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DMMs with coils

Discussion in 'Electronic Design' started by [email protected], Mar 26, 2007.

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  1. Guest

    How do DMMs measure resistance? I was testing some relay coils that
    are specced at 50 ohms resistance, but they read 0.5ohms on both
    "Brand X" DMMs we have at work. If the DMM was in manual ranging in K
    mode, it would have read 0.05. The coils don't have built-in snubbers.

    Naturally I will confiscate these meters Monday morning, but I want to
    know what's going on here.

    At home, on a 10$ Mastercraft made in China analog meter, they read 50
    ohms. On a Tektronix 249 DMM, they read 50 ohms.

    The last time I put a scope across a DMM while it was measuring
    resistance I saw narrow pulses. This must cause the coil to react and
    cause false readings on naive DMMs?
     
  2. The inductance in relay coils can cause DMMs, especially with autoranging,
    to give erratic readings. Usually they will settle down to a correct value,
    and in manual ranging there should be no problem. Maybe there is a
    protection circuit kicking in and causing the current to cycle on and off?

    Paul
     
  3. Robert Baer

    Robert Baer Guest

    *All* DMMs that i know of use a simple battery and resistor in series
    with the unknown the metering circuit.
    Only the capacitive measuring circuit may use pulses.
    Methinks you may have done something incorrectly...
     
  4. Phil Allison

    Phil Allison Guest


    ** By using dual slope integration, a reference resistor and determining the
    ratio of voltages.


    ** The auto-ranging function is going haywire.


    ** But on lots of other, auto-ranging DMMs they will not.

    Even Flukes.


    ** Yes - back emfs from the coil ( or a transformer primary) upset the
    meter.

    Either lock in the low ohms range or add a suitable C across the coil.



    ....... Phil
     
  5. Eeyore

    Eeyore Guest

    I've know the reactance of some transformers to defeat a DC resistance
    measurement with a Fluke 77.

    Graham
     
  6. Boris Mohar

    Boris Mohar Guest

    Watch out. That Mastercraft, although quite accurate has 1M input
    resistance for DC range of 20V I did not check other ranges.
    Part# 052-0060-2
     
  7. Robert Baer

    Robert Baer Guest

    THe "reactance" of the meter to the "resistance" *must* have been a
    "fluke".
     
  8. Jamie

    Jamie Guest

    that's strange, i have no problems with the meters i use.
    i normally use FLuke Extech etc..

    But i need to put them in Manual mode other wise, they just
    keep floppy all over.
     
  9. Jim Yanik

    Jim Yanik Guest

    Probably ALL the volts ranges are 1 MegR.
    The Harbor Freight $3 DMM I have is 1 MegR. I was thinking about changing
    it to a 10 MegR string,just for fun.
     
  10. Guest

    That ain't analog. I've got the cheap needle movement that moves when
    the meter is moved. Crap. Great way to test tunnel diodes though since
    it only uses a AA cell, no nasty 9V supply waiting to blow up delicate
    junctions.
     
  11. Robert Baer

    Robert Baer Guest

    All decent DMMs use a 9V battery and *none* of them will stress, much
    less "blow up" any junction, delecate or otherwise.
     
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