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.

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.

"Pooh Bear" wrote in message


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.

oops!

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 with the non-linear load.

Been there, done that.

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

Perhaps.

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

Interesting. What were these new dummy loads composed of, exactly?

The biggest problem I've found with my dummy loads is variation of actual DC
resistance with lnger-term heating and cooling.

I have acquired a stash of precision NI wirewound resistors from Mouser for
my next pass at the problem.

Arny Krueger wrote:

"Pooh Bear" wrote in message


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.

oops!

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 with the non-linear load.

Been there, done that.

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

Perhaps.

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

Interesting. What were these new dummy loads composed of, exactly?

2 of these in series to make a 600W 4 ohm load. Tubular ceramic wirewound type.

http://uk.farnell.com/jsp/endecaSear...=1840290&N=401

They're still fine for soak tests.


The biggest problem I've found with my dummy loads is variation of actual DC
resistance with lnger-term heating and cooling.

I have acquired a stash of precision NI wirewound resistors from Mouser for
my next pass at the problem.

These take my fancy. Supposed to be non-inductive.

http://uk.farnell.com/jsp/endecaSear...=3067920&N=401

Hugely expensive though.


Graham

In article , Pooh Bear
wrote:


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.

Was the 'suspect load' THD high at HF or at LF?

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

Agreed. I think I know the type of loads you mean, and if so, they are the
sort I used to use many years ago. However the reason I ask the above
question is that I recall a review in HFN of the Armstrong 600 range where
the reviewer got much higher THD values than the company had measured.
Investigation lead to us deciding that the reviewer's load had a high
series inductance which was changing the distortion. (However it may have
been a thermal effect, despite our conclusion at the time.)

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 article , Pooh Bear
wrote:

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.

Was the 'suspect load' THD high at HF or at LF?

Seemed to be pretty much independent of frequency IIRC which seemed odd. I was
more interested in just sorting it to spend too much time though.


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

Agreed. I think I know the type of loads you mean, and if so, they are the
sort I used to use many years ago. However the reason I ask the above
question is that I recall a review in HFN of the Armstrong 600 range where
the reviewer got much higher THD values than the company had measured.
Investigation lead to us deciding that the reviewer's load had a high
series inductance which was changing the distortion. (However it may have
been a thermal effect, despite our conclusion at the time.)

I used to think that too. I even measured the inductive component of some of
our loads. It wasn't that high.

The ceramic tubular loads appear to be the ones with the problem. More recent
ones seem worse too. Different resistance wire ?

Graham

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.