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Bridge Failures ...

Discussion in 'Electronic Repair' started by Arfa Daily, Sep 18, 2007.

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  1. Arfa Daily

    Arfa Daily Guest

    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
     
  2. Eeyore

    Eeyore Guest

    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
     
  3. Arfa Daily

    Arfa Daily Guest

    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
     
  4. Eeyore

    Eeyore Guest

    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
     
  5. hr(bob)

    hr(bob) Guest

    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
     
  6. Arfa Daily

    Arfa Daily Guest

    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
     
  7. Eeyore

    Eeyore Guest

    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
     
  8. Eeyore

    Eeyore Guest

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

    Graham
     
  9. Eeyore

    Eeyore Guest

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

    So what ?

    No, that would be voodoo thinking.

    Check out Kirchoff's Laws.

    Graham
     
  10. msg

    msg Guest

    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
     
  11. Adrian C

    Adrian C Guest

    Arfa Daily wrote:
    Explain why it's always the
    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.
     
  12. Arfa Daily

    Arfa Daily Guest

    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
     
  13. Arfa Daily

    Arfa Daily Guest

    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
     
  14. N Cook

    N Cook Guest

    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. ?
     
  15. Eeyore

    Eeyore Guest

    Adrian C's explanation sounds quite convincing.

    Graham
     
  16. Eeyore

    Eeyore Guest

    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
     
  17. Adrian C

    Adrian C Guest

    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....
     
  18. Ron(UK)

    Ron(UK) Guest

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

    Ron(UK)
     
  19. Adrian C

    Adrian C Guest

    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.
     
  20. Arfa Daily

    Arfa Daily Guest

    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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