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Zener diode reverse characteristic curves.

Discussion in 'Electronic Design' started by James Meyer, Jul 21, 2003.

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  1. James Meyer

    James Meyer Guest

    I was told that there are no knees associated with zener diodes if the
    characteristic curves were plotted with the "correct" scales on the V-I axis.
    Take a look at figure 5 on page 3 for the CZRA series diodes whose datasheets
    can be found at and tell me
    what sorts of axis ate needed to reduce the curve to a straight line.

  2. Jim Thompson

    Jim Thompson Guest

    A French curve ?:) Sent by "couriel" ?:)

    ...Jim Thompson

  3. There are a lot of problems with this datasheet. One sees this a lot
    these days. Why when I was a boy, a datasheet was a datasheet.
    I still remember the one for then 2N107........ Oh, what? Where am I?

    First of all, these are in no way ZENER diodes. The actual zener
    phenomenon only happens below 5 volts or so. Anything over that is
    just your regular avalanche phenomenon.

    Secondly, these don't seem to be specified as (non-zener) REGULATOR diodes.
    They seem to conduct somewhere around 120% of rating, so they're
    more likely to be snubber diodes, which you might expect to have much
    looser and variable curves than a true REGULATOR diode that you plan
    to use to generate a REGULATED voltage.

    Now back to your question, If I Recall Correctly, a diode's forward
    curve is an exponential one, so if you plot the forward voltage linearly on
    the X axis
    and plot the LOG of conducted current on the Y axis, you get pretty
    straight line.
    You can see a bunch of these in one place in the back of Bob Pease's
    book, "Troubleshooting Analog circuits". This exponential curve goes all
    the way down,
    way down down down below what we usually think of the lower limit of
    arounf 0.6 volts for silicon diodes.

    Now back to your REAL question, about reverse conduction (which is
    called "forward" or "normal" conduction for Zener (and Zener-workalike)
    It's probably not far off an exponential curve, with perhaps the x intercept
    shifted by
    the Zener voltage.

    Maybe you could look at some Zener SPICE models and see what they use?
  4. James Meyer

    James Meyer Guest

    Whenever I see a post that uses any of the words "no, never, all, and
    always", I'm almost 99 percent sure that an exception exists somewhere. :cool:

    That's why I usually try to include a few "weasle-words" in my posts.
    Most of the time. :cool:

  5. James Meyer

    James Meyer Guest

    Most of the Google hits I got had to do with SPICE models. I was wary
    of quoting them because I had the feeling that they were developed to show the
    designer what he felt he *wanted* to see rather than what actually existed.

    My response was to a previous message that stated categorically that
    there were *NO* knees associated with diodes, not even with the reverse
    characteristics of "zener" diodes.

    That's clearly a specious statement.

  6. George wrote...
    Actually, from measurements and studies I have made, the
    common "avalanche" zener diodes (all those above say 8V)
    are generally VERY far off from any type of exponential

    For example at low currents where the zener knee is first
    encountered (yes there is a definite knee for most types)
    the zener voltage may actually drop as the zener current
    is increased. Furthermore, after the conduction current
    is well established, the voltage drop at higher currents
    can look more like a fixed unvarying zener breakdown in
    series with a fixed internal series resistance. This is
    clearly seen at least in pulsed high-current testing.

    And to further complicate matters, this "fixed" breakdown
    voltage has a positive temperature coefficient (which can
    be quite large at high zener voltages), so along with the
    series resistance this changes the measured voltage as the
    current and the dissipated power is increaseed, and adds
    time course and package mounting issues to the situation.

    Whew, all this talk of breakdown makes me hungry, I'm off
    to grab some lunch.

    - Win
  7. Jon Elson

    Jon Elson Guest

    Figure 5 is the voltage drift with respect to temperature.

    Figure 4 shows the current vs. reverse voltage, and the knee on this
    curve is quite obvious. Do you mean you need a linearized temperature
    coefficient, or linearized voltage/current? Although you can't get it in
    a 100+ V range, there are some great reference 'diodes', like the LM399,
    LM4040, etc. that have much improved characteristics.

  8. John Larkin

    John Larkin Guest

    Wake up, George. It's 2003. Nobody proof-reads data sheets any more.

  9. Jim Meyer

    Jim Meyer Guest

    Here's why I can't take SPICE models and simulations seriously....

    In the listing below, change the two lines as indicated, run the
    simulation again, and then explain why there is a *negative* 390 mV
    potential across D1 at low current levels.

    SYMATTR Value 1N750
    TEXT -320 80 Left 0 !.dc dec I1 .01e-9 200e-3
  10. Jim Thompson

    Jim Thompson Guest

    I don't have time to load your listing right now... I'm scurrying to
    finish a job before my chassis fan dies... the monitor pops up every
    thirty minutes or so when the speed drops seriously below spec :-(

    But my guess... Mikey uses behavioral diode models which are a wee bit
    hokey... they're a voltage source/ideal diode gimmick.

    ...Jim Thompson
  11. Jim Thompson

    Jim Thompson Guest

    I think you'll find that the "true-Spice" models are pretty reliable
    if you choose IS, RS, BV and IBV judiciously.

    Those models which use a DC source plus an ideal diode can do weird

    ...Jim Thompson
  12. Ben Bradley

    Ben Bradley Guest

    The proper terminology about something which there may be more than
    significant doubt is "take it with a grain silo of salt." At least
    that's they way I like to say it.
  13. My tests have shown fairly close to exponential behaviour for zeners with
    nominal voltages say below 4.7V, above that voltage they are like you describe.

  14. This was a reply to Win's post. It doesn't look that way on my browser!
  15. Robert Strand wrote...
    If you search the s.e.d. archives on Google using the words zener
    oscillation, you should uncover the half-dozen threads exploring
    this subject. Several of us spent far too much time on the issue,
    but the interesting physics we found makes for interesting reading.
    Some of the posts feature my attempts at ascii oscilloscope traces.

    - Win
  16. If you search the s.e.d. archives on Google using the words zener
    Win, thanks for the tip. I've have read some of those over the years,
    probably not all of them though.

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