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Bridge rectifier replacement

Discussion in 'Electronic Repair' started by [email protected], Apr 12, 2007.

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  1. Guest

    I have to replace a KBPC602 bridge rectifier. It is rated 200VRMS at
    6.0 A. It is wired into the circuit using only the + and - terminals.
    The AC connections are not used. If I were to build a bridge out of
    four 6.0 amp 100V diodes, would I be able to directly replace this
    device? Does the circuit basically require a 12.0A 200V device?
    Conversely if I were to place four 3.0 amp 200V diodes in parallel
    would that arrangement work also? Thanks. Lenny Stein, Barlen
  2. GregS

    GregS Guest

    Probably just fine. You loose the sinking unless you use tab diodes.

  3. Eeyore

    Eeyore Guest

    In which case it's not being used as a bridge rectifier !

    What's the application ?

  4. Arfa Daily

    Arfa Daily Guest

    That's a strange use of a bridge. You've effectively got two diodes in
    series plus two diodes in series, with the two strings formed, in parallel.

  5. Possibly 400V 6A instead.
  6. Sofie

    Sofie Guest

    How is the bridge wired into the circuit? ..... as a half wave rectifier?
    .... or ??
    Might I suggest that hooking up diodes in parallel without low ohm
    (equalizing) series resistors is not a good practice.... in fact, it is not
    a good practice to hook up diodes in parallel even with series resistors.
    Also if diodes are hooked up in series to increase voltage rating... fairly
    high ohm parallel equalizing resistors should be used.
    All of this is needed because no two semiconductor junctions are the same
    and one or the other diode will hog the current or voltage. Most good
    designs AVOID these kind of hook ups altogether and go with a single diode
    You should look at the circuit and determine what the rough current draw is
    and what voltage it should handle...... if the circuit current permits a
    single 6 amp "barrel" diode may suffice or a heat sinkable diode in a To-220
    or Stud-mount case with a higher current rating could be used.
    Daniel Sofie
    Electronics Supply & Repair
    - - - - - - - - - -
  7. Guest

    Captain Video,
    This is a strange way to connect a rectifier diode, unless the design
    called for a high current diode and only a bridge was handy.
    Is this a "commercial" piece of equipment or is is someone's homebrew
    design. .
    Take a close look at the circuitry and components and fairly easily
    you should be able to determine the approximate voltage and current
    needed for the replacement (single) diode if in fact it is used as a
    rectifier in a half wave configuration.

    If there is something else going on here with the circuitry and it is
    a commericial design and it has worked that way for years then why not
    just replace it with an "original" 6A, 200V bridge or even a higher
    voltage and current rating as these bridges are commonly available,
    even from scrapped televisions and other devices.

    In either case, you should consider Sofie's textbook argument
    regarding parallel and series diode connections, he is absolutely
    correct with what he stated.

  8. Arfa Daily

    Arfa Daily Guest

    Yeah, I was thinking that too, but couldn't quite get my head around it. I'm
    not sure how they rate bridges. When it's being used normally, two diodes
    are in conduction at any one time, in what amounts to a series circuit, so
    the quoted current rating must only be the rating of any one diode. However,
    as there are two diodes series'd into the circuit, the voltage rating should
    be twice that of any one diode, so when a bridge is quoted at 200v, is each
    individual diode 100v ? That would make the arrangement that's being
    enquired about, still 200v, but at twice the current, as all four diodes
    will be in conduction. Either way, should be easy enough to pick up either a
    direct replacement bridge, or 200v 3A diodes, which should be plenty enough.

  9. I imagine it's all about dissipation so the amps should be the same.
  10. Guest

    The application is an Elmo school overhead projector. The unit uses an
    82v 360W bulb. It looks like the bridge, that is, two sets of diodes
    in series with those two sets then in parallel, is in series with the
    lamp. When these bridges short, (usually just one diode in the bridge
    shorts) the bulbs burn out. In this application it is used as a half
    wave rectifier. I thought that perhaps by using a bridge in this
    manner they were attempting to get a higher voltage drop across the
    unit and less voltage to the bulb as there would be two junction
    drops instead of one. However with those junctions then paralleled
    with the other diodes maybe it would be a wash. Lenny.
  11. Arfa Daily

    Arfa Daily Guest

    So is this arrangement hooked directly to the incoming line voltage ( I
    guess 110v in your neck of the woods ? ) The drops across the diodes will be
    insignificant in this application, I feel, and it will be all about 'area
    under the curve' from the half wave rectification that the diode is
    performing. A bulb rated at 82v ( RMS ?? ) 360W will pull about four and a
    half amps RMS when hot, more when it's cold. I guess that this is what this
    is all about. The designer figured that it was a good way to get a high
    voltage high current diode in a compact package with enough surface area to
    self-cool. As it's a schools unit and probably subject to all sorts of
    health and safety regulations, I think that I would feel inclined to stick
    with the way it was designed originally, possibly uprating the specs of the
    bridge, if they are a regular failing point.

  12. Jitt

    Jitt Guest

    It may be the other way around. I have seen a small bridge used as
    protecion. When the big projector bulb fails, there's a big current
    spike, blowing the diode.

    FWIW on ESP website (Rod Elliot), the 100W guitar amp
    project, power supply schematic shows a "protector" bridge.
  13. Why do I get the idea that the designer was seeking a proprietary lamp
    voltage so that they could soak the school boards for $200 replacement

    Joe Leikhim K4SAT
    "The RFI-EMI-GUY"©

    "Treason doth never prosper: what's the reason?
    For if it prosper, none dare call it treason."

    "Follow The Money" ;-P
  14. Guest

    Funny that the back EMF theory as a cause of destroyed bridges had
    occurred too. Maybe I should install a small snubber, (perhaps a 1.0
    ohm resistor in series with say a .10uf capacitor across the bulb. BTW
    the bulb is very common and about 9.00 dollars. Lenny.
  15. Guest

    Using devices in series parallel is an effective way to fail-safe
    things. Any one of those diodes can short or go o/c and the rectifier
    still works as a rectifier.

    And you say its failed before - sounds like the designer was trying to
    reduce the failure rate instead of getting to the bottom of the cause.

    I dont know what type of bulb it uses, or why the BR is failing, but
    if its a filament bulb, arc over will draw huge currents at fail time.
    Fitting an mcb in the line to the bulb may save the diode sometimes,
    maybe. This must be a magnetic one, not a thermal one.

    Of course you could eliminate the problem entirely by using a mains
    voltage bulb and losing the BR.

    I'm not sure a CR across the bulb would help, as its current surge
    thats killing the BR. But if you do that then 100 ohms and 0.1u in
    series is typical.

    Another solution would be to lose the BR and fit a capacitor dropper
    with a fixed R to drop a bit of the power. Now the C&R will prevent
    any monster currents when the bulb dies. Just a small R, not much
    Pdiss and an X rated cap. The R helps limit peak i, though with the
    lightbulb's R you should be fine with just the C alone.

  16. James Sweet

    James Sweet Guest

    For some reason a lot of projectors seem to use oddball voltage bulbs.
    If the bridge is failing a lot, I'd try installing a buck transformer if
    there's room. You'd probably have to series a couple of secondary
    windings to get the odd 38V drop though.
  17. Sofie

    Sofie Guest

    Now that you have finally given us more information I would be inclined to
    suggest that you replace the bridge with a "like and kind" exact OEM part or
    a possibly upgraded voltage and current version.
    Since this is in a commercially designed overhead projector and used in a
    school, you would be best advised NOT to CHANGE the original design or use
    anything other than a "factory" part.
    Bridges of this nature and rating are readily available so stop wasting time
    posting to the newsgroup and get online and order the proper part.... get
    the proper and safe part that was originally installed.

    The one word that comes to my mind is "liability".

    Daniel Sofie
    Electronics Supply & Repair
    - - - - - - - -
  18. Guest

    I talked to a tech at Elmo this morning and asked about the use of the
    bridge as opposed to using a large single diode. He said that the
    engineers found that the DC when looked at with a scope from the
    bridge was cleaner than the output of just a single diode. Does that
    sound possible? As far as wasting anyones time, if I was I appoligize.
    I thought we were having an interesting discussion. Lenny
  19. Arfa Daily

    Arfa Daily Guest

    I absolutely agree, and already said exactly the same several posts above

  20. Guest

    We were and are having an interesting discussion and thread and no,
    you are NOT wasting anyone's time.
    Instead of trying to find ways to jury rig a substitute for a common
    and readily available bridge rectifier you should have just procured
    one and installed it and be done with it.... saying that, perhaps the
    only person's time "wasted" was your own.
    Until your later posting that revealed it was for a commercially
    manufactured overhead projector being used in a school environment you
    had everyone guessing about how the bridge was being used in the
    circuit, in fact, you never revealed the circuit in any kind of detail
    as it related to equipment, configuration, voltage, amps, etc... even
    after several reply posts asked you for details about it.
    As you know, none of us can read minds and no one has a working
    crystal ball on the newsgroup so the more information an OP can give,
    the better and more straight forward will be the answers.
    The best answer for fixing the overhead projector is to replace the
    bridge with an OEM part..... obviously other parts and configurations
    will work, BUT if something goes wrong and the projector malfunctions
    and causes smoke and fire damage... particuarly in a school
    environment.... your LIABILITY will be much more exposed if you
    DIDN'T use replacement parts that were used in the original
    manufacture. Enough said.
    Lenny, please feel free to start up more "interesting"
    conversations... it sure beats the spammers and the trolls and their
    filthy subject matter. Electronics discussions are much more
    enjoyable than that crap.
    Best Regards,
    Daniel Sofie
    Electronics Supply & Repair
    - - - - - - - -

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