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Zener Diode Dilemma

Discussion in 'Electrical Engineering' started by jjk, Oct 18, 2013.

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  1. jjk

    jjk Guest

    Zener Diode Dilemma

    I purchased 50 1N4728 3.3v Fairchild zener diodes and trying to test them before installing in a circuit I'm building.

    The diode test function on my meter shows 0.6v forward biased and .9v reversed biased. Performing the same tests on a 5.6v zener, I get 0.6v forward biased and when reversed biased I see the same reading as not connected to the diode at all - which is what I would expect.

    Further, I inserted the diodes in a test circuit described below.

    vcc -> 560 ohm resistor -> milliamp meter -> zener cathode, zener anode to ground.

    When vcc voltage was applied, current was measured as shown below with the diode conducting all the time, and I could never get the voltage across thezener to reach 3.3v without smoking the resistor.

    1.5v 0.1mA
    2.0v 0.6mA
    2.5v 1.1mA
    2.7v 1.5mA
    2.9v 1.7mA
    3.0v 1.8mA

    All of the 3.3v parts I tested exhibited the same behavior. Suspecting a bad lot of diodes I purchased a few 3.3v diodes made by NTE. They all too exhibit the same behavior - conducting well before their breakdown voltage.

    Trying the above with a 5.6v zener shows no current flow until the voltage across the diode reached the device rated breakdown voltage. I also put a scope on the power supply to verify it was stable.

    Any guidance as to what I may be doing wrong would be appreciated.

  2. Tom Biasi

    Tom Biasi Guest

    First, the diode function on your multimeter will not give you any
    useful information about zeners.
    Second, Your test looks strange. Is your supply current limiting perhaps?
  3. JJK-

    Plotting your Voltage vs current data suggests you may be measuring the
    forward-bias curve of several diode junctions connected in series. (It
    looks more like a constant-Voltage curve than constant-current.)

    The only 3.3 Volt Zener in my collection is a 1N5575A. It checks good
    as a diode on the diode ranges of a Fluke 8020A multimeter. I did not
    run your current vs Voltage test, but would expect it to behave as well
    as your 5.6 Volt test.

    I wonder if both your source of the 1N4728 diode and NTE, get their
    diodes from the same factory? There could have been a batch that were
    mis-labeled, or they could have fallen victim to counterfeit parts.

  4. Tom Biasi

    Tom Biasi Guest

    The curve is linear until he gets to 2.7 volts then there is a bump
    (reading error?). This looks like a forward conducting diode and the
    readings are across the resistor.
  5. jjk

    jjk Guest

  6. JW

    JW Guest

    Where are you getting these diodes?
  7. Tom Biasi

    Tom Biasi Guest

  8. jjk

    jjk Guest

    I received a package of 50 Fairchild devices from
    Then I purchased 5 NTE devices from a local electronics store.
    All of the ones I have tested from the above failed.
    I contacted Jameco and they sent 50 replacements and all that I tested failed in the same way. I've been in contact with the Jameco technical rep and they are looking into this and trying to obtain devices from another lot from their supplier.
  9. jjk

    jjk Guest

  10. JJK-

    I did not see how you were using the 3.3 V Zeners. If you are
    generating 3.3 Volt power, you might consider using the LM-317. They
    are available in several packages including surface mount, and can be
    adjusted from 1.2 to 39 Volts using a pair of resistors.

  11. Tom Biasi

    Tom Biasi Guest

  12. jjk

    jjk Guest

    Thanks for your comments. The potentiometer circuit I described was for testing an alternate power supply source to rule out a problem with my bench supply.
    I intend to use the zener for over voltage pin protection on 3.3v devices.
  13. jjk

    jjk Guest

  14. jjk

    jjk Guest

  15. Tom Biasi

    Tom Biasi Guest

    I guess Jameco doesn't have any incoming QA inspection.
  16. This exactly matches the datasheet, which says it needs at least 76mA
    in order to act as a zener voltage reference.

    This is a common issue with low voltage zeners - the current they
    need to operate is high (compared with higher voltage zeners).

    If you want a low voltage reference at low current (e.g. for battery
    operated equipment), an LED can often be used (forward biased) at a
    much lower operating current (1mA or less), although not quite as
    stable a reference.
  17. jjk

    jjk Guest

    Thanks for your comments Andrew.
    I'm wondering why the zener would conduct at all at voltages as low as 1.4v?
    My goal was not to use the zener as a regulator, but to protect 3.3v devices from potentially external over voltage events (microcontroller driving 5vcircuits).
  18. Guest

    Doesn't Jameco still sell floor sweepings? I sure wouldn't trust any
    component bought from them.
  19. Tom Biasi

    Tom Biasi Guest

    I don't know their practices.
  20. Zener voltage regulation happens in reverse breakdown mode.
    All zeners leak reverse current at voltages below the breakdown voltage.
    With low voltage zeners, this leakage current is very high, up to 76mA
    for this part number. This makes low voltage zeners unsuitable for
    many applications, where you might expect a theoretically perfect
    zener to work.
    If the 50mA or so leakage at 3V is an unacceptable issue for you,
    you might instead consider a potential divider across the 3V supply,
    with the centerpoint connected to an SCR gate to crowbar the supply.
    Adjust the potential divider ratio to give the right tripping voltage.
    A red led (forward biased) added at the top of the potential divider
    might make it more accurate/sensitive.

    I haven't actually tried this at such a low voltage, but I suspect it
    may work better than a zener.
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