Arny Krueger wrote:

"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.

There's an old JAES paper by Greiner of the University of Wisconson that
charted nonlinear distoriton due to fuse thermal effects. Speaker voice
coils do this as well.


Recently acquired some new dummy loads for amplifier testing.

The measured THD figures seemed rather high. Swapping to an earlier dummy
load reduced the THD.

Concluded that the resistance element was thermally modulating on a cycle by
cycle basis. The cable to the load acted like a potential divider in series
withe lon-linear load.

Measuring THD directly at the amplifer output itself proved the point.

Not all dummy loads are equal it seems ! The best I've found in this respect
are the alumium clad bolt down types.

Graham

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.

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