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zener diode clamping Q

Discussion in 'Electronic Design' started by Yzordderex, Feb 13, 2004.

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  1. Yzordderex

    Yzordderex Guest

    I have just tested two 36v zeners(1N5258) to see where they clamp.
    One thru-hole part (Diodes inc)and one mini-melf (Vishay). I charged
    a 10uf cap and discharged into zener through a 10ohm resistor.

    The thru-hole part clamped at 40v with 2 amps of current. The melf
    part went to 55v with only 400ma.

    What's up with these two supposedly identical parts?

    regards,
    Bob
     
  2. Don Pearce

    Don Pearce Guest

    Given a maximum dynamic resistance of 70 ohms at nominal current
    (which you are exceeding by a long way), then 55 volts at 400mA is OK.
    ..4 times 70 is 28 volts. Add that to 36 volts and you get 64 volts.
    You are well inside that figure. No problem with the MELF part, or
    the through-hole. This is just normal component tolerance.

    d

    _____________________________

    http://www.pearce.uk.com
     
  3. You might find TVS parts to be more appropriately specified if your
    test is indicative of actual requirements. Eg. 1.5KE36A (1N6285A) with
    34.2-37.8 Vbr(1mA) and clamping to 49.9V at 30A. The SMT (SMC) version
    1.5SMC36AT3 has the same specs as the above.

    Best regards,
    Spehro Pefhany
     
  4. Where he's using it, it's off the graphs. They quit at 20mA!

    Best regards,
    Spehro Pefhany
     
  5. Ben Bradley

    Ben Bradley Guest

    Is the current you're putting through these diodes within their
    current limits? It sure doesn't look like it. If I read the Vishay
    data sheet correctly, the absolute maximum current is that which
    causes maximum power dissipation. 0.5W/36V=13.9mA max current. I would
    have thought it would have some pulse-current rating that's higher
    than that, but I don't see it. Regardless, that's a whole lot less
    current than what you're putting through it.
    After you get the current within the rating, the (Vishay MiniMELF
    case datasheet) data sheet shows a "test current" of 3.4mA, and this
    is with a 5 percent tolerance. At that current, the voltage can be as
    low as 36*.95=34.2V or as high as 36*1.05=37.8V.

    In short, what's up with not reading the data sheet?
     
  6. Jamie

    Jamie Guest

    you know if you look at most Zener Catalogs they do mention
    RZ which is the lowest resistance the diode is to shut down
    to how ever.
    with different style packages you can get some altered affects
    especially when you exceed the expected shunting resistance.
    i am sure if you stay with in the limits the diode will perform
    as indicated.
    if you are still finding large margins even with in the suggested
    specs then i would suspect that the manf had some bad runs.
    P.s.
    i rarely run components to their limits.
     
  7. Yzordderex

    Yzordderex Guest

    Thanks for all the fine answers, but, my question still lingers.

    (Life was so much better when GI was GI and Vishay was... who cares)

    I suppose what I'm really after is an explaination as to why two
    supposedly identical parts (according to the data sheets which I have
    read{Mr. Bradley}) would behave in such different manners. The carrier
    multiplication of the through hole part seems to be orders of
    magnitude greater than other said part. If in fact the parts have
    similar impedances at two places along the curve ([email protected] &
    [email protected])then shouldn't they be similar everywhere? Yes the
    impedances are specified as maximum values which in all probability,
    and as we see in fact, have little correlation to real world parts.
    But still (whine) I would expect parts to behave in a *more* similar
    manner.

    Disclaimer follows.

    I DID NOT design this sloppy circuit. I've been called in to perform
    a design review to determine if circuit is worthy to be mass produced.
    Heheh, I think we all know the answer to that question.

    Thanks again fellas.

    regards,
    Bob
     
  8. Ben Bradley

    Ben Bradley Guest

    I can see this point, but I can only speculate that the two
    manufacturers use different techniques or processes that result in
    in-tolerance values within the specs, yet are divergent outside those
    specs. Maybe someone familiar with semiconductor manufacturing
    knowledge/experience can answer this.
    In that case, I apologize for my derisive comment about not reading
    the data sheet, OTOH I had no indication of what you were doing or
    what was your relationship with this circuit.
     
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