Fuse Failure Machanism ...??

A

Arfa Daily

Guest
How do three fuses fail simultaneously ? I have just repaired a small
external switcher from an LCD TV for one of my trade customers. 12V @ 5A -
you know the sort of thing. It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds. It
also had a 3A plugtop fuse fitted, this being in the UK. So that's three
fuses all effectively in series. Two of them directly so, and one in the
other (return) side of the input circuit. All three were open, and the cause
of this appeared to be that the main filter cap was bulged and open,
although I'm not entirely sure why this should have led to excess input
current, as I have seen open main filter caps many times, and fuse failure
from this alone, is not the usual symptom.

However, vagaries of switcher behaviour aside, how can three fuses -
especially considering that one of them is a higher rated plugtop fuse, not
known for their accuracy - that are all in series, all be open ? Clearly,
as soon as one fails, in theory at least, all current through the other two
is interupted ...

As soon as the cap and three fuses were replaced, operation of the supply
was normal, and it powered the TV without issue. Thoughts anyone ?

Arfa
 
On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> wrote:

How do three fuses fail simultaneously ? I have just repaired a small
external switcher from an LCD TV for one of my trade customers. 12V @ 5A -
you know the sort of thing. It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds. It
also had a 3A plugtop fuse fitted, this being in the UK. So that's three
fuses all effectively in series. Two of them directly so, and one in the
other (return) side of the input circuit. All three were open, and the cause
of this appeared to be that the main filter cap was bulged and open,
although I'm not entirely sure why this should have led to excess input
current, as I have seen open main filter caps many times, and fuse failure
from this alone, is not the usual symptom.

However, vagaries of switcher behaviour aside, how can three fuses -
especially considering that one of them is a higher rated plugtop fuse, not
known for their accuracy - that are all in series, all be open ? Clearly,
as soon as one fails, in theory at least, all current through the other two
is interupted ...

As soon as the cap and three fuses were replaced, operation of the supply
was normal, and it powered the TV without issue. Thoughts anyone ?

Arfa

If you were here in the US midwest, I would say it was a nearby
Click to expand...
lightning strike and the fuses kept arcing after the first one opened.
Anything like that possible in your area?
 
On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> wrote:

How do three fuses fail simultaneously ? I have just repaired a small
external switcher from an LCD TV for one of my trade customers. 12V @ 5A -
you know the sort of thing. It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds. It
also had a 3A plugtop fuse fitted, this being in the UK. So that's three
fuses all effectively in series. Two of them directly so, and one in the
other (return) side of the input circuit. All three were open, and the cause
of this appeared to be that the main filter cap was bulged and open,
although I'm not entirely sure why this should have led to excess input
current, as I have seen open main filter caps many times, and fuse failure
from this alone, is not the usual symptom.

However, vagaries of switcher behaviour aside, how can three fuses -
especially considering that one of them is a higher rated plugtop fuse, not
known for their accuracy - that are all in series, all be open ? Clearly,
as soon as one fails, in theory at least, all current through the other two
is interupted ...

As soon as the cap and three fuses were replaced, operation of the supply
was normal, and it powered the TV without issue. Thoughts anyone ?

Arfa
Click to expand...
I can only say: good fuses! <g>

A massive overcurrent, I'm sure, will blow all. I've seen two blow
like this, I suspect one fails, but the failure arc is enough to cause
the other (compromised at this point) to blow as well.
 
The lightning strikes sounds plausible.

I think there's another possibility...

Suppose the overload were enough to severely _bend_ the fuses, rather than
blow them outright. When one finally popped, the others flexed back and
cracked. So... Are they clear glass? Do you seen vaporized metal?

Another possibility is that the overload was _so_ severe that they really
_did_ all blow at the same time. The fuse wire, however small it is, has
thermal inertia. It can still break, even after the power has been removed,
if it was hot enough a millisecond or two before.

Regardless, it's an interesting question.
 
On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
<arfa.daily@ntlworld.com> wrote:

It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds.
Click to expand...
Are those slo-blow fuses? If the fuse case is clear, it will show a
spring and a resitor inside.

If it is a slow blow, these take a while to open after seeing an
overload. It depends on the amount of overload and resistor heating.
There's a small delay during which time the other fuses are also
receiving the full benefit of the overload. Even if the load is
removed during this time, thermal inertia (the time it take for heat
to make it from the resistor to the fuse melt area) will cause the
other fuses to blow.

However, if it's NOT a slo-blow fuse, then there's insufficient mass
in the fuse wire to produce any thermal inertia effects, so this
mechanism is unlikely.



--
Jeff Liebermann jeffl@cruzio.com
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558
 
"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:v0daa5tnb7vik4n0b5k7pp9fitsfck75m1@4ax.com...
On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> wrote:

It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds.

Are those slo-blow fuses? If the fuse case is clear, it will show a
spring and a resitor inside.

If it is a slow blow, these take a while to open after seeing an
overload. It depends on the amount of overload and resistor heating.
There's a small delay during which time the other fuses are also
receiving the full benefit of the overload. Even if the load is
removed during this time, thermal inertia (the time it take for heat
to make it from the resistor to the fuse melt area) will cause the
other fuses to blow.

However, if it's NOT a slo-blow fuse, then there's insufficient mass
in the fuse wire to produce any thermal inertia effects, so this
mechanism is unlikely.



--
Jeff Liebermann jeffl@cruzio.com
Click to expand...

Ok. Few points from various people to answer here. The lightning strike
scenario is a possibility. There have been a couple of storms in the area in
the last few weeks, but in general, it's been a pretty quiet season here in
the UK this year. The O/C main filter cap might just be a red herring here.
I must admit that I replaced it at the same time as the fuses, as it was
obviously bulging both top and bottom, and read open on the ESR meter. It
could be that it was on its last legs ESR-wise, but still had enough 'guts'
to perform its basic function, until the surge that took the fuses out,
finished it off. That might add support to the lightning strike theory.

As to the fuse types. I take it the term "Wickman fuse" doesn't have any
meaning in the U.S.? If not, allow me to explain. Wickmans are a type of
wire-ended fuse for direct PCB mounting. They are a plastic cylinder about
10mm diameter and maybe 10 -12mm high. They have two leads coming out of the
bottom pitched at probably 5mm. Imagine a small stubby radial leaded
electrolytic, if you like. The plastic is usually either black or brown.
They have always been popular in Philips gear, although other manufacturers
do use them. They come in both fast and "T" rated types. The ones that were
fitted in this particular supply, were "T" type, so yes, slo-blo's. However,
with them being plastic, you get no indications of how they have failed. I
am of course very familiar with the three normal failure modes of
vapourised, melted, and broken, and if you could see how these had failed,
I'm sure it would have given a clue as to what was going on. To be honest,
I'm not even sure of how these fuses are constructed internally. I might try
and break into one tomorrow to get a look.

As to the plugtop fuse, these are ceramic bodied, so again, you can't see
the failure mode. I guess you might not have ever even seen one in the U.S.
I've no idea what the fusing characteristic of them is. They come in 3A, 5A,
10A and 13A flavours, but most people would consider them to be little more
than a calibrated nail. I have seen them stand up to overloads where you
could almost hear the substation transformer jumping off its mountings, and
yet they will also readily fail when a light bulb pops its clogs.

I quite like the thermal inertia idea as an explanation, but even though the
Wickmans are "T" rated, I'm not sure that there is sufficient wire in that
little case (assuming that it is in some way a 'conventional' wire fuse
inside) to actually have that much inertia ...

Thanks all for your thoughts (including bz who responded (accidentally ?)
directly to me). It's an interesting little puzzle.

Arfa
 
On Tue, 8 Sep 2009 01:53:05 +0100, "Arfa Daily" <arfa.daily@ntlworld.com> wrote:

:
:"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
:news:v0daa5tnb7vik4n0b5k7pp9fitsfck75m1@4ax.com...
:> On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
:> <arfa.daily@ntlworld.com> wrote:
:>
:>>It had two 2.5A Wickman fuses just south of the
:>>line input connector, so that's one each in the live and neutral feeds.
:>
:> Are those slo-blow fuses? If the fuse case is clear, it will show a
:> spring and a resitor inside.
:>
:> If it is a slow blow, these take a while to open after seeing an
:> overload. It depends on the amount of overload and resistor heating.
:> There's a small delay during which time the other fuses are also
:> receiving the full benefit of the overload. Even if the load is
:> removed during this time, thermal inertia (the time it take for heat
:> to make it from the resistor to the fuse melt area) will cause the
:> other fuses to blow.
:>
:> However, if it's NOT a slo-blow fuse, then there's insufficient mass
:> in the fuse wire to produce any thermal inertia effects, so this
:> mechanism is unlikely.
:>
:>
:>
:> --


:> Jeff Liebermann jeffl@cruzio.com
:
:
:Ok. Few points from various people to answer here. The lightning strike
:scenario is a possibility. There have been a couple of storms in the area in
:the last few weeks, but in general, it's been a pretty quiet season here in
:the UK this year. The O/C main filter cap might just be a red herring here.
:I must admit that I replaced it at the same time as the fuses, as it was
:eek:bviously bulging both top and bottom, and read open on the ESR meter. It
:could be that it was on its last legs ESR-wise, but still had enough 'guts'
:to perform its basic function, until the surge that took the fuses out,
:finished it off. That might add support to the lightning strike theory.
:
:As to the fuse types. I take it the term "Wickman fuse" doesn't have any
:meaning in the U.S.? If not, allow me to explain. Wickmans are a type of
:wire-ended fuse for direct PCB mounting. They are a plastic cylinder about
:10mm diameter and maybe 10 -12mm high. They have two leads coming out of the
:bottom pitched at probably 5mm. Imagine a small stubby radial leaded
:electrolytic, if you like. The plastic is usually either black or brown.
:They have always been popular in Philips gear, although other manufacturers
:do use them. They come in both fast and "T" rated types. The ones that were
:fitted in this particular supply, were "T" type, so yes, slo-blo's. However,
:with them being plastic, you get no indications of how they have failed. I
:am of course very familiar with the three normal failure modes of
:vapourised, melted, and broken, and if you could see how these had failed,
:I'm sure it would have given a clue as to what was going on. To be honest,
:I'm not even sure of how these fuses are constructed internally. I might try
:and break into one tomorrow to get a look.
:
:As to the plugtop fuse, these are ceramic bodied, so again, you can't see
:the failure mode. I guess you might not have ever even seen one in the U.S.
:I've no idea what the fusing characteristic of them is. They come in 3A, 5A,
:10A and 13A flavours, but most people would consider them to be little more
:than a calibrated nail. I have seen them stand up to overloads where you
:could almost hear the substation transformer jumping off its mountings, and
:yet they will also readily fail when a light bulb pops its clogs.
:
:I quite like the thermal inertia idea as an explanation, but even though the
:Wickmans are "T" rated, I'm not sure that there is sufficient wire in that
:little case (assuming that it is in some way a 'conventional' wire fuse
:inside) to actually have that much inertia ...
:
:Thanks all for your thoughts (including bz who responded (accidentally ?)
:directly to me). It's an interesting little puzzle.
:
:Arfa
:

I would be more concerned with what caused the filter cap to bulge...

If it is a well known brand and correctly rated for the application then it
should not have degenerated to that state as long as normal input voltage range
was experienced. I also doubt that a possible short term lightning event would
have caused the cap to bulge - this symptom is more likely due to a longer term
overvoltage event.

If all the major input circuitry and components are ok I would suspect a leaky
diode in the bridge rectifier.
 
"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
news:6siba59ouaan92agjvoshtibvit2l670va@4ax.com...
On Tue, 8 Sep 2009 01:53:05 +0100, "Arfa Daily" <arfa.daily@ntlworld.com
wrote:

:
:"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
:news:v0daa5tnb7vik4n0b5k7pp9fitsfck75m1@4ax.com...
:> On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
:> <arfa.daily@ntlworld.com> wrote:
:
:>>It had two 2.5A Wickman fuses just south of the
:>>line input connector, so that's one each in the live and neutral feeds.
:
:> Are those slo-blow fuses? If the fuse case is clear, it will show a
:> spring and a resitor inside.
:
:> If it is a slow blow, these take a while to open after seeing an
:> overload. It depends on the amount of overload and resistor heating.
:> There's a small delay during which time the other fuses are also
:> receiving the full benefit of the overload. Even if the load is
:> removed during this time, thermal inertia (the time it take for heat
:> to make it from the resistor to the fuse melt area) will cause the
:> other fuses to blow.
:
:> However, if it's NOT a slo-blow fuse, then there's insufficient mass
:> in the fuse wire to produce any thermal inertia effects, so this
:> mechanism is unlikely.
:
:
:
:> --


:> Jeff Liebermann jeffl@cruzio.com
:
:
:Ok. Few points from various people to answer here. The lightning strike
:scenario is a possibility. There have been a couple of storms in the area
in
:the last few weeks, but in general, it's been a pretty quiet season here
in
:the UK this year. The O/C main filter cap might just be a red herring
here.
:I must admit that I replaced it at the same time as the fuses, as it was
:eek:bviously bulging both top and bottom, and read open on the ESR meter. It
:could be that it was on its last legs ESR-wise, but still had enough
'guts'
:to perform its basic function, until the surge that took the fuses out,
:finished it off. That might add support to the lightning strike theory.
:
:As to the fuse types. I take it the term "Wickman fuse" doesn't have any
:meaning in the U.S.? If not, allow me to explain. Wickmans are a type of
:wire-ended fuse for direct PCB mounting. They are a plastic cylinder
about
:10mm diameter and maybe 10 -12mm high. They have two leads coming out of
the
:bottom pitched at probably 5mm. Imagine a small stubby radial leaded
:electrolytic, if you like. The plastic is usually either black or brown.
:They have always been popular in Philips gear, although other
manufacturers
:do use them. They come in both fast and "T" rated types. The ones that
were
:fitted in this particular supply, were "T" type, so yes, slo-blo's.
However,
:with them being plastic, you get no indications of how they have failed.
I
:am of course very familiar with the three normal failure modes of
:vapourised, melted, and broken, and if you could see how these had
failed,
:I'm sure it would have given a clue as to what was going on. To be
honest,
:I'm not even sure of how these fuses are constructed internally. I might
try
:and break into one tomorrow to get a look.
:
:As to the plugtop fuse, these are ceramic bodied, so again, you can't see
:the failure mode. I guess you might not have ever even seen one in the
U.S.
:I've no idea what the fusing characteristic of them is. They come in 3A,
5A,
:10A and 13A flavours, but most people would consider them to be little
more
:than a calibrated nail. I have seen them stand up to overloads where you
:could almost hear the substation transformer jumping off its mountings,
and
:yet they will also readily fail when a light bulb pops its clogs.
:
:I quite like the thermal inertia idea as an explanation, but even though
the
:Wickmans are "T" rated, I'm not sure that there is sufficient wire in
that
:little case (assuming that it is in some way a 'conventional' wire fuse
:inside) to actually have that much inertia ...
:
:Thanks all for your thoughts (including bz who responded (accidentally ?)
:directly to me). It's an interesting little puzzle.
:
:Arfa
:

I would be more concerned with what caused the filter cap to bulge...

If it is a well known brand and correctly rated for the application then
it
should not have degenerated to that state as long as normal input voltage
range
was experienced. I also doubt that a possible short term lightning event
would
have caused the cap to bulge - this symptom is more likely due to a longer
term
overvoltage event.

If all the major input circuitry and components are ok I would suspect a
leaky
diode in the bridge rectifier.
Click to expand...
Presumably, you don't see too many small high current switchers ? This is a
small supply - probably around 6" x 3" x 1.5", and fully enclosed in a
plastic case. The cap in question was a CapXon 120uF at 450v, and a 105 deg
type, so a common enough make, more than adequately rated for the job. As is
ever the case, it is the way that the designer has used it, that has led to
its demise. It was layed over flat on the board, jammed in by the heatsink
that carries the secondary side reccies etc. This is a very typical
scenario, which anyone who works with them a lot, will tell you leads to
drying out, bulging, and ultimate failure.

I'm not sure where you got it from that a lightning event was thought to be
responsible for this bulging, Ross. I don't think I actually said that. The
supply was dead as received. When I opened it up, the cap was obviously
bulging, and very 'tired' looking, as I think I said. When checked, the two
Wickman fuses in the front end, were both open. Without thinking too much
about it, I associated the two 'problems', and went ahead and replaced all
three items. When the supply still didn't work, I checked the input volts,
and there wasn't any, because the plugtop fuse had also failed. It was only
then that I started to question exactly what the failure mechanism had been,
and how, or even *if* the cap had in some way been responsible. I came to
the conclusion that the two sets of failures might be unrelated. Clearly,
before the supply failed completely, the main filter cap must have been
working to a degree, allbeit in its bulged condition, which I'm quite sure
was a long term thing caused by poor ventillation and positioning. It really
*is* a very common occurence, which most often is due to poor mechanical
design of a supply, and rarely anything to do with defective bridges in my
day to day experience. I have of course had leaky diodes in bridges, but I
normally find that they tend to produce more catastrophic failures than just
a gently cooking cap. I must say that I didn't check the bridge specifically
for this. I guess it will soon come bouncing back, if there is any such
problem ... :)

For all I know, the cap might still have been working enough to run the
supply. As I had already replaced it, I guess we'll never know. When I
checked it, it was only on the ESR meter, and only a quick check, as it was
obviously in a bit of a state. It might not have been totally open, just
outside the measurement range of the meter. The only place then, that the
lightning came into it, was that I thought that as the cap might have been
at *almost* the point of total failure from its slow heat degeneration, a
large surge might have been enough to take it that last gasp to failure.

Thanks for you input anyway. It's all interesting stuff.

Arfa
 
Arfa Daily <arfa.daily@ntlworld.com> wrote in message
news:TVhpm.185288$156.21456@newsfe14.ams2...
"Jeff Liebermann" <jeffl@cruzio.com> wrote in message
news:v0daa5tnb7vik4n0b5k7pp9fitsfck75m1@4ax.com...
On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> wrote:

It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds.

Are those slo-blow fuses? If the fuse case is clear, it will show a
spring and a resitor inside.

If it is a slow blow, these take a while to open after seeing an
overload. It depends on the amount of overload and resistor heating.
There's a small delay during which time the other fuses are also
receiving the full benefit of the overload. Even if the load is
removed during this time, thermal inertia (the time it take for heat
to make it from the resistor to the fuse melt area) will cause the
other fuses to blow.

However, if it's NOT a slo-blow fuse, then there's insufficient mass
in the fuse wire to produce any thermal inertia effects, so this
mechanism is unlikely.



--
Jeff Liebermann jeffl@cruzio.com


Ok. Few points from various people to answer here. The lightning strike
scenario is a possibility. There have been a couple of storms in the area
in
the last few weeks, but in general, it's been a pretty quiet season here
in
the UK this year. The O/C main filter cap might just be a red herring
here.
I must admit that I replaced it at the same time as the fuses, as it was
obviously bulging both top and bottom, and read open on the ESR meter. It
could be that it was on its last legs ESR-wise, but still had enough
'guts'
to perform its basic function, until the surge that took the fuses out,
finished it off. That might add support to the lightning strike theory.

As to the fuse types. I take it the term "Wickman fuse" doesn't have any
meaning in the U.S.? If not, allow me to explain. Wickmans are a type of
wire-ended fuse for direct PCB mounting. They are a plastic cylinder about
10mm diameter and maybe 10 -12mm high. They have two leads coming out of
the
bottom pitched at probably 5mm. Imagine a small stubby radial leaded
electrolytic, if you like. The plastic is usually either black or brown.
They have always been popular in Philips gear, although other
manufacturers
do use them. They come in both fast and "T" rated types. The ones that
were
fitted in this particular supply, were "T" type, so yes, slo-blo's.
However,
with them being plastic, you get no indications of how they have failed. I
am of course very familiar with the three normal failure modes of
vapourised, melted, and broken, and if you could see how these had failed,
I'm sure it would have given a clue as to what was going on. To be honest,
I'm not even sure of how these fuses are constructed internally. I might
try
and break into one tomorrow to get a look.

As to the plugtop fuse, these are ceramic bodied, so again, you can't see
the failure mode. I guess you might not have ever even seen one in the
U.S.
I've no idea what the fusing characteristic of them is. They come in 3A,
5A,
10A and 13A flavours, but most people would consider them to be little
more
than a calibrated nail. I have seen them stand up to overloads where you
could almost hear the substation transformer jumping off its mountings,
and
yet they will also readily fail when a light bulb pops its clogs.

I quite like the thermal inertia idea as an explanation, but even though
the
Wickmans are "T" rated, I'm not sure that there is sufficient wire in that
little case (assuming that it is in some way a 'conventional' wire fuse
inside) to actually have that much inertia ...

Thanks all for your thoughts (including bz who responded (accidentally ?)
directly to me). It's an interesting little puzzle.

Arfa
Click to expand...
As far as mains glass-barrel fuses , its annoying when the owner throws away
the blown fuse.
Inspection can tell a lot, from
2 remaining sagged pieces of wire with a small break in the middle, could be
due to fair wear and tear in effect, sagging over time and current carrying
capacity going down over time.
via still extant wires at the ends and one molten ball , no blacking (x2 or
so overload)
via a few sputter balls in the tube , some blacking (x5 or so overload ?)
to totally black and cracked glass (> x10 overload?)

Its so long since I came across a Wickman I don't remember if I ground
into/cracked it to explore its construction/ failure character.



--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
 
Genrad 1659 RLC tester (Quadtech ) use them, which are American I think. Now
I remember it was very short duration overlaods, from static discharges,
that kept blowing them ,showing as very neat breaks as though cut with
endsnips.
After active stuff had been destroyed of course

If anyone knows the USA term I will add to my translator
http://www.divdev.fsnet.co.uk/tool_terms.htm


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
 
"Arfa Daily" <arfa.daily@ntlworld.com> wrote in message
news:STopm.22457$uC1.12185@newsfe18.ams2...
When checked, the two Wickman fuses in the front end, were both
open. Without thinking too much about it, I associated the two
'problems', and went ahead and replaced all three items. When the
supply still didn't work, I checked the input volts, and there
wasn't any, because the plugtop fuse had also failed. > Arfa
Arfa,
Click to expand...

I always used to check the plug top fuse first even though the
customer invariably said that they had tried the fuse. In my
experience customers never throw a ceramic plug top fuse away even the
blown ones. They usually end up in a drawer to be tried again just in
case it wasnt faulty! Not to mention the people who fit 13 Amp fuses
to everything, or worse, wrap the fuse in tinfoil.

Roy
 
N_Cook wrote:
If anyone knows the USA term I will add to my translator
http://www.divdev.fsnet.co.uk/tool_terms.htm
Click to expand...
I don't know of a standard term, but browsing through DigiKey
datasheets, the terms 'subminiature', 'micro', 'cartridge', and 'IEC
60127-3' are commonly used.

Littlefuse/Wickmann:
http://www.littelfuse.com/data/en/Data_Sheets/374_LF_011906.pdf
Littlefuse:
http://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/663SERIES.pdf
Belfuse:
http://www.belfuse.com/Data/Datasheets/MRT.pdf
Cooper/Bussmann:
http://www.cooperbussmann.com/pdf/1231b004-2213-4ce1-bcc3-a975cdc90d96.pdf
Schurter:
http://www.schurterinc.com/pdf/english/typ_mst_250.pdf

I've seen similar fuses on PCI extender/debug cards.

TM
 
On Tue, 8 Sep 2009 09:48:47 +0100, "Arfa Daily" <arfa.daily@ntlworld.com> wrote:

:
:"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
:news:6siba59ouaan92agjvoshtibvit2l670va@4ax.com...
SNIP

:>
:> I would be more concerned with what caused the filter cap to bulge...
:>
:> If it is a well known brand and correctly rated for the application then
:> it
:> should not have degenerated to that state as long as normal input voltage
:> range
:> was experienced. I also doubt that a possible short term lightning event
:> would
:> have caused the cap to bulge - this symptom is more likely due to a longer
:> term
:> overvoltage event.
:>
:> If all the major input circuitry and components are ok I would suspect a
:> leaky
:> diode in the bridge rectifier.
:
:presumably, you don't see too many small high current switchers ?

Not in the last few months. No. However I was repairing small and large SMPS
quite frequently in the past. The symptoms you described are exactly the same as
I found in small 5" x 3" format SMPS I recently repaired, although there was
only one series fuse and a current inrush limiter - both of which had died. The
main filter cap was also bulging, as in your case, and the fault was traced to a
dud diode in the bridge rectifier. The suggestion was made as a possibility
since I can't actually see the unit myself and all particulars were not
disclosed initially.

:This is a
:small supply - probably around 6" x 3" x 1.5", and fully enclosed in a
:plastic case. The cap in question was a CapXon 120uF at 450v, and a 105 deg
:type, so a common enough make, more than adequately rated for the job. As is
:ever the case, it is the way that the designer has used it, that has led to
:its demise. It was layed over flat on the board, jammed in by the heatsink
:that carries the secondary side reccies etc. This is a very typical
:scenario, which anyone who works with them a lot, will tell you leads to
:drying out, bulging, and ultimate failure.

The fully enclosed nature of your unit does help to explain a possible reason
for long term heating as a possible cause for the cap bulging. Pity you didn't
include those details before since it is well known that heat is a major killer
of caps in SMPS.

:
:I'm not sure where you got it from that a lightning event was thought to be
:responsible for this bulging, Ross. I don't think I actually said that.

The lightning suggestion was offered by greenpjs@neo.rr.com in his response and
that is what I was referring to.

:The
:supply was dead as received. When I opened it up, the cap was obviously
:bulging, and very 'tired' looking, as I think I said. When checked, the two
:Wickman fuses in the front end, were both open. Without thinking too much
:about it, I associated the two 'problems', and went ahead and replaced all
:three items. When the supply still didn't work, I checked the input volts,
:and there wasn't any, because the plugtop fuse had also failed. It was only
:then that I started to question exactly what the failure mechanism had been,
:and how, or even *if* the cap had in some way been responsible. I came to
:the conclusion that the two sets of failures might be unrelated. Clearly,
:before the supply failed completely, the main filter cap must have been
:working to a degree, allbeit in its bulged condition, which I'm quite sure
:was a long term thing caused by poor ventillation and positioning. It really
:*is* a very common occurence, which most often is due to poor mechanical
:design of a supply, and rarely anything to do with defective bridges in my
:day to day experience. I have of course had leaky diodes in bridges, but I
:normally find that they tend to produce more catastrophic failures than just
:a gently cooking cap. I must say that I didn't check the bridge specifically
:for this. I guess it will soon come bouncing back, if there is any such
:problem ... :)
:
:For all I know, the cap might still have been working enough to run the
:supply. As I had already replaced it, I guess we'll never know. When I
:checked it, it was only on the ESR meter, and only a quick check, as it was
:eek:bviously in a bit of a state. It might not have been totally open, just
:eek:utside the measurement range of the meter. The only place then, that the
:lightning came into it, was that I thought that as the cap might have been
:at *almost* the point of total failure from its slow heat degeneration, a
:large surge might have been enough to take it that last gasp to failure.
:
:Thanks for you input anyway. It's all interesting stuff.
:
:Arfa
:
 
Tony Matt <tonym@theworld.com> wrote in message
news:h86s1s$kn1$1@news.eternal-september.org...
N_Cook wrote:
If anyone knows the USA term I will add to my translator
http://www.divdev.fsnet.co.uk/tool_terms.htm

I don't know of a standard term, but browsing through DigiKey
datasheets, the terms 'subminiature', 'micro', 'cartridge', and 'IEC
60127-3' are commonly used.

Littlefuse/Wickmann:
http://www.littelfuse.com/data/en/Data_Sheets/374_LF_011906.pdf
Littlefuse:
http://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/663SERIES.pdf
Belfuse:
http://www.belfuse.com/Data/Datasheets/MRT.pdf
Cooper/Bussmann:
http://www.cooperbussmann.com/pdf/1231b004-2213-4ce1-bcc3-a975cdc90d96.pdf
Schurter:
http://www.schurterinc.com/pdf/english/typ_mst_250.pdf

I've seen similar fuses on PCI extender/debug cards.

TM
Click to expand...

I will go with
IEC 60127-3, Littelfuse 369 series

incidently
( Littelfuse & Wickman spellings)


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/
 
"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
news:05aea5poetv1gir68od7l5gh3hi914j48d@4ax.com...
On Tue, 8 Sep 2009 09:48:47 +0100, "Arfa Daily" <arfa.daily@ntlworld.com
wrote:

:
:"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
:news:6siba59ouaan92agjvoshtibvit2l670va@4ax.com...
SNIP

:
:> I would be more concerned with what caused the filter cap to bulge...
:
:> If it is a well known brand and correctly rated for the application
then
:> it
:> should not have degenerated to that state as long as normal input
voltage
:> range
:> was experienced. I also doubt that a possible short term lightning
event
:> would
:> have caused the cap to bulge - this symptom is more likely due to a
longer
:> term
:> overvoltage event.
:
:> If all the major input circuitry and components are ok I would suspect
a
:> leaky
:> diode in the bridge rectifier.
:
:presumably, you don't see too many small high current switchers ?

Not in the last few months. No. However I was repairing small and large
SMPS
quite frequently in the past. The symptoms you described are exactly the
same as
I found in small 5" x 3" format SMPS I recently repaired, although there
was
only one series fuse and a current inrush limiter - both of which had
died. The
main filter cap was also bulging, as in your case, and the fault was
traced to a
dud diode in the bridge rectifier. The suggestion was made as a
possibility
since I can't actually see the unit myself and all particulars were not
disclosed initially.

:This is a
:small supply - probably around 6" x 3" x 1.5", and fully enclosed in a
:plastic case. The cap in question was a CapXon 120uF at 450v, and a 105
deg
:type, so a common enough make, more than adequately rated for the job. As
is
:ever the case, it is the way that the designer has used it, that has led
to
:its demise. It was layed over flat on the board, jammed in by the
heatsink
:that carries the secondary side reccies etc. This is a very typical
:scenario, which anyone who works with them a lot, will tell you leads to
:drying out, bulging, and ultimate failure.

The fully enclosed nature of your unit does help to explain a possible
reason
for long term heating as a possible cause for the cap bulging. Pity you
didn't
include those details before since it is well known that heat is a major
killer
of caps in SMPS.
Click to expand...

Sorry Ross - wasn't getting at you. When I said "presumably you don't see
....etc" I was being quite genuine in asking. Although I didn't specifically
say that it was a fully enclosed type, I did say in the original post that
it was "...a small external switcher from an LCD TV." I assumed that anyone
who was regularly involved in the repair of such items, would immediately
recognise that description as being a small, plastic, fully enclosed type.
Interesting that you had identical symptoms on a similar one, and that you
actually found a leaky diode in the bridge. Can you from memory, put a value
on the amount of leakage ? I figure it must have been quite small, as any
leaky bridges I've had, have had a much more catastrophic effect than just
over-working the main filter cap for long enough to dry it out. I was
originally thinking U.S. 110v input and whether the reduced voltage might
allow such a leak to fester away without exploding anything, but thinking
about it some more, you're in Aus-land aren't you ? Line voltage nominally
230 - ish same as here in the U.K. ??


:
:I'm not sure where you got it from that a lightning event was thought to
be
:responsible for this bulging, Ross. I don't think I actually said that.

The lightning suggestion was offered by greenpjs@neo.rr.com in his
response and
that is what I was referring to.
Click to expand...
Ah, OK. I misunderstood you. I thought that you thought that I thought (
!!! ) that the lightning had led to the bulging. I was actually not
associating the bulging with any lightning event, but considering the
possibility that a surge on the line voltage from a nearby strike, as
suggested by the poster that you cite, might have been responsible for the
failure of the fuses, and for possibly 'finishing off' the cap which had
already suffered long term damage from overheating.

Again, sorry if you thought I was being arsey with you, me old mate. I
really wasn't. :)

Arfa
 
greenpjs@neo.rr.com wrote:
On Mon, 7 Sep 2009 13:11:51 +0100, "Arfa Daily"
arfa.daily@ntlworld.com> wrote:

How do three fuses fail simultaneously ? I have just repaired a small
external switcher from an LCD TV for one of my trade customers. 12V @ 5A -
you know the sort of thing. It had two 2.5A Wickman fuses just south of the
line input connector, so that's one each in the live and neutral feeds. It
also had a 3A plugtop fuse fitted, this being in the UK. So that's three
fuses all effectively in series. Two of them directly so, and one in the
other (return) side of the input circuit. All three were open, and the cause
of this appeared to be that the main filter cap was bulged and open,
although I'm not entirely sure why this should have led to excess input
current, as I have seen open main filter caps many times, and fuse failure
from this alone, is not the usual symptom.
However, vagaries of switcher behaviour aside, how can three fuses -
especially considering that one of them is a higher rated plugtop fuse, not
known for their accuracy - that are all in series, all be open ? Clearly,
as soon as one fails, in theory at least, all current through the other two
is interupted ...

As soon as the cap and three fuses were replaced, operation of the supply
was normal, and it powered the TV without issue. Thoughts anyone ?

Arfa

If you were here in the US midwest, I would say it was a nearby
lightning strike and the fuses kept arcing after the first one opened.
Click to expand...
That'd be my guess too. A nearby ground-strike will arc over a zapped
fuse with no trouble at all.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
 
Tony Matt wrote:
N_Cook wrote:
If anyone knows the USA term I will add to my translator
http://www.divdev.fsnet.co.uk/tool_terms.htm

I don't know of a standard term, but browsing through DigiKey
datasheets, the terms 'subminiature', 'micro', 'cartridge', and 'IEC
60127-3' are commonly used.

Littlefuse/Wickmann:
http://www.littelfuse.com/data/en/Data_Sheets/374_LF_011906.pdf
Littlefuse:
http://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/663SERIES.pdf
Belfuse:
http://www.belfuse.com/Data/Datasheets/MRT.pdf
Cooper/Bussmann:
http://www.cooperbussmann.com/pdf/1231b004-2213-4ce1-bcc3-a975cdc90d96.pdf
Schurter:
http://www.schurterinc.com/pdf/english/typ_mst_250.pdf

I've seen similar fuses on PCI extender/debug cards.
Click to expand...
They sound like the fuses that used to be common on SCSI cards. They
look very much like small value electrolytics, except hard plastic
cases, with the value on the top, rather than on the side.

--
W
. | ,. w , "Some people are alive only because
\|/ \|/ it is illegal to kill them." Perna condita delenda est
---^----^---------------------------------------------------------------
 
On Wed, 9 Sep 2009 09:37:54 +0100, "Arfa Daily" <arfa.daily@ntlworld.com> wrote:

:
:"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
:news:05aea5poetv1gir68od7l5gh3hi914j48d@4ax.com...
:> On Tue, 8 Sep 2009 09:48:47 +0100, "Arfa Daily" <arfa.daily@ntlworld.com>
:> wrote:
:>
:> :
:> :"Ross Herbert" <rherber1@bigpond.net.au> wrote in message
:> :news:6siba59ouaan92agjvoshtibvit2l670va@4ax.com...
:> SNIP
:>
:> :>
:> :> I would be more concerned with what caused the filter cap to bulge...
:> :>
:> :> If it is a well known brand and correctly rated for the application
:> then
:> :> it
:> :> should not have degenerated to that state as long as normal input
:> voltage
:> :> range
:> :> was experienced. I also doubt that a possible short term lightning
:> event
:> :> would
:> :> have caused the cap to bulge - this symptom is more likely due to a
:> longer
:> :> term
:> :> overvoltage event.
:> :>
:> :> If all the major input circuitry and components are ok I would suspect
:> a
:> :> leaky
:> :> diode in the bridge rectifier.
:> :
:> :presumably, you don't see too many small high current switchers ?
:>
:> Not in the last few months. No. However I was repairing small and large
:> SMPS
:> quite frequently in the past. The symptoms you described are exactly the
:> same as
:> I found in small 5" x 3" format SMPS I recently repaired, although there
:> was
:> only one series fuse and a current inrush limiter - both of which had
:> died. The
:> main filter cap was also bulging, as in your case, and the fault was
:> traced to a
:> dud diode in the bridge rectifier. The suggestion was made as a
:> possibility
:> since I can't actually see the unit myself and all particulars were not
:> disclosed initially.
:>
:> :This is a
:> :small supply - probably around 6" x 3" x 1.5", and fully enclosed in a
:> :plastic case. The cap in question was a CapXon 120uF at 450v, and a 105
:> deg
:> :type, so a common enough make, more than adequately rated for the job. As
:> is
:> :ever the case, it is the way that the designer has used it, that has led
:> to
:> :its demise. It was layed over flat on the board, jammed in by the
:> heatsink
:> :that carries the secondary side reccies etc. This is a very typical
:> :scenario, which anyone who works with them a lot, will tell you leads to
:> :drying out, bulging, and ultimate failure.
:>
:> The fully enclosed nature of your unit does help to explain a possible
:> reason
:> for long term heating as a possible cause for the cap bulging. Pity you
:> didn't
:> include those details before since it is well known that heat is a major
:> killer
:> of caps in SMPS.
:
:
:Sorry Ross - wasn't getting at you. When I said "presumably you don't see
:...etc" I was being quite genuine in asking. Although I didn't specifically
:say that it was a fully enclosed type, I did say in the original post that
:it was "...a small external switcher from an LCD TV." I assumed that anyone
:who was regularly involved in the repair of such items, would immediately
:recognise that description as being a small, plastic, fully enclosed type.
:Interesting that you had identical symptoms on a similar one, and that you
:actually found a leaky diode in the bridge. Can you from memory, put a value
:eek:n the amount of leakage ? I figure it must have been quite small, as any
:leaky bridges I've had, have had a much more catastrophic effect than just
:eek:ver-working the main filter cap for long enough to dry it out. I was
:eek:riginally thinking U.S. 110v input and whether the reduced voltage might
:allow such a leak to fester away without exploding anything, but thinking
:about it some more, you're in Aus-land aren't you ? Line voltage nominally
:230 - ish same as here in the U.K. ??

That's OK Arfa.. words alone don't convey facial expressions unfortunately. I
have certainly been guilty of unintentionally upsetting people with terse
remarks on the NG's :)

Actually, when the SMPS came to me the diode had actually gone fully short
circuit. I guessed that if it had catastrophically failed in one hit the
probable result would have been that the fuse and inrush limiter would have
failed rapidly and the cap would not have been overheated long enough to cause
bulging. I reasoned that the diode had started failing (become leaky) which
gradually applied overvoltage and incorrect polarity pulses to the cap which
caused it to overheat and begin bulging. Only when the diode had gone almost
short did it cause the fuse and inrush limiter to rupture but by then the cap
had also ad enough and it was not in good shape either.

Our mains voltage, while nominally quoted as 230V, actually sits around 255V in
most cases so components in SMPS here are stressed quite a bit.
:
:
:>
:> :
:> :I'm not sure where you got it from that a lightning event was thought to
:> be
:> :responsible for this bulging, Ross. I don't think I actually said that.
:>
:> The lightning suggestion was offered by greenpjs@neo.rr.com in his
:> response and
:> that is what I was referring to.
:
:Ah, OK. I misunderstood you. I thought that you thought that I thought (
:!!! ) that the lightning had led to the bulging. I was actually not
:associating the bulging with any lightning event, but considering the
:possibility that a surge on the line voltage from a nearby strike, as
:suggested by the poster that you cite, might have been responsible for the
:failure of the fuses, and for possibly 'finishing off' the cap which had
:already suffered long term damage from overheating.

Yes, it is possible that this sequence of events could have occurred. I
mistakenly assumed that given all operational conditions were normal, and based
on my experience, the cap would be in good order and not already bulging. I
reasoned that if an induced lightning splat did occur, it might blow the fuses
but it is unlikely it would have caused the bulging cap at that time. I failed
to take into account that since you hadn't seen the unit until after it had
completely failed you couldn't have known what sequence of events had
transpired. The cap may have been already bulging due to its normal operating
environment before the fatal over-current (whatever the cause) blew the fuses. I
sincerely apologise for this inexcusable oversight.

In most cases in my experience it is rare for failure of the main filter cap/s
following the bridge in universal SMPS to occur, since they are usually well
selected types for the application and physically large enough to dissipate any
heat generated in their vicinity. I think I can truthfully say that I have had
to change only about 2 such capacitors out of the several hundred small SMPS I
have repaired. In most cases the Tan-Delta of the insitu caps have exhibited
extremely low values even though they have already seen many years of service.

It is also not unreasonable that the cap in your unit is not as good as it
should be and this has lead to excessive heating and subsequent bulging.

I am sure that you would now have checked the individual diodes in the bridge
rectifier with your DVM and found them to be ok, hence your question on "how
much leakage" did I find in my case. In my case there was a shorted diode and I
reasoned it hadn't failed short circuit suddenly but had caused the bulging cap
over a period of time before finally failing S/C and taking out the fuse and
inrush limiter. I don't know if there was any lightning event at the time the
unit actually failed, but while this is also a possibility, I doubt that it
would have been the real cause of the failure.

If all your diodes test ok on the DVM I don't think anyone can know for certain
what the failure mechanism was in this case. And since testing for diode leakage
over the full working voltage range is not all that easy I can only suggest that
you change the bridge rectifier (or all individual diodes in the bridge) just to
be on the safe side.


:
:Again, sorry if you thought I was being arsey with you, me old mate. I
:really wasn't. :)
:
:Arfa
:
:
:

No problem Arfa...
 
:
:Again, sorry if you thought I was being arsey with you, me old mate. I
:really wasn't. :)
:
:Arfa
:
:
:

No problem Arfa...
Click to expand...
We are pretty much on the same page with this I think d;~}

Arfa
 
Bob Larter wrote:
Tony Matt wrote:
N_Cook wrote:
If anyone knows the USA term I will add to my translator
http://www.divdev.fsnet.co.uk/tool_terms.htm

I don't know of a standard term, but browsing through DigiKey
datasheets, the terms 'subminiature', 'micro', 'cartridge', and 'IEC
60127-3' are commonly used.

Littlefuse/Wickmann:
http://www.littelfuse.com/data/en/Data_Sheets/374_LF_011906.pdf
Littlefuse:
http://media.digikey.com/pdf/Data%20Sheets/Littelfuse%20PDFs/663SERIES.pdf
Belfuse:
http://www.belfuse.com/Data/Datasheets/MRT.pdf
Cooper/Bussmann:
http://www.cooperbussmann.com/pdf/1231b004-2213-4ce1-bcc3-a975cdc90d96.pdf
Schurter:
http://www.schurterinc.com/pdf/english/typ_mst_250.pdf

I've seen similar fuses on PCI extender/debug cards.

They sound like the fuses that used to be common on SCSI cards. They
look very much like small value electrolytics, except hard plastic
cases, with the value on the top, rather than on the side.
Click to expand...

That sounds like a 'Picofuse'.


--
You can't have a sense of humor, if you have no sense!
 
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