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reliably testing diodes in circuit ?

Discussion in 'Electronic Components' started by robb, Nov 15, 2007.

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  1. robb

    robb Guest

    while checking diodes **in circuit** with diode setting on DMM

    i ran across a unexpected results where diode test at (.567 V) in
    one direction then swap probes around and expect (0 V) but i
    get unexpected (1.897 V) reading.

    are there conditions where this is a valid reading that does
    not indicate a circuit error or fault with diode?

    thanks,
    robb
     
  2. mc

    mc Guest

    Certainly. Your meter can't tell the electrons to *only* to through the
    diode. They will go through whatever else is in the vicinity.

    A diode by itself is not .567 one way and 0 the other. It is .567 (or so)
    one way and infinity ("overload") the other way, indicating that there is no
    path for the electrons.

    In this case the electrons are finding a path through other components.
     
  3. Eeyore

    Eeyore Guest

    Why would you expect 0V ? That would indicate a short !

    0.567V is about the right forward voltage for a diode check.

    That's the non-conducting direction for the diode. The 1.897 V will come from
    other components in parallel with it. It's hardly unexpected !

    I think you need to practice measuring some diodes before drawing incorrect
    conclusions as at present.

    Graham
     
  4. Yes.



    Best regards,
    Spehro Pefhany
     
  5. Eeyore

    Eeyore Guest

    LOL !

    That's an unusually terse response from you there Spehro.

    Graham
     
  6. Phil Allison

    Phil Allison Guest

    "robb"

    ** You meant the meter shows over-range, not " 0.000 " - OK ?

    ** As it will with a resistor of about 10 - 20 kohms

    ** It is generally not possible to make "reliable " tests on diodes that are
    still in circuit - certainly not with a DMM set to " diode test".

    I find an old fashioned analogue multimeter to be far more useful ( since
    its has a test current of circa 50mA), but even it will not reliably find
    leaky diodes in circuit.



    ....... Phil
     
  7. robb

    robb Guest

    Thanks for help and explanation.

    regards,
    robb
     
  8. robb

    robb Guest

    apology, i mistyped. i meant to type 0L, at least that is what
    the meter displays when i use diode test on the DMM

    Thanks for help.
     
  9. robb

    robb Guest

    thanks Spehro.
     
  10. robb

    robb Guest

    apology, yes, 0L on DMM display not 0V .
    thanks for helpful explanation Phil.
     
  11. Dave Platt

    Dave Platt Guest

    while checking diodes **in circuit** with diode setting on DMM
    I ran into a situation like that when trying to figure out what a
    little four-leaded device was. Turns out it's a diode ring - four
    diodes connected anode-to-cathode - intended for use in a balance
    mixer/modulator. For any two adjacent leads, it would read about .5
    volts with the leads in one orientation and three times that voltage
    in the other direction... I was actually reading the forward voltage
    drop of either one or three diodes.

    In your case, I expect that you're probably reading the forward
    voltage drop of the diode in one direction, and reading the effect of
    other components (diodes with the opposite orientation, shunt
    resistors, etc.) in the other. Since you're reading in-circuit, you
    cannot be certain that either reading is due solely to the diode you
    think you're measuring, and the actual number displayed is going to
    depend somewhat on the measurement technique used by the meter.
     
  12. Chris

    Chris Guest

    Hi, Robb. When on diode check, the DMM typically puts out a current
    of a couple of mA, and then measures the voltage across the probes.

    As mentioned elsewhere, doing a diode check on a diode in-circuit also
    measures everything else in parallel with the diode. The readings may
    have little or no meaning.

    One exception -- if you're measuring a diode's forward voltage drop in-
    circuit, the reading has to be less than or equal to the diode drop
    out of circuit. If the reading is greater, that reliably indicates a
    problem with the diode. Since diodes do sometimes fail open, that
    data point can be of some use.

    Cheers
    Chris
     
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