400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs of
added solder over the track for current carrying, the other polarity trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of
what rating ?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:

400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .

How did that happen ? The 'cabinet' is supposed to prevent such things, it's an
IEC safety requirement.


The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.

Sounds like an incompetent manufacturer to me. 8 amps will provide a continuous
2kW almost ! Given typical fusing characteristics, consider that up to 4kW for
maybe 10 seconds.


Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through

IEC 60065 failure very likely.


and that had 2 runs of
added solder over the track for current carrying, the other polarity trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of
what rating ?

Probably something like 3.15A but you may need to add a surge-gard type NTC in
the mains input.

What make / model was it ?

Graham

N Cook wrote:
400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs of
added solder over the track for current carrying, the other polarity trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of
what rating ?

The power supply to your amp probably has massively oversize filter
capacitors.

Average current draw at 400W out might be only two amps, but because
of the rectifier-filter construction it doesn't draw that two amps
continuously. It draws it in very brief gulps 120 times a second.
Typical conduction angle might be 20 percent in a well designed
supply; with massively oversize filter caps it could be as small as 5
percent. Massively oversized filter caps have been all the rage in
consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

And remember, fuses aren't really there to protect your electronics,
they're in there to prevent fires.

If you want a supply that doesn't have such a small conduction angle,
you go to choke-input supplies, which for some bizarre reason aren't
nearly as popular as they were 50 or 70 years ago :-).

Tim.

Tim Shoppa wrote in message
oups.com...
N Cook wrote:
400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating,
which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs
of
added solder over the track for current carrying, the other polarity
trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other
rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added
fuses of
what rating ?

The power supply to your amp probably has massively oversize filter
capacitors.

Average current draw at 400W out might be only two amps, but because
of the rectifier-filter construction it doesn't draw that two amps
continuously. It draws it in very brief gulps 120 times a second.
Typical conduction angle might be 20 percent in a well designed
supply; with massively oversize filter caps it could be as small as 5
percent. Massively oversized filter caps have been all the rage in
consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

And remember, fuses aren't really there to protect your electronics,
they're in there to prevent fires.

If you want a supply that doesn't have such a small conduction angle,
you go to choke-input supplies, which for some bizarre reason aren't
nearly as popular as they were 50 or 70 years ago :-).

Tim.


The 2 reservoir caps are 6800uF, 80V rating with presumably about 65V on
each , when all is in working order.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Meat Plow wrote in message
...
On Sat, 14 Jul 2007 17:32:33 +0100, N Cook wrote:

400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating,
which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs
of
added solder over the track for current carrying, the other polarity
trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other
rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added
fuses of
what rating ?

So what you are saying is that this catastrophic failure of a 400 watt amp
didn't blow an 8 amp fuse? I find that really hard to believe not that you
are making this up mind you.


Well semi-catastrophic, the positive side in a sorry state but the negative
rail side cold-tests ok so far. I was wrong when I said about the overheated
trace it was the ground return not the negative rail, the + trace burned
through as a "fuse", no deliberate necking at those points.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

Tim Shoppa wrote:

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

Absolute utter complete and total rubbish.

'Oversized' caps will only slightly change the conduction angle. It's readily
shown by calculation. Certainly no way whatever will the rms current be 8A !

Graham

Meat Plow wrote:

Anyway, I think fusing the rails is feasible.

It's good practice IMHO.

Unfortunately it's unlike to *prevent* further 'burn ups' although it might help
mitigate them slightly.

To actually prevent burn-ups you need to design for that and the small Chinese
companies I've come across who offer cheap OEM products don't have that level of
design skill. With bipolar output amplifiers you'll normally need to use a number
of fusible resistors (or else rate the output devices more conservatively - which
cheap designs never do of course).


Graham

N Cook wrote:

The 2 reservoir caps are 6800uF, 80V rating with presumably about 65V on
each , when all is in working order.

That's a reasonable size. They won't be causing excessive switch on current.

Is the power transformer toroidal or EI ? That's where the big difference is.
Toroids will take a considerable switch-on current (it's largely due to their
magnetic characteristics).

Graham

Tim Shoppa wrote:

N Cook wrote:

400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs of
added solder over the track for current carrying, the other polarity trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of
what rating ?


The power supply to your amp probably has massively oversize filter
capacitors.

Average current draw at 400W out might be only two amps, but because
of the rectifier-filter construction it doesn't draw that two amps
continuously. It draws it in very brief gulps 120 times a second.
Typical conduction angle might be 20 percent in a well designed
supply; with massively oversize filter caps it could be as small as 5
percent. Massively oversized filter caps have been all the rage in
consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

That shouldn't be terribly hard to calculate, assuming a conduction
angle of your choice. So how about adding some support for the
assertion?


And remember, fuses aren't really there to protect your electronics,
they're in there to prevent fires.

If you want a supply that doesn't have such a small conduction angle,
you go to choke-input supplies, which for some bizarre reason aren't
nearly as popular as they were 50 or 70 years ago :-).

Tim.



--
The e-mail address in our reply-to line is reversed in an attempt to
minimize spam. Our true address is of the form .

Eeyore wrote in message
...


N Cook wrote:

The 2 reservoir caps are 6800uF, 80V rating with presumably about 65V on
each , when all is in working order.

That's a reasonable size. They won't be causing excessive switch on
current.

Is the power transformer toroidal or EI ? That's where the big difference
is.
Toroids will take a considerable switch-on current (it's largely due to
their
magnetic characteristics).

Graham


Toroidal , not E-I lamination transformer
I'm coming around to thinking 5 amp anti-surge in the mains fuse-holder and
a 5 amp fuse shunted in the + and - rail traces from the DC side of the
bridge rectifier before the reservoir caps, but undecided whether quick-blow
or anti-surge ones there.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

CJT wrote:
Tim Shoppa wrote:

N Cook wrote:

400 watt RMS amp for use on 240 V mains.
Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.
Shorted power trannies, burnt low power trannies and even a piece of 3mm
trace from the main bridge rectifier burnt through and that had 2 runs of
added solder over the track for current carrying, the other polarity trace
overheated but not ruptured
I'm thinking of bridging that gap with a fuse and another on the other rail
after cutting it.
The mains transformer is rated at 2x 47V,5 amp.
So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses of
what rating ?


The power supply to your amp probably has massively oversize filter
capacitors.

Average current draw at 400W out might be only two amps, but because
of the rectifier-filter construction it doesn't draw that two amps
continuously. It draws it in very brief gulps 120 times a second.
Typical conduction angle might be 20 percent in a well designed
supply; with massively oversize filter caps it could be as small as 5
percent. Massively oversized filter caps have been all the rage in
consumer audio for too many decades now.

The size of fuse you have to put in depends not on simple average
current (2 amps) but the root-mean-square (RMS) current (which could
indeed be 8 amps at full load).

That shouldn't be terribly hard to calculate, assuming a conduction
angle of your choice. So how about adding some support for the
assertion?

10% conduction angle gives peak current of 20A, RMS current of 6.3A,
not too unreasonable to choose an 8A fuse in that case.

6.25% conduction angle gives 8A RMS, but that'd blow the fuse way too
often.

It takes big capacitors and toroidal transformers with really stiff
windings but many folks are building audio power supplies that way. I
personally don't like razor-thin conduction angles like 10% but that's
the style of other folks, not me!

Tim.

"N Cook"

400 watt RMS amp for use on 240 V mains.

** Is that 400 watts per channel or 200 per channel ?


Destructed due to metal dropping in and shorting the amp .
The only fuse is on the mains rated at 8 amp, maker's design rating, which
shows no sag , discolour or anything like that, ie untouched.


** Then the AC supply current did not exceed 8 amps by much for more than
a fraction of a second.


Shorted power trannies, burnt low power trannies


** Err - " trannies " = transistors ??

The mains transformer is rated at 2x 47V,5 amp.


** Say it is rated at 500 VA, for simplicity.

So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses
of
what rating ?


** From another post I see it is a toroidal type.

Couple of facts:

1. With secondary shorted ( before or after the bridge) primary current will
rise to circa 42 amps rms - so bye bye to any 8 amp fuse real quick.

2. At switch on, the peak supply current will regularly exceed +/- 100
amps for the first half cycle - diminishing to the idle value over the
next 10 - 15 cycles or so.

3. At 400 watts output, the AC current draw will be around 4 amps rms -
assuming this is a typical, low bias, class AB amplifier design. With hard
clipping the figure will rise to over 6 amps rms.

So - the maker's choice of an 8 amp AC fuse is not unreasonable, given the
above facts.

The knee jerk reaction of matching the fuse rating to the VA rating of the
AC tranny does not work in practice - just try using a 2 amp fuse if in
doubt.

Maybe try a 6.3 amp " anti-surge " fuse - the kind with spiral wound fuse
wire.

Keep a few spares handy.



....... Phil

"N Cook"

So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added fuses
of
what rating ?


** Be very wary of adding +/- DC rail fues to any power amp designed without
them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.

Recipe for fried speakers.

You have been warned.




........ Phil

Phil Allison wrote:

snip
** Be very wary of adding +/- DC rail fues to any power amp designed without
them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.
snip

One could add overcurrent shutdown circuitry on each rail designed to shut
down the entire supply, or for antique simplicity use 'indicator fuses' which
have a spring-loaded plunger which makes contact with an endstop terminal
when they blow -- the endstop is often connected to a crowbar which blows
main fuses upstream. These are small cartridge fuses with the same form
factor as standard varieties; I had to replace lots of them in old line
printers where they were used in the hammer drivers -- in this
application they did not crowbar but instead were wired to status circuits
that reported the failure.

Regards,

Michael

"msg"
Phil Allison wrote:

** Be very wary of adding +/- DC rail fuses to any power amp designed
without them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.
snip

One could add overcurrent shutdown circuitry on each rail designed to shut
down the entire supply,


** How idiotic !


or for antique simplicity use 'indicator fuses' which
have a spring-loaded plunger which makes contact with an endstop terminal
when they blow -- the endstop is often connected to a crowbar which blows
main fuses upstream.


** Or simply use no DC rail fuses and let the AC supply fuse blow when
overloaded - as originally intended by the designer.

Those power amps that DO use +/-DC rail fuses, almost invariably have them
fitted in line with the output devices but NOT in line with low power,
voltage amplification circuitry.

It must be possible to remove either DC ( or both) rail fuse with the amp
under load and drive and have no circuit damage occur. Any competent amp
designer can arrange things so this is the case - but not likely the OP.





........ Phil

Phil Allison wrote:

"msg"

Phil Allison wrote:


** Be very wary of adding +/- DC rail fuses to any power amp designed
without them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.

snip

One could add overcurrent shutdown circuitry on each rail designed to shut
down the entire supply,



** How idiotic !



or for antique simplicity use 'indicator fuses' which
have a spring-loaded plunger which makes contact with an endstop terminal
when they blow -- the endstop is often connected to a crowbar which blows
main fuses upstream.



** Or simply use no DC rail fuses and let the AC supply fuse blow when
overloaded - as originally intended by the designer.

Evidently the designer did not account for the sort of catastrophic
failure described by the OP. The large capacitive reservoir in the supply
is very much like that in the line printers that employed the sorts of
protection I described, including the "idiotic" independent supply rail
overcurrent protection scheme with very fast response times (faster than
any fusible link).

Those power amps that DO use +/-DC rail fuses, almost invariably have them
fitted in line with the output devices but NOT in line with low power,
voltage amplification circuitry.

Indeed that was implicit in my response; the rails in question are the
power amp rails, _not_ low-power rails.


It must be possible to remove either DC ( or both) rail fuse with the amp
under load and drive and have no circuit damage occur. Any competent amp
designer can arrange things so this is the case - but not likely the OP.


Yes, but the OP did not design the amp and short of re-engineering it he
would need workable solutions.

Regards,

Michael

"msg"
Phil Allison


** Be very wary of adding +/- DC rail fuses to any power amp designed
without them.

Very likely if one or other DC fuse blows or is removed, the amp's
output will swing fully to the rail with its fuse still intact.

snip

One could add overcurrent shutdown circuitry on each rail designed to
shut
down the entire supply,


** How idiotic !


or for antique simplicity use 'indicator fuses' which
have a spring-loaded plunger which makes contact with an endstop terminal
when they blow -- the endstop is often connected to a crowbar which blows
main fuses upstream.


** Or simply use no DC rail fuses and let the AC supply fuse blow when
overloaded - as originally intended by the designer.

Evidently the designer did not account for the sort of catastrophic
failure described by the OP.


** Nonsense.

Output stage S/C failure is intended to blow the AC supply fuse.

The circuit damage is already done when it failed.

( snip drivel)



Those power amps that DO use +/-DC rail fuses, almost invariably have
them fitted in line with the output devices but NOT in line with low
power, voltage amplification circuitry.

Indeed that was implicit in my response; the rails in question are the
power amp rails, _not_ low-power rails.


** No - you have got it WRONG again !!!

It is the same two DC rails, split to service the output stage devices via
fuses.

YOU have NO clue whatever about power amplifier circuitry.

Better you learned to shut the **** up.



It must be possible to remove either DC ( or both) rail fuse with the amp
under load and drive and have no circuit damage occur. Any competent amp
designer can arrange things so this is the case - but not likely the OP.


Yes, but the OP did not design the amp and short of re-engineering it he
would need workable solutions.


** Well, he ain't gonna get any from a posturing ASS like you then.

The only *problem* the OP ever had was due to dropping a metal object
inside the amp - to which the solution is damn obvious.




....... Phil

Phil Allison wrote:

"msg"
Phil Allison

snip

Those power amps that DO use +/-DC rail fuses, almost invariably have
them fitted in line with the output devices but NOT in line with low
power, voltage amplification circuitry.

Indeed that was implicit in my response; the rails in question are the
power amp rails, _not_ low-power rails.



** No - you have got it WRONG again !!!

Huh? I was agreeing with you and pointing out that my previous post
was based on that assumption...

snip


YOU have NO clue whatever about power amplifier circuitry.

I will withdraw from this discussion at this point; I have designed
my share of power amps, mostly involving large computer peripherals
and have experience with big servo amps and shaker tables as well.

Regards,

Michael

"msg"
Phil Allison


Those power amps that DO use +/-DC rail fuses, almost invariably have
them fitted in line with the output devices but NOT in line with low
power, voltage amplification circuitry.

Indeed that was implicit in my response; the rails in question are the
power amp rails, _not_ low-power rails.



** No - you have got it WRONG again !!!

Huh?


** **** off - you snipping IMBECILE.


I was agreeing with you and pointing out that my previous post
was based on that assumption...


** Total ********.



YOU have NO clue whatever about power amplifier circuitry.

I will withdraw from this discussion at this point; I have designed
my share of power amps, mostly involving large computer peripherals
and have experience with big servo amps and shaker tables as well.


** But obviously YOU have never studied how typical audio power amps are
made or have the tiniest a clue why.

**** OFF

- BLOODY FOOL




......... Phil

Phil Allison wrote:

I was agreeing with you and pointing out that my previous post
was based on that assumption...

** Total ********.

Wonderful. You're arguing with people who are agreeing with you.

Quite entertaining.

You're not trying to approach the notoriety of Rod Speed are you? He might
be different, but he still makes sense.

You on the other hand make no sense whatsoever. Still entertaining though...
--
Linux Registered User # 302622
http://counter.li.org

"John Tjerkshis ****wit "

( snip autistic, verbal drool )


** Those with the IQ of a donkey only understand donkey speak.


Hee - haw, hee - hawww......





...... Phil

Phil Allison wrote in message
...

"N Cook"

So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added
fuses
of
what rating ?


** Be very wary of adding +/- DC rail fues to any power amp designed
without
them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.

Recipe for fried speakers.

You have been warned.




....... Phil





I'm just trying to engineer a more respectable equivalent of fuses for trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.
Then as the common return almost fused and is the same dimension but in the
even worse case then carries twice the current , halving this trace rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?
If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.
Anyone know the term for laying double traces of solder along copper traces
to increase current carrying , so I can research it ?
I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.

Anyway measurements:
Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder then
area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and major
axes of .15mm and .8mm.
From tables of fusing currents for copper, also tin and lead (NB in the form
of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps so 18/4 = 4.5 amps conventional fuse rating.
Presumably the circular to sheet allowance would up this 4.5 amp figure ,
but by how much ?
What sort of correction factor for thin sheet/non-circular heating then
rupture allowance?

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:

I'm just trying to engineer a more respectable equivalent of fuses for trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.
Then as the common return almost fused and is the same dimension but in the
even worse case then carries twice the current , halving this trace rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?
If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.
Anyone know the term for laying double traces of solder along copper traces
to increase current carrying , so I can research it ?

Beefing up!

I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.

When we used to build classic 'Hitachi' mosfet amps, we would lay a
thickness of solder along the DC rails to the outputs transistors and to
the output terminal. It`s a bit like welding, once you get the hang of
it, you can lay down a respectable run of solder that looks quite
professional. I spose you could beef up with tinned copper wire, but it
wouldn`t look so neat. Some amps have hard wired connections directly to
the output trannies, or, the DC feed from the PSU go directly to the pcb
lands upon which the outputs are soldered/bolted.


Anyway measurements:
Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder then
area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and major
axes of .15mm and .8mm.
From tables of fusing currents for copper, also tin and lead (NB in the form
of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps so 18/4 = 4.5 amps conventional fuse rating.
Presumably the circular to sheet allowance would up this 4.5 amp figure ,
but by how much ?
What sort of correction factor for thin sheet/non-circular heating then
rupture allowance?

Did you ever tell us what make and model of amplifier this is?

Is it really worth all this effort to redesign? You're never going to be
able to protect against stray metal object clattering around inside, and
I`m sure the original designers never gave it a thought.


Ron(UK)

Ron(UK) wrote in message
...
N Cook wrote:

I'm just trying to engineer a more respectable equivalent of fuses for
trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.
Then as the common return almost fused and is the same dimension but in
the
even worse case then carries twice the current , halving this trace
rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?
If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.
Anyone know the term for laying double traces of solder along copper
traces
to increase current carrying , so I can research it ?

Beefing up!

I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.

When we used to build classic 'Hitachi' mosfet amps, we would lay a
thickness of solder along the DC rails to the outputs transistors and to
the output terminal. It`s a bit like welding, once you get the hang of
it, you can lay down a respectable run of solder that looks quite
professional. I spose you could beef up with tinned copper wire, but it
wouldn`t look so neat. Some amps have hard wired connections directly to
the output trannies, or, the DC feed from the PSU go directly to the pcb
lands upon which the outputs are soldered/bolted.


Anyway measurements:
Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder
then
area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and
major
axes of .15mm and .8mm.
From tables of fusing currents for copper, also tin and lead (NB in the
form
of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps so 18/4 = 4.5 amps conventional fuse rating.
Presumably the circular to sheet allowance would up this 4.5 amp figure
,
but by how much ?
What sort of correction factor for thin sheet/non-circular heating then
rupture allowance?

Did you ever tell us what make and model of amplifier this is?

Is it really worth all this effort to redesign? You're never going to be
able to protect against stray metal object clattering around inside, and
I`m sure the original designers never gave it a thought.


Ron(UK)

These beefed-up traces are far too regular to be done by hand, also precise
45 degree X+Y plot lines so some sort of plotter type delivery system
following the traces/etch artwork co-ordinates. But its not just the solder
lines but the amount of solder delivered that is so uniform.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

N Cook wrote:
Ron(UK) wrote in message
...
N Cook wrote:

I'm just trying to engineer a more respectable equivalent of fuses for
trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.
Then as the common return almost fused and is the same dimension but in
the
even worse case then carries twice the current , halving this trace
rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?
If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.
Anyone know the term for laying double traces of solder along copper
traces
to increase current carrying , so I can research it ?
Beefing up!

I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.
When we used to build classic 'Hitachi' mosfet amps, we would lay a
thickness of solder along the DC rails to the outputs transistors and to
the output terminal. It`s a bit like welding, once you get the hang of
it, you can lay down a respectable run of solder that looks quite
professional. I spose you could beef up with tinned copper wire, but it
wouldn`t look so neat. Some amps have hard wired connections directly to
the output trannies, or, the DC feed from the PSU go directly to the pcb
lands upon which the outputs are soldered/bolted.

Anyway measurements:
Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder
then
area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and
major
axes of .15mm and .8mm.
From tables of fusing currents for copper, also tin and lead (NB in the
form
of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps so 18/4 = 4.5 amps conventional fuse rating.
Presumably the circular to sheet allowance would up this 4.5 amp figure
,
but by how much ?
What sort of correction factor for thin sheet/non-circular heating then
rupture allowance?
Did you ever tell us what make and model of amplifier this is?

Is it really worth all this effort to redesign? You're never going to be
able to protect against stray metal object clattering around inside, and
I`m sure the original designers never gave it a thought.


Ron(UK)

These beefed-up traces are far too regular to be done by hand, also precise
45 degree X+Y plot lines so some sort of plotter type delivery system
following the traces/etch artwork co-ordinates. But its not just the solder
lines but the amount of solder delivered that is so uniform.

Ah you're talking abput the hatched (scotched) type thing, I`m sure
that`s solder paste screen printed on during manufacture of the pcb.

Ron(UK)

Ron(UK) wrote in message
...
N Cook wrote:
Ron(UK) wrote in message
...
N Cook wrote:



These beefed-up traces are far too regular to be done by hand, also
precise
45 degree X+Y plot lines so some sort of plotter type delivery system
following the traces/etch artwork co-ordinates. But its not just the
solder
lines but the amount of solder delivered that is so uniform.

Ah you're talking abput the hatched (scotched) type thing, I`m sure
that`s solder paste screen printed on during manufacture of the pcb.

Ron(UK)

None of that on this one but I know what you mean. But some extrusion of
solder paste process via a vinyl sign cutter type plotter mechanism in place
of the cutter would be more likely than molten solder. Then fused onto the
board before populating with components but would that take solder bath
soldering of the components afterwards ?
The "beefing" is not applied after the main component soldering operation.

--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/

On 15 Jul, 09:23, "N Cook" wrote:

Toroidal , not E-I lamination transformer
I'm coming around to thinking 5 amp anti-surge in the mains fuse-holder and

whatever fuse the toroidal gets needs to be antisurge. Toroids are
known for their sometimes heavy inrush.

This will give no protection to the output devices of course, it
really will be nothing more than basic fire protection.


a 5 amp fuse shunted in the + and - rail traces from the DC side of the
bridge rectifier before the reservoir caps, but undecided whether quick-blow
or anti-surge ones there.

You'd get much better fuse response if you put them after the
reservoirs, because they dont then need to handle high peak cap
charging currents.


I'm just trying to engineer a more respectable equivalent of fuses for trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.

Then as the common return almost fused and is the same dimension but in the
even worse case then carries twice the current

worst case is 1x not 2x. The common psu rail carries near zero if
similar fault currents flow in both + and - rails.


, halving this trace rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?

If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.

then there would be near zero current in the common rail.

Anyone know the term for laying double traces of solder along copper traces
to increase current carrying , so I can research it ?
I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.

I dont think I've ever seen any attempt to do it to accurate
dimensions. Neither flow soldering nor hand soldering work like that.


Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps

No, because cu and slobber blow at very different temps. 18A probably
wont even be close.


I'm left wondering precisely what youre trying to achieve. If youre
trying to protect output devices, none of the above will do it. If
youre trying to protect speakers, the above are not whats wanted. If
all you want is to prevent charring of pcb due to repeated blowing,
its going to be cheaper in the end to prevent the repeat blowing.
Really whats propsed so far doesnt seem to solve any real world issue.
The amp sounds roughly designed, intended for a limited service life,
and to change that you'd need to add a full set of current and safe
area protection for the output devices, almost none of which can be
achieved with fuses.

What are you really looking to achieve?


NT

wrote in message
ups.com...
On 15 Jul, 09:23, "N Cook" wrote:



I'm left wondering precisely what youre trying to achieve. If youre
trying to protect output devices, none of the above will do it. If
youre trying to protect speakers, the above are not whats wanted. If
all you want is to prevent charring of pcb due to repeated blowing,
its going to be cheaper in the end to prevent the repeat blowing.
Really whats propsed so far doesnt seem to solve any real world issue.
The amp sounds roughly designed, intended for a limited service life,
and to change that you'd need to add a full set of current and safe
area protection for the output devices, almost none of which can be
achieved with fuses.

What are you really looking to achieve?


NT


I'll repeat it without the clutter.

I'm just trying to fashion a more respectable equivalent of fuses instead of
trace burning and rupture.

msg wrote:

Phil Allison wrote:

** Be very wary of adding +/- DC rail fues to any power amp designed without
them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.

One could add overcurrent shutdown circuitry on each rail designed to shut
down the entire supply,

He's *repairing* an amplifier not redesigning it.

A decent amplifier has some element of overcurrent protection built in anyway.

Graham

msg wrote:

Phil Allison wrote:
"msg"

** Or simply use no DC rail fuses and let the AC supply fuse blow when
overloaded - as originally intended by the designer.

Evidently the designer did not account for the sort of catastrophic
failure described by the OP.

You don't have much experience of high power audio amplifiers do you ?

Graham

N Cook wrote:

Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.

You'll find it was probably getting on for 100 amps or so.

Graham

N Cook wrote:

Phil Allison wrote in message
...

"N Cook"

So drop the 8A fuse (for 2 KWatt !) to what value ? and the 2 added
fuses
of
what rating ?


** Be very wary of adding +/- DC rail fues to any power amp designed
without
them.

Very likely if one or other DC fuse blows or is removed, the amp's output
will swing fully to the rail with its fuse still intact.

Recipe for fried speakers.

You have been warned.




....... Phil





I'm just trying to engineer a more respectable equivalent of fuses for trace
rupture.
Eventually one DC rail ruptured as a fuse. I've now decided to do some
calculation to determine what the rupture current for that trace was.
Then as the common return almost fused and is the same dimension but in the
even worse case then carries twice the current , halving this trace rupture
current to use as the fuse rupture current and reduce that for current
carying capacity , by half again ?
If all output trannies fused short circuit all round so loading both DC
rails the combined common return would have ruptured at whatever that
rupture current is for the trace that did burn up.
Anyone know the term for laying double traces of solder along copper traces
to increase current carrying , so I can research it ?
I'm intrigued how you lay quite accurate molten solder runs and parallel
over and with existing traces.

Anyway measurements:
Copper trace 3.5 x .02mm and as 2 half approximated elipses of solder then
area of 1 elipse of tin+lead which is Pi x a x b , a and b minor and major
axes of .15mm and .8mm.
From tables of fusing currents for copper, also tin and lead (NB in the form
of circular wires ) for different diameters.

Copper fusing current of the trace = 12 amps
Lead+Tin elipse then 6 amps (not as much as I would have intuitively
thought)
Total 18 amps so 18/4 = 4.5 amps conventional fuse rating.
Presumably the circular to sheet allowance would up this 4.5 amp figure ,
but by how much ?
What sort of correction factor for thin sheet/non-circular heating then
rupture allowance?

Don't waste time.

The customer's not paying for you to redesign the amp. Fit some wire mesh or
whatever to stop things being dropped inside it in future as per safety regs.

Graham

"Ron(UK)" wrote:

You're never going to be able to protect against stray metal object clattering
around inside,

Quite !

and I`m sure the original designers never gave it a thought.

In which case it shouldn't have a CE sticker.

Graham

N Cook wrote:

Ron(UK) wrote

Ah you're talking abput the hatched (scotched) type thing, I`m sure
that`s solder paste screen printed on during manufacture of the pcb.


None of that on this one but I know what you mean. But some extrusion of
solder paste process via a vinyl sign cutter type plotter mechanism in place
of the cutter would be more likely than molten solder. Then fused onto the
board before populating with components but would that take solder bath
soldering of the components afterwards ?

No.

You've gone completely berserk.

Graham

N Cook wrote:

I'm just trying to fashion a more respectable equivalent of fuses instead of
trace burning and rupture.

You can't.

Fix it and tell the owner not to drop metal bits inside it.

Graham

Eeyore wrote:

"Ron(UK)" wrote:

You're never going to be able to protect against stray metal object clattering
around inside,

Quite !

and I`m sure the original designers never gave it a thought.

In which case it shouldn't have a CE sticker.

Graham


I would imagine that the stray object originated inside the amp,
probably a panel screw or captive nut never secured in the first place.

How often you you pick up an item of gear and hear something loose
rolling around inside


Ron(UK)

Eeyore wrote:

N Cook wrote:

Ron(UK) wrote
Ah you're talking abput the hatched (scotched) type thing, I`m sure
that`s solder paste screen printed on during manufacture of the pcb.

None of that on this one but I know what you mean. But some extrusion of
solder paste process via a vinyl sign cutter type plotter mechanism in place
of the cutter would be more likely than molten solder. Then fused onto the
board before populating with components but would that take solder bath
soldering of the components afterwards ?

No.

You've gone completely berserk.

Graham


As I understand it, in modern pcb production, the majority of the solder
is 'silk screened' as a paste, onto the board as part of the process,
then once the majority of components have been positioned, the whole
assembly is passed between radiant heaters to melt the solder paste and
fuse the whole lot together.

Of course, I could be wrong.

It's a miracle that any electronic stuff works at all these days!


Ron(UK)

In article ,
"N Cook" wrote:

Anyone know the term for laying double traces of solder along copper traces
to increase current carrying , so I can research it ?

No, but it's a dumb idea. It's fairly easy to dump solder "accurately"
on a trace (if not covered by soldermask, hopefully that's obvious) -
just heat and feed solder, it will follow the trace, and you can build
it up quite a ways simply from surface tension. If you need more current
capacity on the trace, bend up a chunk of copper wire that actually
conducts well and solder that on. Solder is a lousy conductor, compared
to copper. The copper will hold more solder on there, but most of the
current will be carried by the copper.

--
Cats, coffee, chocolate...vices to live by

"Ron(UK)" wrote:

Eeyore wrote:
"Ron(UK)" wrote:

You're never going to be able to protect against stray metal object clattering
around inside,

Quite !

and I`m sure the original designers never gave it a thought.

In which case it shouldn't have a CE sticker.


I would imagine that the stray object originated inside the amp,
probably a panel screw or captive nut never secured in the first place.

I thought the OP said something did actually fall inside ?


How often you you pick up an item of gear and hear something loose
rolling around inside

Thankfully not too often. I do recall one time about 30 yrs ago when I withdrew a
Bulgin mains lead and the earth prong of the equipment connector came out with it
though ! They were just held in place with screws !

Graham

"Ron(UK)" wrote:

Eeyore wrote:
N Cook wrote:
Ron(UK) wrote

Ah you're talking abput the hatched (scotched) type thing, I`m sure
that`s solder paste screen printed on during manufacture of the pcb.

None of that on this one but I know what you mean. But some extrusion of
solder paste process via a vinyl sign cutter type plotter mechanism in place
of the cutter would be more likely than molten solder. Then fused onto the
board before populating with components but would that take solder bath
soldering of the components afterwards ?

No.

You've gone completely berserk.


As I understand it, in modern pcb production, the majority of the solder
is 'silk screened' as a paste, onto the board as part of the process,
then once the majority of components have been positioned, the whole
assembly is passed between radiant heaters to melt the solder paste and
fuse the whole lot together.

With surface mount that's the typical way. It's called infra-red reflow.


Of course, I could be wrong.

It's a miracle that any electronic stuff works at all these days!

See my comment about lead-free (wrt your Behringer EP2500) in a.ap.l-s.

Graham