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Bridge Failures ...

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

  1. 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.
     
  2. Eeyore

    Eeyore Guest

    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
     
  3. Meat Plow

    Meat Plow Guest

    You can blow something away with an "if" answer?
    Sorry Graham I just have to laugh.
     
  4. 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.
     
  5. Eeyore

    Eeyore Guest

    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
     
  6. Arfa Daily

    Arfa Daily Guest

    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
     
  7. hr(bob)

    hr(bob) Guest

     
  8. Guest

    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
     
  9. Eeyore

    Eeyore Guest

    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
     
  10. Eeyore

    Eeyore Guest

    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
     
  11. clifto

    clifto Guest

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

    John Fields Guest

     

  13. 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
     
  14. 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
     

  15. 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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