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minimum zener current for break down.

Discussion in 'Electronic Design' started by [email protected], Mar 22, 2013.

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  1. Guest

    Is there any minimum zener current required for it to break down or is it only dependent on voltage? For example, MMSZ5V1T1G (datasheet below) break down voltage Izt1 is given at test condition as 5ma.

    The following is our application and current should be limited to the voltage sensing circuitry to less than 100uA. So after break down voltage of 5.6V, only few tens of uA will flow through the zener. Is this sufficient for the zener to break down and maintain the Vs - Vz for the voltage sensing circuitry?

    5V-30V o---/\/\/\---|<|----------/\/\/\-----GND
    1K Z5V6 | 47K
    |
    |_ _ _ /\/\/\---->> Sensing Circuit
    330K


    http://www.onsemi.com/pub_link/Collateral/MMSZ2V4T1-D.PDF

    regards
    Sridhar
     
  2. Guest

    5.6 volt regulator diodes do not have a sharp breakdown knee. They
    are Zener diodes. Higher voltage diodes are avalanche diodes and have
    a sharp knee. if you need a sharp knee, you may have to go to
    something like TL431.


    Dan
     
  3. Robert Macy

    Robert Macy Guest

    May be important, zeners turn on SLOWLY!

    To demonstrate, try to use two zeners in the feedback of an amplifier
    to make a 'square wave' generator. You'll see the zener's true
    characteristics.
     
  4. Guest

    [/QUOTE]

    Thanks. That graph doesn't have any values for zener break down current. How do I find this from the datasheet?
     
  5. Claimed to be sub-ns.
     
  6. Robert Baer

    Robert Baer Guest

    Zeners "work" from zero to the max rated current (and above that, but
    do not do that).
    The problem is, that all voltages will have a noisy and negative
    resistance region.
    You better characterize the particular voltage and manufacturer that
    you think you want to use; you may find that there are NONE that do not
    have these undesirable characteristics in such low current regions.
    That is why the specs AVOID low currents and use "operating" or
    similar terms, and have NO I/V curves of value.
     
  7. Most of the Japanese parts are well specified, and the Iz currents are
    reasonable (like 0.5mA and not 20mA or whatever).

    Eg.

    http://media.digikey.com/pdf/Data Sheets/Panasonic Semiconductors ICs PDFs/DZ2J056.pdf
    5.6V nominal at 5mA, 'typical' curve down to 1uA, up to 100mA

    http://rohmfs.rohm.com/en/products/databook/datasheet/discrete/diode/zener/udzs5.6b.pdf
    5.6V nominal at 500uA, 'typical' curve down to 1uA, up to 10mA

    Seems many designers these days would rather use a $3 chip than a
    $0.03 zener.

    Just guessing about what you are actually trying to do, but the Rohm
    part looks like it might 'typically' be suitable. Maybe you could use
    a higher voltage zener if your input switches between 5V and 30V- the
    5.6V unit is guaranteed to have < 0.5uA at 2.5V. A 9.1V zener from
    that series is guaranteed to have a reverse current of < 0.5uA at 6V
    and will drop no more than 9.2V at 5mA, so at 5V in you'd have less
    than 24mV to your sensing circuit, and at 30V you'd have more than
    about 21V.
     
  8. Guest

    On Friday, March 22, 2013 8:48:54 AM UTC-4, wrote:

    No one can tell anything from your question, you don't even specify what impedance or voltage is at the sensing circuit, you don't mention how much loading the 5-30V source can handle, you don't tell anything about whether you're trying to detect just the presence of input in 5-30V range or actuallymeasure its value or how the sensor determines it. The circuit you have drawn is HIGHLY unusual for a voltage clamp with the 47K to GND.
     
  9. Jamie

    Jamie Guest

    That's a rather strange way to detect anything over 5 volts? I
    mean, that is what it looks like you're doing?

    If that is your aim, I would use an NPN transistor with an emitter
    bias of 4.5 volts for example and have the collector bias the next
    stage. THe base input would be the input from the 5-30 volts via a
    R ofcourse.. Put a 35 volt TVS diode on the front end for protection..
     
  10. miso

    miso Guest

    Characterizing components is always a good thing. Depending on the
    numbers is another story. If it is not in the datasheet, the number can
    always change. So you play the "what if" game. What if XXX doubled? What
    if XXX were cut in half? That is, you determine the sensitivity of your
    design to this undocumented parameter.

    This presumes some product you will be selling in the hundreds of
    thousands to millions per year over the next decade. If you just need a
    few units, then characterization data should be fine.
     
  11. Guest

    All I am trying to do is to find the voltage is above 6V approx or not by sensing the remaining voltage after the zener drop.
     
  12. Robert Baer

    Robert Baer Guest

    A 5.6V zener essentially does not count WRT to negative resistance
    and noise at low currents; I should have been more eXplicit - should
    have said "avalanche".
    Note that 9V zener does "count"; runs in avalanche mode and current
    spec is much higher than what the OP mentioned.
     
  13. Fred Abse

    Fred Abse Guest

    From the table below the graph:

    [email protected], [email protected]
     
  14. Fred Abse

    Fred Abse Guest

    It's Google, expect the unexpected.
     
  15. josephkk

    josephkk Guest

    In that case a micropower variant of a TLV431, a few resistors and a
    mocropower comparator would be a far better choice. You should be able to
    get your power under 10 uA easy, and near or even less that 1 uA if you
    try hard.

    ?-)
     
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