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Bridge Failures ...

A

Ancient_Hacker

Jan 1, 1970
0
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.
 
E

Eeyore

Jan 1, 1970
0
Adrian said:
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
 
M

Meat Plow

Jan 1, 1970
0
I can blow that one away instantly.

If <snip>

You can blow something away with an "if" answer?
Sorry Graham I just have to laugh.
 
A

Andrew Erickson

Jan 1, 1970
0
msg said:
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.
 
E

Eeyore

Jan 1, 1970
0
Andrew said:
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
 
A

Arfa Daily

Jan 1, 1970
0
Andrew Erickson said:
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.


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?


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.

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
 
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
 
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
 
E

Eeyore

Jan 1, 1970
0
hr(bob) [email protected] said:
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
 
E

Eeyore

Jan 1, 1970
0
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
 
C

clifto

Jan 1, 1970
0
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.
 
M

Michael A. Terrell

Jan 1, 1970
0
Arfa said:
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
 
M

Michael A. Terrell

Jan 1, 1970
0
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
 
M

Michael A. Terrell

Jan 1, 1970
0
N said:
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
 
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