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

--
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

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

"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.

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.

"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.

In article , Don Pearce
wrote:
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


[snip]

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.

Yes, I'd agree this may well be so, particularly for LF or LF+HF intermod
effects. Not seen measurements, etc, on this. But I avoided using o/p fuses
in the past precisely because I feared such things might occur.

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.

Yes. In fact IIRC some of our older sinewave oscillators in the teaching
labs use a small incandescent lamp to stabilise the gain of the oscillator.
(That makes me also recall that I think the old 'Sound Technology THD
analyser I used to use also did something similar.)

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

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

In article , Graham Holloway
wrote:
[snip]


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.

Yes. I ended up also deciding that the best place for fuses is in the power
rails rather than the o/p.

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

"Graham Holloway" wrote
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.

One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.


Martin
--
M.A.Poyser Tel.: 07967 110890
Manchester, U.K. http://www.fleetie.demon.co.uk

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


A colleague of mine did his PhD on fuses about 30 years ago.

Ian

--
Ian Bell

Fleetie wrote:

"Graham Holloway" wrote
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.

One of the fuses will blow before the other.

What happens to the voltage at the speaker output then? And could it
toast the speaker?

Sounds well dangerous to me.

Far more reliable is to fit 'crowbar protection' on the output to guard against
excessive DC. Whatever else - the speaker won't see prolonged DC. Supply rail
fuses will blow.


Graham