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multimeter question

Discussion in 'Electronic Repair' started by slight, Dec 19, 2004.

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  1. slight

    slight Guest

    When i measure resistances with my dmm, i get a different reading if i
    swap the leads around.
    Is this normal?
     
  2. Noah Little

    Noah Little Guest

    Sometimes.

    If you want a more useful answer, you'll need to provide more
    information with the question.
     
  3. slight

    slight Guest

    an example would be measuring a 10k resistor in-circuit.
    by reversing the lead polarity i get two readings, 10k and 5.8k.
    im guessing that is because the meters i have use 3.3v across the leads
    and this is triggering a diode or something in the circuit.
    are all meters like this? the ones i have are pretty cheap.
    do the flukes do this? been looking at one for Christmas.
     
  4. N Cook

    N Cook Guest

    Different contact potentials due to different metals could give an added
    V in one sense and a subtracted V to the Vp of the voltage on the probes.

    electronic hints and repair briefs , schematics/manuals list on
    http://homepages.tcp.co.uk/~diverse
     
  5. Roby

    Roby Guest

    Your guess is exactly right: resistance is calculated by measuring the
    current that flows in response to the applied voltage. To my knowledge,
    all ohmmeters work this way.

    As you found out, measuring a resistance "in-circuit" doesn't give you the
    result you want. Doing it with the circuit powered up is likely to destroy
    your meter.

    Roby
     
  6. John Bachman

    John Bachman Guest

    Measuring resistance in circuit is not a very good practice as other
    components in the circuit are going to affect the reading. And, yes,
    lead polarity will get different results due to semiconductor
    junctions in the circuit.

    If you are measuring small values then an ESR meter can do the trick
    because they all use small voltages - too small to turn on any
    junctions. But they will still be affected by passive components in
    the circuit.

    John
    AnaTek Corporation
    The Electronic Repair Center at www.anatekcorp.com
    Moderatedd electronic epair discussion group at
    www.anatekcorp.com/elrepair/elrepair.htm
     
  7. Noah Little

    Noah Little Guest

    You broke the code. Most likely reason for different readings when the
    leads are reversed is a diode (or other semiconductor junction) that's
    in the circuit with the resistor under test. You should find if you
    lift one leg of the resistor that the readings are the same in both
    directions.

    Cheap meters, expensive meters, they'll behave about the same in this
    regard. But you'll be happy you went with the Fluke.

    Best,
     
  8. Ol' Duffer

    Ol' Duffer Guest

    Maybe. Depends on what you are measuring. A pure resistance
    should measure the same either way. If you have non-linear
    parts in the circuit (semiconductors, electrolytic capacitors)
    or residual voltages present (powered circuits, batteries,
    capacitors) it will affect readings. And in some cases, the
    presence of noise (RF, etc.) will affect the meter differently
    depending on polarity. Interpretation is the responsibility
    of the user.
     
  9. Jerry G.

    Jerry G. Guest

    What are you measuring? It must be some type of device that is
    polarity sensitive.


    Jerry G.
    ======
     
  10. Asimov

    Asimov Guest

    "slight" bravely wrote to "All" (19 Dec 04 21:58:00)
    --- on the heady topic of "multimeter question"

    sl> From: slight <>
    sl> Subject: multimeter question
    sl> Xref: aeinews sci.electronics.repair:3876

    sl> When i measure resistances with my dmm, i get a different reading if i
    sl> swap the leads around.
    sl> Is this normal?

    Stray AC being picked up by nearby objects and your body. The DMM
    analog to digital converter should integrate the AC to zero but it
    gets rectified differently depending on which part of the complicated
    external path is effectively grounded. The problem is worse with
    higher value resistors as you noticed. Try to mount a grounded sheet
    of metal under your desk or rubber mat to form a ground plane. This
    tends to help a lot. In-circuit readings will obviously be distorted
    by non-linear junctions (diodes, electros, etc). Most DMM's use a very
    low voltage source to avoid this and better DMM's have a setting to
    bias diodes if needed.

    A*s*i*m*o*v

    .... Of course it's grounded! ...watch, ...YEEEEEAAAAAAAAHHHHHHHH!!!!
     
  11. James Sweet

    James Sweet Guest

    If there's a capacitor in the circuit it can be charged by the ohm meter and
    give a false reading when the leads are reversed. You can often get a
    ballpark reading in circuit, but you have to remove a resistor at least at
    one end to get an accurate reading.
     
  12. budgie

    budgie Guest

    Not really. Most half-decent DMMs have some ranges which use a source voltage
    below the conduction threshold of (at least silicon) semiconductor junctions.
    There is ususally some marking on the DMM face which will indicate which ranges
    will/won't bias a diode into conduction.

    OTOH most el cheapo DMMs lack this feature.
     
  13. Jim Adney

    Jim Adney Guest

    You are correct about the reason for the difference.

    All Ohmmeters put some voltage across the circuit, then measure the
    current and calculate the resistance. All Ohmmeters do not, however,
    use the same test voltage.

    Many of the Flukes (as well as many other good quality meters) use a
    test voltage that is below a semiconductor breakover voltage. This is
    done precisely so that you are more likely to get a useful reading
    when you try to measure something in-circuit.

    Which model Fluke have you been looking at?

    -
     
  14. slight

    slight Guest

    the 187/189 looks really good but a bit beyond the budget.
    probably the 73. negotiations with santa continue.
     
  15. Jim Adney

    Jim Adney Guest

    The 73 is a good choice. I also like the 23, if they still make it.
    Both use low voltage for the resistance tests, and use a higher
    voltage to test P-N junctions.

    -
     
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