Fuses
 

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.

Fuses
 

"Pooh Bear" wrote in message
...
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


If either fuse blew, the output would float to zero.

We used it because it was cheap. In fact, the 100W amplifier modules cost
under one UKP (circa 1982) in parts, including the heatsink.

Graham Holloway.

Fuses
 

On Mon, 29 Nov 2004 16:24:29 +0000 (GMT), Jim Lesurf
wrote:

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

I have a home-built Wien bridge oscillator stabilized by a thermistor
- a tiny bead sealed in a glass tube. It is very good above a hundred
Hz, but its attempts to stabilize the oscillator down around 10Hz are
really pretty pathetic.

d

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

Fuses
 

In article , Fleetie
wrote:


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.

I have wondered about this. However whenever I tested by driving an amp
into a low impedance until fuse failure *both* fuses blew almost at the
same instant. (i.e. I didn't see any time delay) This may well depend upon
the amp design, though.

One point to bear in mind is that you have to use surprisingly low-value
fuses. Thus you tend to end up with a situation where you can get quite
high music and sinewave powers through line fuses that then blow quickly
with a comparable steady level, etc.

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

Fuses
 

In article , Pooh Bear
wrote:
Fleetie wrote:



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.

I avoided using 'protection' circuits as I always had the feeling that:

1) They would fire when not needed, thus either blowing fuses or
interfering with the music when it wasn't warranted.

2) They could also go wrong, and took design effort and cost away from the
actual amp.

However I agree that by avoiding them, the designer takes a risk. Swings
and roundabouts. :-)

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

Fuses
 

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

Fuses
 

"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.

Fuses
 

"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.

Fuses
 

"Fleetie" wrote in message

"Graham Holloway" wrote

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.

Agreed.

What happens to the voltage at the speaker output then?

Depends, but its possible and often done so that the output of the amp does
not swing to the other rail. This may take some care, as I've definately
seen amps with the output firmly stuck on one of the power supplies.

And could it toast the speaker?

Yes, hence the desireability of some additional other kind of protection -
often a relay or a thyristor.

Fuses
 

"Jim Lesurf" wrote in message


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.

Case in point most really old oscillators, including the amateur's fave -
the Heath IG-18

(That makes me also recall that I think the old
'Sound Technology THD analyser I used to use also did something
similar.)

The alternatives use either photocells (typcially CdS) or later on, FETs.

I published a CdS cell update for the IG-18 a few decades ago in Audio
Amateur.

Audio Magazine published a series of articles for construction of a modern
THD analysis rig that used a FET to stabilize its oscillator.