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2N3904 in zener mode

Discussion in 'Electronic Design' started by Frederic, Jan 29, 2007.

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  1. Frederic

    Frederic Guest

    Hi everyone,

    I was checking a dozen of 2N3904 I got in stock in the zener mode (with 15V
    and 10K limiting resistor) and they all give the same voltage, about 8.2V.
    And quite a sharp knee.

    Anyone can confirm if this voltage is always the same from batch to batch?

    Frederic
     
  2. Jim Thompson

    Jim Thompson Guest

    In a given process run ("batch") they'll be the same. Next "batch",
    probably could differ by as much as a volt.

    ...Jim Thompson
     
  3. Eeyore

    Eeyore Guest

    It's avalanche breakdown btw.

    Graham
     
  4. Frederic

    Frederic Guest

    Mmmh... not too bad after all.
    Thanks!
     
  5. Eeyore

    Eeyore Guest

    FWIW when I tried this I got ~ 7V.

    Nat Semi also couldn't or wouldn't guarantee the long term viability of the
    device in this mode, although if you don't exceed Ib(max) I imagine you'd be OK.

    Graham
     
  6. I think the zener (or avalanche, if you prefer) is pretty
    stable, but using the transistor in this mode will
    cumulatively ddegrade the beta.
     
  7. Didi

    Didi Guest

    In a given process run ("batch") they'll be the same. Next "batch",
    This reminds me an old (12 years or so) design of mine, where I
    needed variable gain cells, DC to say 10 MHz, 12 bit, 1:50 factor or
    so.
    I used paired 2N4391-s; one for compensation feedback, the other
    as a variable resistor (having the gates shorted was handy as
    it allowed me to solder the cans together for temperature coupling).
    Buying about a 100 transistors inevitably bought me most if not
    all of them from the same wafer and I could find decent pairs
    easily enough.
    Now obviously I did not get 12-bit temperature stability, but I had
    anticipated this sort of trouble and had put an AD792 temperature
    sensor next to them :). I had an MCU and it did turn the fine gain
    to compensate; this gave me a 10x+ advantage over competitors
    (who were _awarded_ all major deals in spite of that, but that's
    another story).
    Now the rest of the story is that I got no 12-bit integral non-
    linearity,
    either. Obviously even the 4391, in the reasonable low-voltage
    range where I used it, was about 0.5 to may be 1% non-linear,
    which was not acceptable. Fortunately I had enough processing
    power (on a 16.67 MHz 68340 :) to acquire the non-linear
    data into a somewhat longer spectrum (8192 was the norm;
    I did something like 8500 or so) and continuously (IIRC 10 times
    a second) converted it down to a linearized version.... Worked
    wonderfully, some are still in use.
    Got carried away by just a phrase, I guess, but I see this
    as a "heroic" design of mine (there are more reasons except
    the above for me to think so :) and I felt like telling about it
    although being only very remotely related to the context...

    Dimiter
     
  8. Robert Baer

    Robert Baer Guest

    The zener voltage for a given transistor type does not seem to change
    from batch to batch; sometimes one can even change vendors.
    Changes seen seem to be less than 1%.
    In the case cited, i believe you will not see oscillation or negative
    resistance effects.
    Run the current down toward zero to see some spetacular "bazz-fazz".
     
  9. Robert Baer

    Robert Baer Guest

    Zener operation of the E-V junction "injects" lattice defects and
    thus reduces beta (at minimum). Baking in an oven (300-350F) for a few
    hours will anneal out the damage.
     
  10. Robert Baer

    Robert Baer Guest

    Yup!
    That is why the uA709s used in Apollo had problems; seems that the
    "testing" lab for NASA did not know what an op-amp was or how to test them.
    Did not follow the datasheet for temp/stress testing: used voltage
    follower configuration and a 5V pulse on input (good old slew rate
    caused brakedown on inputs).
    Hell, they even tried to measure gain and offset !open loop!.
     
  11. Ken Smith

    Ken Smith Guest

    I'll say it isn't the same batch to batch and maker to maker. Some
    Darlingtons have a very cute EB breakdown. They oscillate.
     
  12. Ken Smith

    Ken Smith Guest

    No, you need to stay way below Ib(Max). There is a lot more heating.
     
  13. What the hell is bazz-fazz??

    Jim
     
  14. No, there's no guarantee of this between batches. It's only
    guaranteed to be above a certain minimum- that's what's on the spec
    sheet.

    But if they decide to cook the next batch a little longer, or change
    the device geometry to improve some other spec, the zener voltage
    could really change a lot. As long as it remains above the spec, you
    have nothing to legitimately complain about if the zener voltage zooms
    up to 15 volts in the next batch.

    As an example, I have a batch of MPSA42's, spec says 300 volts
    maximum, but a half-dozen I tested work fine up to about 830 volts
    plus or minus about 10 volts. Does this mean I can depend on this?
    No.


    Better to not go into production if you're depending on this!
     
  15. John Larkin

    John Larkin Guest

    Some NPN transistors seem to be very consistant at about 5.5 volts Vbe
    breakdown. If you then use them in collector-emitter zener mode, you
    get a nearly zero TC 6.2 volt "reference zener."

    John
     
  16. Frederic

    Frederic Guest

    Indeed!

    Fred
     
  17. Frederic

    Frederic Guest

    bazz-fazz????
     
  18. Eeyore

    Eeyore Guest

    If you need a higher voltage MPSA42/43 check out the little known MPSA44/45.

    Graham
     
  19. Guest

    I remember doing that as an experiment in college. My recollection was
    you only need 100uA or so. Maybe up the resistor a bit, say 47k to be
    safe that it breaks down. [Of course, you need to do the
    experiment. ;-) ]
     
  20. Robert Baer

    Robert Baer Guest

    I strongly do not recommend darlingtons for that; too variable and a
    high zener due to two E-B junctions to break down.
    I have tested a number of transistors, and for a given brand and
    type, the berakdown voltage (in the *stable* region for the part) is
    remarkably "constant" between devices and between batches (even when a
    year or so seperated).
    All transistor E-B junctions have a range of currents where they
    exhibit negative resistance as well as oscillate; some are really nasty.
    It seems that a majority *start* with a large negative resistance
    characteristic (even in the nanoamp region), and slowly go towards zero
    then finally positive.
    Then again, most zeners that are made also have similar problems.
     
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