When a diode power bridge - discrete devices or four-legged package - fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the four
failing at random ?

Arfa

Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package - fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit.

No it isn't.

Any thoughts on why this should be, rather than any one of the four
failing at random ?

Your observation is obviously flawed. All the diodes pass the same current. By
chance you've simply experienced a statistically anomalous percentage of such
failures.

Graahm

"Eeyore" wrote in message
...


Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit.

No it isn't.

Any thoughts on why this should be, rather than any one of the four
failing at random ?

Your observation is obviously flawed. All the diodes pass the same
current. By
chance you've simply experienced a statistically anomalous percentage of
such
failures.

Graahm


Oh, sorry. I must be more stupid than I thought then. Do you honestly think
that if I had not genuinely observed this, that I would be taking the
trouble to post the question ? Sometimes, Graham (how long have you not been
able to spell your own name BTW ? ) you can be a real contentious prat.

Arfa

Arfa Daily wrote:

"Eeyore" wrote
Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails, it's almost invariably one diode in the 'negative' arm which goes
short
circuit.

No it isn't.

Any thoughts on why this should be, rather than any one of the four
failing at random ?

Your observation is obviously flawed. All the diodes pass the same
current. By chance you've simply experienced a statistically anomalous
percentage of
such failures.


Oh, sorry. I must be more stupid than I thought then. Do you honestly think
that if I had not genuinely observed this, that I would be taking the
trouble to post the question ? Sometimes, Graham (how long have you not been
able to spell your own name BTW ? ) you can be a real contentious prat.

And what makes you so sure you haven't just experienced a statistical anomaly ?

I mean, you'd have to believe in black magic or voodoo / whatever otherwise.
There's certainly no possible scientific explanation.

What amazes me as much as anything is that you'd keep a record of this !

Graham

On Sep 18, 6:47 am, Eeyore
wrote:
Arfa Daily wrote:
"Eeyore" wrote
Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails, it's almost invariably one diode in the 'negative' arm which goes
short
circuit.

No it isn't.

Any thoughts on why this should be, rather than any one of the four
failing at random ?

Your observation is obviously flawed. All the diodes pass the same
current. By chance you've simply experienced a statistically anomalous
percentage of
such failures.

Oh, sorry. I must be more stupid than I thought then. Do you honestly think
that if I had not genuinely observed this, that I would be taking the
trouble to post the question ? Sometimes, Graham (how long have you not been
able to spell your own name BTW ? ) you can be a real contentious prat.

And what makes you so sure you haven't just experienced a statistical anomaly ?

I mean, you'd have to believe in black magic or voodoo / whatever otherwise.
There's certainly no possible scientific explanation.

What amazes me as much as anything is that you'd keep a record of this !

Graham- Hide quoted text -

- Show quoted text -

Graham:

You obviously don't know very nmuch about electronic componentgs and
circuits. It is very possible that there are components that are
associated with the bridge, either on the input or output side, that
are not completely symmetrical and that might lead to Arfa's
observations.

I can think of several components, such as small value bypass
capacitors, bleed resistors, that are not cymmetrical, and that could
have an effect, especially during surge events.
Look at any TV schematic and you should be able to find these
components.

Don't dismiss someone else's observations with such disdain when you
don't have firm evidence that you are 100% correct.
I have been doing electronics servicing for 50 years, from vacuum
tubes to IC's, and there are many strange things that I have seen, but
closer investigation has usually allowed me to figure out what the
likely cause was, sometimes not at all what things seemed to be at
first glance.

H. R. Hofmann

"Eeyore" wrote in message
...


Arfa Daily wrote:

"Eeyore" wrote
Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails, it's almost invariably one diode in the 'negative' arm which
goes
short
circuit.

No it isn't.

Any thoughts on why this should be, rather than any one of the four
failing at random ?

Your observation is obviously flawed. All the diodes pass the same
current. By chance you've simply experienced a statistically anomalous
percentage of
such failures.


Oh, sorry. I must be more stupid than I thought then. Do you honestly
think
that if I had not genuinely observed this, that I would be taking the
trouble to post the question ? Sometimes, Graham (how long have you not
been
able to spell your own name BTW ? ) you can be a real contentious prat.

And what makes you so sure you haven't just experienced a statistical
anomaly ?

I mean, you'd have to believe in black magic or voodoo / whatever
otherwise.
There's certainly no possible scientific explanation.

What amazes me as much as anything is that you'd keep a record of this !

Graham


Well, all right then. Just *what* qualifies *you* to tell *me* that I'm
wrong ? I have probably changed more bridge rectifiers in the 35 years that
I've been repairing stuff for a living, than you have ever even seen. Do you
think that you are teaching me something by coming out with 'big' words like
"statistical" and "anomaly" ? And what qualifies *you* to be so certain that
there is "no possible scientific explanation" ? Explain why it's always the
neutral terminal screw that comes loose in a plugtop, for instance. Same
current flows in both pins.

I don't "keep a record" of this. If you knew the first thing about the
practical world of service, instead of just pretending that you do all the
time, you would know that this is just the sort of thing that sticks in a
proper service engineer's head. As it happens, the reason that I brought it
up was that a colleague of mine, just last week, made the comment to me, and
I agreed with him. Then, this morning, the very first job on the bench, had
a faulty bridge in it, and what d'ya know - it's one of the diodes in the
negative arm again that's short circuit.

Whilst the magnitude of the current in all diodes is theoretically the
same - and even that might not be quite true if there is any residual DC
magnetism in the core of the transformer - the current which flows in the
diodes in the negative arm, flows in the opposite direction to that in the
positive arm. Also, the arse end of the diodes in the negative arm,
typically go to the chassis mass, which may well be tied to power ground
(mains earth) so I think that there might very well be a "possible
scientific explanation".

Anyone with a better understanding of the real world than Graham care to
comment ?

Arfa

"hr(bob) " wrote:

Graham:

You obviously don't know very nmuch about electronic componentgs and
circuits.

It must be my imagination that I'm an electronics design consultant in that case !


It is very possible that there are components that are
associated with the bridge, either on the input or output side, that
are not completely symmetrical and that might lead to Arfa's
observations.

Name ONE !


I can think of several components, such as small value bypass
capacitors, bleed resistors, that are not cymmetrical,

Utter RUBBISH ! There is nothing asymmetrical about any such parts.


and that could have an effect, especially during surge events.

Nope.


Look at any TV schematic and you should be able to find these
components.

Why TV ?


Don't dismiss someone else's observations with such disdain when you
don't have firm evidence that you are 100% correct.
I have been doing electronics servicing for 50 years, from vacuum
tubes to IC's, and there are many strange things that I have seen, but
closer investigation has usually allowed me to figure out what the
likely cause was, sometimes not at all what things seemed to be at
first glance.

If you look closely enough there will normally be a perfectly scientific exaplanation
waiting to be found.

Statistical anomalies included.

Graham

Arfa Daily wrote:

Explain why it's always the neutral terminal screw that comes loose in a
plugtop, for instance.

Is it ? Is this a 'well known fact' or something ? Are you actually serious ?

Graham

Arfa Daily wrote:

Whilst the magnitude of the current in all diodes is theoretically the
same

It IS the same.

- and even that might not be quite true if there is any residual DC
magnetism in the core of the transformer

Any residual magnetism won't affect the load current one tiny bit.


- the current which flows in the diodes in the negative arm, flows in the
opposite direction to that in the
positive arm.

So what ?


Also, the arse end of the diodes in the negative arm,
typically go to the chassis mass, which may well be tied to power ground
(mains earth) so I think that there might very well be a "possible
scientific explanation".

No, that would be voodoo thinking.

Check out Kirchoff's Laws.

Graham

Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package - fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the four
failing at random ?


An interesting observation; some thoughtful speculation and perhaps a few
experiments may be useful.

1. perhaps the dominant failure mode in some equipment happens on the
negative half cycle of input power, especially in switching applications.

2. perhaps in some areas, power line glitches occur more often on the
negative half cycle.

3. perhaps the devices are manufactured with a deviation of thermal
characteristics in the region of the negative arm making them more
prone to failure there.

It would make an interesting science project for a student to accumulate
a number of encapsulated bridge rectifiers (I'll leave it to others
to outline the statistics) and build a shorting fixture that randomly
shorts the device (in a symmetrical fashion, making sure that it is not
synchronized with the power line) and run the experiment. If the
anomaly is verified, then deeper investigation is in order such as
microscopic examination of intact good devices and post mortems on
bad ones. What with the _vast_ amount of trashed electronics these
days, it shouldn't be difficult to accumulate hundreds of rectifiers
for the endeavor.

Also a 'net search on "bridge rectifier" "failure modes" may turn
up something.

Regards,

Michael

Arfa Daily wrote:
Explain why it's always the
neutral terminal screw that comes loose in a plugtop, for instance. Same
current flows in both pins.

I'd say the reason is mechanical.

The neutral pin shakes about by the action of the (UK) plug being placed
in / removed from the socket. This pin probably shakes about more than
the earth pin, as the user is more careful to locate that pin first
before slamming the rest in.

The Live connection is at the other side of the fuseholder which takes
up some of the movement energy.

--
Adrian C

"Eeyore" wrote in message
...


Arfa Daily wrote:

Explain why it's always the neutral terminal screw that comes loose in a
plugtop, for instance.

Is it ? Is this a 'well known fact' or something ? Are you actually
serious ?

Graham


Deadly. I check the mains plug on every bit of kit that comes across my
bench, and in at least 50% of cases, one terminal screw will be loose, and
that is *almost* invariably the neutral one. There has been discussion about
this in trade magazines, so amongst proper service engineers, it would seem
to be well known.

Arfa

"Eeyore" wrote in message
...


"hr(bob) " wrote:

Graham:

You obviously don't know very nmuch about electronic componentgs and
circuits.

It must be my imagination that I'm an electronics design consultant in
that case !


It is very possible that there are components that are
associated with the bridge, either on the input or output side, that
are not completely symmetrical and that might lead to Arfa's
observations.

Name ONE !


I can think of several components, such as small value bypass
capacitors, bleed resistors, that are not cymmetrical,

Utter RUBBISH ! There is nothing asymmetrical about any such parts.


and that could have an effect, especially during surge events.

Nope.


Look at any TV schematic and you should be able to find these
components.

Why TV ?


Don't dismiss someone else's observations with such disdain when you
don't have firm evidence that you are 100% correct.
I have been doing electronics servicing for 50 years, from vacuum
tubes to IC's, and there are many strange things that I have seen, but
closer investigation has usually allowed me to figure out what the
likely cause was, sometimes not at all what things seemed to be at
first glance.

If you look closely enough there will normally be a perfectly scientific
exaplanation
waiting to be found.

Statistical anomalies included.

Graham


Ah, so "statistical anomalies" *are* a scientific reason now then. I thought
you said 5 minutes ago that "There's certainly no possible scientific
explanation". Make your mind up.

Arfa

Arfa Daily wrote in message
...

"Eeyore" wrote in message
...


Arfa Daily wrote:

Explain why it's always the neutral terminal screw that comes loose in
a
plugtop, for instance.

Is it ? Is this a 'well known fact' or something ? Are you actually
serious ?

Graham


Deadly. I check the mains plug on every bit of kit that comes across my
bench, and in at least 50% of cases, one terminal screw will be loose, and
that is *almost* invariably the neutral one. There has been discussion
about
this in trade magazines, so amongst proper service engineers, it would
seem
to be well known.

Arfa



There's a simple explanation for that one.
The neutral retaining screw is connected to the prong that goes in and out
of socket each time and vibrated each time.
The live one is decoupled from a lot of that vibration by the fuse.

But why do christmas tree lights always fail to light when reused the next
year but were fine before packing away, at least one bulb is always loosened
over that 11 months. ?

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

Arfa Daily wrote:

"Eeyore" wrote
Arfa Daily wrote:

Explain why it's always the neutral terminal screw that comes loose in a
plugtop, for instance.

Is it ? Is this a 'well known fact' or something ? Are you actually
serious ?

Deadly. I check the mains plug on every bit of kit that comes across my
bench, and in at least 50% of cases, one terminal screw will be loose, and
that is *almost* invariably the neutral one. There has been discussion about
this in trade magazines, so amongst proper service engineers, it would seem
to be well known.

Adrian C's explanation sounds quite convincing.

Graham

Arfa Daily wrote:

Ah, so "statistical anomalies" *are* a scientific reason now then. I thought
you said 5 minutes ago that "There's certainly no possible scientific
explanation". Make your mind up.

Statistical anomalies are simply a fact of life.

It's why if you toss a coin 100 times, it's no freak if it comes up heads 75
times and tails 25 times for sake of argument..

Graham

N Cook wrote:

But why do christmas tree lights always fail to light when reused the next
year but were fine before packing away, at least one bulb is always loosened
over that 11 months. ?

Vibration....

It is a sad ;-( moment when the feast of Christmas has finished, so the
lights are gently, carefully and slowly placed into the original
wrapping (cardboard cutouts to hold each lamp are a bit fiddly to do in
a rush). When the time comes to have them out again - bash, crash,
wallop, ping....

--
Adrian C

Arfa Daily wrote:
When a diode power bridge - discrete devices or four-legged package - fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the four
failing at random ?

Arfa


Maybe the same reason that it`s always the negative wire from a battery
pack that corrodes away?

Ron(UK)

Arfa Daily wrote:
When a diode power bridge - discrete devices or four-legged package - fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the four
failing at random ?

Arfa


Ok Arfa,

I think now that I have posted physical theories to the last two
(plugtop; xmas light) I might have a similar *physical* answer to the
diode anomaly.

Sinking away heat from the diode PN junction is more efficient with a
large amount of ground plane to connect to which is the case with the
negative half of the bridge. These diodes will run with a higher Vf and
hence a higher power dissipation than the remaining diodes on the bridge
given that both positive and negative halves will be conducting the same
current. Where fault conditions exist on the DC side of the circuit,
these harder running diodes may be the first to fail?

Dunno.

On the other hand, I tried looking for conditions for thermal runaway in
bridge rectifier diodes and came up with the following link
http://www.freepatentsonline.com/6731030.html
but can't say from that which half of the bridge would be suceptable to
shorted failure.

--
Adrian C

"Eeyore" wrote in message
...


Arfa Daily wrote:

"Eeyore" wrote
Arfa Daily wrote:

Explain why it's always the neutral terminal screw that comes loose in
a
plugtop, for instance.

Is it ? Is this a 'well known fact' or something ? Are you actually
serious ?

Deadly. I check the mains plug on every bit of kit that comes across my
bench, and in at least 50% of cases, one terminal screw will be loose,
and
that is *almost* invariably the neutral one. There has been discussion
about
this in trade magazines, so amongst proper service engineers, it would
seem
to be well known.

Adrian C's explanation sounds quite convincing.

Graham


I think it was actually Mr Cook who ventured the detailed explanation, with
Adrian's being just "vibration", but yes, it does indeed seem reasonable. So
you see, with a bit of thought, not everything is as black and white as you
suppose. Before seing an explanation that appealed to you as a possibility,
you had already started to rubbish me on that one too ...

Arfa

One wild guess-- the negative side diodes are likely to be better heat-
sunk by their chassis side leads.

That means there is a larger temperature drop across the diode,
perhaps leading to more stress on the junction.


The positive side diodes are more likely to be nice and warm, about
equally on both sides of the junction.

Adrian C wrote:

Arfa Daily wrote:
When a diode power bridge - discrete devices or four-legged package - fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the four
failing at random ?


I think now that I have posted physical theories to the last two
(plugtop; xmas light) I might have a similar *physical* answer to the
diode anomaly.

Sinking away heat from the diode PN junction is more efficient with a
large amount of ground plane to connect to which is the case with the
negative half of the bridge. These diodes will run with a higher Vf and
hence a higher power dissipation than the remaining diodes on the bridge
given that both positive and negative halves will be conducting the same
current. Where fault conditions exist on the DC side of the circuit,
these harder running diodes may be the first to fail?

I can blow that one away instantly.

If there is a larger amount of pcb foil, it'll help cool those 2 diodes. Since
they fail from over heating, that would suggest the positive diodes ought to fail
first.

Graham

In article ,
msg wrote:

Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the
four
failing at random ?


An interesting observation; some thoughtful speculation and perhaps a few
experiments may be useful.

1. perhaps the dominant failure mode in some equipment happens on the
negative half cycle of input power, especially in switching applications.

Doubtful for switchmode power supplies, as the switching regulator
generally operates asynchronously to the line frequency. Possible for
other switching control methods (e.g. triac or SCR phase control
circuits), but most applications of those don't tend to involve bridge
rectifiers.

2. perhaps in some areas, power line glitches occur more often on the
negative half cycle.

That's very unlikely to be the case in the USA, where the two halves of
the usual 110/220 residential power feed are at 180 degree phase angles
to each other. Half of the devices in any average house would see a
positive glitch, and half a negative glitch, of approximately the same
magnitude and form.

Things may be different in e.g. the UK where the typical household power
supply isn't composed of two out-of-phase legs. Are the secondaries of
UK power distribution transformers center-tapped 440V windings, with
different houses getting opposite phases, or just a single 220V winding?

3. perhaps the devices are manufactured with a deviation of thermal
characteristics in the region of the negative arm making them more
prone to failure there.

Possible.

It would be helpful to determine just what the usual failure modes are.
If the initial event causing the destruction of the rectifier is a
voltage spike, it implies that sudden spikes on the anode are better
handled than sudden spikes on the cathode of the rectifier, the other
leg being held (very roughly speaking) at a constant voltage. I'm not
sure of a good reason why that would be, but I guess it's not really
surprising that an asymmetric component behaves asymmetrically when
subjected to unusual stresses.

An interesting question indeed.

--
Andrew Erickson

"He is no fool who gives what he cannot keep to gain that which he cannot
lose." -- Jim Elliot

Andrew Erickson wrote:

Are the secondaries of UK power distribution transformers center-tapped 440V
windings, with
different houses getting opposite phases, or just a single 220V winding?

230V actually and quite often still 240V which is what is was in the first place
before Brussels decided it had to change for harmonisation reasons..

The 230V supply is actually a single phase of a 415V ? 3 phase supply. I believe
that the houses in a street are connected to the phases sequentially so the first
property will be on 'red' phase, the next on 'blue', then 'yellow' them back to red
again and so on.

Graham

"Andrew Erickson" wrote in message
...
In article ,
msg wrote:

Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the
four
failing at random ?


An interesting observation; some thoughtful speculation and perhaps a few
experiments may be useful.

1. perhaps the dominant failure mode in some equipment happens on the
negative half cycle of input power, especially in switching
applications.

Doubtful for switchmode power supplies, as the switching regulator
generally operates asynchronously to the line frequency. Possible for
other switching control methods (e.g. triac or SCR phase control
circuits), but most applications of those don't tend to involve bridge
rectifiers.

2. perhaps in some areas, power line glitches occur more often on the
negative half cycle.

That's very unlikely to be the case in the USA, where the two halves of
the usual 110/220 residential power feed are at 180 degree phase angles
to each other. Half of the devices in any average house would see a
positive glitch, and half a negative glitch, of approximately the same
magnitude and form.

Things may be different in e.g. the UK where the typical household power
supply isn't composed of two out-of-phase legs. Are the secondaries of
UK power distribution transformers center-tapped 440V windings, with
different houses getting opposite phases, or just a single 220V winding?

3. perhaps the devices are manufactured with a deviation of thermal
characteristics in the region of the negative arm making them more
prone to failure there.

Possible.

It would be helpful to determine just what the usual failure modes are.
If the initial event causing the destruction of the rectifier is a
voltage spike, it implies that sudden spikes on the anode are better
handled than sudden spikes on the cathode of the rectifier, the other
leg being held (very roughly speaking) at a constant voltage. I'm not
sure of a good reason why that would be, but I guess it's not really
surprising that an asymmetric component behaves asymmetrically when
subjected to unusual stresses.

An interesting question indeed.

--
Andrew Erickson

Some interesting thoughts from you and Michael. About the only thing that I
would say is that it seems to be an 'in use' problem of the reccies, rather
than something caused by a downstream failure of another component. I don't
think that I can actually remember ever having a bridge failure - discrete
diode or integrated 4-pin - that had occured in tandem with some other
problem. If a discrete diode bridge has a single diode that's failed, and
there are caps across the diodes, I always replace these as a matter of
course though, just in case, as well as the other three diodes.

One particular commercial board that I work on, has a perfectly conventional
transformer - bridge - resevoir setup, although the cap is separated from
the positive terminal of the bridge by a further diode, leaving a large
ripple at that terminal, which is scaled and then goes off to a micro on the
machine control board, presumably as some kind of sync or zero crossing
signal. The bridge is perfectly well rated for the job in hand, although it
does run quite hot. I repair around 25 of these boards a week, and I would
say that I replace at least one bridge a month. The problem is always a
short circuit diode in the bridge, and I can't remember the last time, if
ever, that it was one of the pair in the positive arm, so that's how common
it seems to be in this particular piece of equipment. Remember also, that
this question was brought up by my colleague, completely unsolicited by me,
and he works mainly on all types of TV set - CRT, back projection, plasma
and LCD, and also VCRs, so if he has experienced a similar situation on the
equipment that he works on, you would have to say that my board is tending
to be a rule rather than an exception.

The board in question does drive some DC motors with brushgear, so sparks
abound, particularly when there is a problem with them, or they are
overloaded by incorrect customer cleaning of the mechanical component that
they drive, so either of those factors could have a hand in the bridge
failing in the first place, but still interesting as to why it always seems
to be the negative arm that fails.

Arfa

On Sep 18, 7:04 pm, "Arfa Daily" wrote:
"Andrew Erickson" wrote in message

...





In article ,
msg wrote:

Arfa Daily wrote:

When a diode power bridge - discrete devices or four-legged package -
fails,
it's almost invariably one diode in the 'negative' arm which goes short
circuit. Any thoughts on why this should be, rather than any one of the
four
failing at random ?

An interesting observation; some thoughtful speculation and perhaps a few
experiments may be useful.

1. perhaps the dominant failure mode in some equipment happens on the
negative half cycle of input power, especially in switching
applications.

Doubtful for switchmode power supplies, as the switching regulator
generally operates asynchronously to the line frequency. Possible for
other switching control methods (e.g. triac or SCR phase control
circuits), but most applications of those don't tend to involve bridge
rectifiers.

2. perhaps in some areas, power line glitches occur more often on the
negative half cycle.

That's very unlikely to be the case in the USA, where the two halves of
the usual 110/220 residential power feed are at 180 degree phase angles
to each other. Half of the devices in any average house would see a
positive glitch, and half a negative glitch, of approximately the same
magnitude and form.

Things may be different in e.g. the UK where the typical household power
supply isn't composed of two out-of-phase legs. Are the secondaries of
UK power distribution transformers center-tapped 440V windings, with
different houses getting opposite phases, or just a single 220V winding?

3. perhaps the devices are manufactured with a deviation of thermal
characteristics in the region of the negative arm making them more
prone to failure there.

Possible.

It would be helpful to determine just what the usual failure modes are.
If the initial event causing the destruction of the rectifier is a
voltage spike, it implies that sudden spikes on the anode are better
handled than sudden spikes on the cathode of the rectifier, the other
leg being held (very roughly speaking) at a constant voltage. I'm not
sure of a good reason why that would be, but I guess it's not really
surprising that an asymmetric component behaves asymmetrically when
subjected to unusual stresses.

An interesting question indeed.

--
Andrew Erickson

Some interesting thoughts from you and Michael. About the only thing that I
would say is that it seems to be an 'in use' problem of the reccies, rather
than something caused by a downstream failure of another component. I don't
think that I can actually remember ever having a bridge failure - discrete
diode or integrated 4-pin - that had occured in tandem with some other
problem. If a discrete diode bridge has a single diode that's failed, and
there are caps across the diodes, I always replace these as a matter of
course though, just in case, as well as the other three diodes.

One particular commercial board that I work on, has a perfectly conventional
transformer - bridge - resevoir setup, although the cap is separated from
the positive terminal of the bridge by a further diode, leaving a large
ripple at that terminal, which is scaled and then goes off to a micro on the
machine control board, presumably as some kind of sync or zero crossing
signal. The bridge is perfectly well rated for the job in hand, although it
does run quite hot. I repair around 25 of these boards a week, and I would
say that I replace at least one bridge a month. The problem is always a
short circuit diode in the bridge, and I can't remember the last time, if
ever, that it was one of the pair in the positive arm, so that's how common
it seems to be in this particular piece of equipment. Remember also, that
this question was brought up by my colleague, completely unsolicited by me,
and he works mainly on all types of TV set - CRT, back projection, plasma
and LCD, and also VCRs, so if he has experienced a similar situation on the
equipment that he works on, you would have to say that my board is tending
to be a rule rather than an exception.

The board in question does drive some DC motors with brushgear, so sparks
abound, particularly when there is a problem with them, or they are
overloaded by incorrect customer cleaning of the mechanical component that
they drive, so either of those factors could have a hand in the bridge
failing in the first place, but still interesting as to why it always seems
to be the negative arm that fails.

Arfa- Hide quoted text -

- Show quoted text -

It seems like Graham's been pretty well subdued by most of the
commenters who see a reasonable number of reasons why one diodes on
one side of the bridge might fail more often. Not that it couldn't be
a statistical anomaly, but there are other possibilities too, and I
hope Graham now understands his trashing response was out of line. We
need helpful comments, not trashing ones.

H. R. Hofmann

On Sep 18, 3:44 pm, Eeyore
wrote:
Andrew Erickson wrote:
Are the secondaries of UK power distribution transformers center-
tapped 440V
windings, with
different houses getting opposite phases, or just a single 220V
winding?

230V actually and quite often still 240V which is what is was in
the first place
before Brussels decided it had to change for harmonisation
reasons..

The 230V supply is actually a single phase of a 415V ? 3 phase
supply. I believe
that the houses in a street are connected to the phases
sequentially so the first
property will be on 'red' phase, the next on 'blue', then 'yellow'
them back to red
again and so on.

Graham

So every residence gets its _own_ pole pig? Where I live in southern
Cal, there are many houses on 1 phase and 10 houses share a pole pig.

GG

"hr(bob) " wrote:

It seems like Graham's been pretty well subdued by most of the
commenters who see a reasonable number of reasons why one diodes on
one side of the bridge might fail more often. Not that it couldn't be
a statistical anomaly, but there are other possibilities too, and I
hope Graham now understands his trashing response was out of line. We
need helpful comments, not trashing ones.

Most of the comments that suggested a relationship seemed to be clutching at
straws quite frankly.

The best explanation I saw was the influence of large areas of copper foil on the
pcb. That will help to cool the diode on that 'leg' and a cool diode is less
likely to fail than a hot one.

However, the idea that there's always more foil on the negative terminal seems
spurious to me. I certainly don't lay out pcbs like that. Your own experience may
differ of course.

Graham

wrote:

Eeyore wrote:
Andrew Erickson wrote:
Are the secondaries of UK power distribution transformers center-
tapped 440V windings, with different houses getting opposite phases,
or just a single 220V winding?

230V actually and quite often still 240V which is what is was in
the first place before Brussels decided it had to change for harmonisation
reasons..

The 230V supply is actually a single phase of a 415V ? 3 phase
supply. I believe that the houses in a street are connected to the phases
sequentially so the first property will be on 'red' phase, the next on
'blue',
then 'yellow' them back to red again and so on.


So every residence gets its _own_ pole pig? Where I live in southern
Cal, there are many houses on 1 phase and 10 houses share a pole pig.

No.

All urban residential circuits are fed by underground cables. The distribution
is, as I explained at 230/415V. Even the majority of telephone/cable TV etc
circuits are underground.

Rural areas here do sometimes have overhead lines and 'pole pigs'.

Graha

wrote:
So every residence gets its _own_ pole pig? Where I live in southern
Cal, there are many houses on 1 phase and 10 houses share a pole pig.

Yeah, but in So Cal, everybody believes that (1) there is no God and
(2) electricity comes from God.

--
If you really believe carbon dioxide causes global warming,
you should stop exhaling.

On Tue, 18 Sep 2007 14:56:02 +0100, Eeyore
wrote:



"hr(bob) " wrote:

Graham:

You obviously don't know very nmuch about electronic componentgs and
circuits.

It must be my imagination that I'm an electronics design consultant in that case !

---
Truer words were never spoken!


--
JF

Arfa Daily wrote:

Oh, sorry. I must be more stupid than I thought then. Do you honestly think
that if I had not genuinely observed this, that I would be taking the
trouble to post the question ? Sometimes, Graham (how long have you not been
able to spell your own name BTW ? ) you can be a real contentious prat.


Only on his good days. If you tell him the sky is blue, he'll insist
that it's orange.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

John Fields wrote:

On Tue, 18 Sep 2007 14:56:02 +0100, Eeyore
wrote:



"hr(bob) " wrote:

Graham:

You obviously don't know very nmuch about electronic componentgs and
circuits.

It must be my imagination that I'm an electronics design consultant in that case !


Even a donkey slips up and gets something right, once in a blue moon.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

N Cook wrote:

There's a simple explanation for that one.
The neutral retaining screw is connected to the prong that goes in and out
of socket each time and vibrated each time.
The live one is decoupled from a lot of that vibration by the fuse.

But why do christmas tree lights always fail to light when reused the next
year but were fine before packing away, at least one bulb is always loosened
over that 11 months. ?


Planned obsolecence. Most people are too lazy to find a bad bulb, so
they get to sell a new string. ;-)

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida