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testing zener diodes in circuit

Discussion in 'Electronic Components' started by [email protected], May 28, 2007.

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  1. Guest

    I have a circuit that I built years ago. Rather than just replacing a
    bunch of zener diodes, is there a way to test them in circuit, without
    powering on the circuit?
    I know that surface mount zeners fail shorted, but they can open if
    there is enough current.
    Can I just do a diode check at each polarity with a DMM to detect a
    bad zener?
  2. debeers

    debeers Guest

    using a multimeter, you can check the zener diodes. first get the true
    values from a good one then compare the values.
    while measuring, first make the forward-biased the diot then reverse-
  3. Arfa Daily

    Arfa Daily Guest

    You can check for only very basic diode functionality, in circuit, and
    without the unit powered. You cannot check for zener voltage, and any which
    were open circuit or leaky, may be difficult to spot. Short circuit ones
    should show up ok, but bear in mind that you will probably have a decoupling
    cap directly across the zener, and the zener may be strung directly across a
    transistor junction if it's part of a 'conventional' linear regulator. If
    either of those additional components was leaky or short, the reading across
    the zener will be directly influenced. On the other hand, zeners that are
    perfectly ok may show what appears to be reverse leakage, but is actually
    the influence of attached circuitry, and if any are very low voltage
    zeners - say 3v3 - depending on the meter's test voltage, they may *just*
    start to conduct in the reverse direction, giving the impression that they
    are reverse leaky, when they're not. Is there a big problem with powering
    the unit ? Do you suspect that it has failed zeners ?

  4. Yes, that is right. But you can only measure the typical Silicon-Voltage
    (approx .5 to .8 Volts). Switch your Multimeter to Diode-Check (that
    supllies a higher Voltage than normal Resistor-check).
    In one Direction you should get overflow "1" and in the other direction
    the Voltage named above. That means: The Diode is OK.
    Sometimes, you will get in both directions any other Voltage or even a
    shortcut. That is mostly caused by other parts connected to your Zener.

    Best Way to solve this Problem: Dissolder one of the Zeners Pins and get
    the right Voltage...

    What you can NOT measure that way, is the real Zener-Voltage.
    You would need a little Test-Circuit (and dissolder the Diode
    Connect The Zeners cathode via a 10K Resistor with e.g.15V.
    Ground the Anode. A Zener is only used in the "wrong direction"
    The get the Voltage above the Zener. That is the real Zener Voltage.

    Many Greetings from Germany,
  5. mike

    mike Guest

    You can find a shorted circuit node with an ohm-meter.
    If you've got stuff in parallel that could be shorted, you have
    to determine where the current is going. Easiest thing is to
    unsolder something. If you put a bunch of current into a shorted
    node, the offending surface mount will desolder itself...sometimes
    explosively. Or use a high-tech infrared thermal imager. Used that
    back in the day to find internal circuit board shorts. Low-tech
    non-contact thermometer works if you can find one with a very narrow
    beam width.

    Another approach is to use a HP current tracer
    and HP current pulser to determine which way the current is going
    in a trace/wire.

    Open zeners are a bit more difficult. Easiest thing there is to power
    the circuit and measure the voltage across the zener. If you can't put
    the circuit in a mode that turns on the zener, it's not likely the cause
    of your symptom...or you didn't need one in the
    first place. A curve tracer can often tell you a lot about the
    condition of your zener, but it depends heavily on whether you can
    reach zener voltage without blowing up something else. YOu can make
    one from an AC wall wart, a resistor and scope. Modems often have AC
    wall warts.

    Would be interesting to know why you can't power it on??

    If you built the circuit, you should be able to get close to the problem
    by analysis of the symptoms.

    And zeners do fail in modes whereby the small signal behavior
    is more like a resistor.
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