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Bridge Failures ...

A

Arfa Daily

Jan 1, 1970
0
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
 
E

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
No it isn't.


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
 
E

Eeyore

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.

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
 
H

hr(bob) [email protected]

Jan 1, 1970
0
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
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
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
 
E

Eeyore

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

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
E

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
M

msg

Jan 1, 1970
0
Arfa said:
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
 
A

Adrian C

Jan 1, 1970
0
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.
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
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
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
It must be my imagination that I'm an electronics design consultant in
that case !



Name ONE !



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



Why TV ?



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
 
N

N Cook

Jan 1, 1970
0
Arfa Daily said:
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. ?
 
E

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
E

Eeyore

Jan 1, 1970
0
Arfa said:
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
 
A

Adrian C

Jan 1, 1970
0
N said:
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....
 
R

Ron(UK)

Jan 1, 1970
0
Arfa said:
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)
 
A

Adrian C

Jan 1, 1970
0
Arfa said:
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.
 
A

Arfa Daily

Jan 1, 1970
0
Eeyore said:
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
 
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