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zener failure question

Discussion in 'Electronic Repair' started by Lee, Jun 1, 2013.

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  1. Lee

    Lee Guest

    How often does a zener have a failure mode where it acts like a good
    (not leaky) much lower voltage zener? I usually see leaky or shorted
    ones rather than substantial drift...

    Fault was 185v line in 'scope low at 170v and slight ripple, failure was
    24v zener which was acting like a good 3.3v zener.
    'Scope was still working, just had slightly reduced intensity and
    thought it better to check voltages before twiddling the sub-intensity
    pot ;)

    Lee
     
  2. Jamie

    Jamie Guest

    That is common for a zener.. excessive heat usually does that.

    From what I understand the zener is fabbed a little differently so
    not to destroy itself when doing the break down. Normal diodes tend
    to not like being used as zeners for any length of time. I guess
    it could be possible you may had a zener that had some impurities in
    the process that could of allowed it to break down like non-zener semi's
    or, it just got wacked some how. If this reference is supplying some
    out side device as a source of energy or ref on the scope? It's possible
    it got whacked.

    Also, if only supplies internal references, check to make sure the
    connecting circuit is still using it. If not, the zener could be
    clamping too much and operating hot..

    Normally the evidence is showed with a over heated board that it's
    attached to.


    Jamie
     
  3. Lee

    Lee Guest


    Thanks.

    I'm not sure I understand the reasons it's used in this application
    either, it's a 5W part and it seems to be used to create a 185v supply
    from a 245v tap (+ bridge obviously) on the mains transformer. This
    185v supply is then regulated by a comparator/series pass transistor to
    160v. The weird thing is that even with the zener failed, the 160v
    supply was still just about right, (it was at 163v).
    Replacing the zener has put the 185v rail back to the proper value and
    the 160v rail is now solid as well, minus the ripple.

    The zener was mounted off-board with fibreglass sleeves on its leads so
    they expected it to get hot ;) Hasn't burnt the board though.
    Replacement runs hot as well, but not uncomfortably so.

    Lee
     
  4. Lee

    Lee Guest

    Ah, penny drops. I had to model the circuit in Spice before I understood
    it ;)
    It's used to create a stable 160v for the positive end of the resistor
    divider at the input to the comparator, the negative end of the chain is
    connected to a separately regulated -12v (from a different transformer
    tapped supply) and the comparator (other input is ground) and series
    pass transistor then regulates the 245v supply from the bridge to 185v.
    Which in turn is dropped by the zener to 160v

    If the zener is shorted then the 160v supply stays at 160v, but with
    lots of ripple. The 185v supply isn't used for anything else other than
    as a marked test point and to supply the 160v rail via the zener. That's
    the thing that confused me originally ;) Why have a series pass
    transistor but then drop that supply through a zener?

    I guess there is a good reason to do it this way, but it confuzzled me ;)

    Lee
     
  5. Jamie

    Jamie Guest

    sounds like a poor design.. maybe it should have a heat sink hanging
    off it.

    Jamie
     
  6. Jamie

    Jamie Guest

    Possibly two things..

    1. The series transistor is under switch control so that it could shut
    off the source or, it could be a current limited.

    2. The series transistor could be stepping down the voltage so that
    zener shunt does not exert so much current.. Just enough to maybe smooth
    out the ripple..

    Jamie
     
  7. Lee

    Lee Guest

    Interesting idea, does sound like they could be doing that ;)
    The comparator is a TL072 and the reference voltage is fed into the
    inverting input...
    With the faulty zener there was a varying 2-3v of 100hz positive-going
    ripple on the 160v line, looked like a failed smoothing cap. With the
    replacement it's a few millivolts.

    Lee
     
  8. Guest

    I would surely try to put someting on the replacement zener to cool
    it, a fin, or maybe clamp the body to a piece of electricallly
    isolated metal. Even mounting the zener with as short leads as
    possible if connected to something on a pwb that would act as a heat
    sink, or maybe even solder some copper tape wings onto the zener
    leads, anything to help it run cooler.
     
  9. Jamie

    Jamie Guest

    The little tap wings works, I've done things like that. But, I also
    do this;
    I keep a roll of copper tape with adhesive around. Cut in narrow
    strip and make little fan folds, stick to the diode body, solder it
    together around the body..

    Use precision tweezers to make the folds. You shouldn't need to solder
    to a lead. You could put a dab of glue on there if that makes you feel
    better.

    Jamie
     
  10. N_Cook

    N_Cook Guest

    Another option, topology dependent, is using the full length replacement
    zener leads, scraping off any coating to the traces and curving the leads
    to match the traces and soldering the leads to the traces for an inch or
    more
     
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