitude compression - I've used a light bulb for this
in the past.

d

Pearce Consulting
http://www.pearce.uk.com


When I worked for ILP Electronics many years ago, they suggested the use of
a fuse in series with the loudspeaker as protection against a DC fault. I
found that distortion was significant due to the modulation of the fuse
resistance by low frequencies. I have no records of the tests I did.
However, I did suggest an alternative, and that was to put a fuse in each (+
and -) supply rails. It was possible to use fuses of a lower rating than
that in the output line because of the lower (half cycles) duty.

Graham Holloway
WPS Electronics.