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Low Resistance Measurement

Discussion in 'Electronic Basics' started by [email protected], Oct 15, 2007.

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  1. Guest

    What is the recommended procedure for measuring low resistances?
    My DMM seems to wander all over the place depending on how I hold the
    probes against the conductors. Is there a better method? It's
    autoranging, so I don't know what range it's working with.
    I seemed to have better results with my old, cheap, analogue meter.

    ps I'm looking at buying a cheap $90 megger for checking insulation
    around the home and workshop. Any problems possible here?
  2. Guest

    Thanks Tom, that's something I did not think of. I remember finding my
    fridge unacceptable many years ago, but was told by a reliable source
    that fridges are notoriously bad at current leakage.
    Then I thought of my new fridge with its box of electronicals on top.
    I won't check that! I was thinking of checking the windings of old
    transformers that I am rewinding. No point wasting electricity melting
    them if I've breached the insulation anywhere :)
    Same goes for old motors that I might want to resurrect.
    And then there is elderly house wiring...
  3. Guest

    Ahh, thanks, Phil.
    Come to daddy, li'l ol' analogue meter :)
    Sort of how you can check earth wiring. Apply a decent load (electric
    radiator eg) between active and earth and see if it works as per

    Are there any DMMs out there that use a larger test current?

  4. Tom Biasi

    Tom Biasi Guest

    Your DMM will certainly have a hard time with measurements below 1 Ohm.
    For precise low ohm measurements a bridge meter designed for that purpose is
    As for problems with the megger, you could fry things if you don't know what
    you are doing.

  5. Phil Allison

    Phil Allison Guest


    ** Use lotsa current.

    ** The analogue jobs use far more test current.

    The simplest way for the odd low R measurement is to use TWO meters at once.

    One for current and the other for voltage.

    ......... Phil
  6. Mike

    Mike Guest

    Your old analog meter seemed to better because it can't display the very small changes in the resistance of
    the connections to the unknown resistance (contact resistance). The digital meter will show these variations
    and as a result the reading will wander quite a bit. A much better method is what is called 4 wire or Kelvin
    sensing. With this method 2 wires are used to supply a regulated constant current through the unknow
    resistance so that the current flowing through the resistance is independent of the contact resistance. The
    other 2 wires are used to measure the resulting voltage across the unknown resistance and that voltage is
    scaled to represent the resistance. The resistance of the circuitry used to measure the voltage is very very
    high compared to the resistance being measured so it has very little effect on the the voltage appearing
    across the the unknown resistance. Some of the more expensive meters have this capability, but you can use
    this method without such a meter by using a vaiable current regulated supply and setting the current through
    the resistance and measuring the voltage across it then calculate the resistance. You can build a simple
    current regulator pretty easiy just for this purpose.


    "I think, therefore GOD is."
    J.P. Moreland
  7. Guest

    I didn't ask you anything, Herr Controller


    ps I can't honestly think of how to create one of these, but then I'm
    just a newbie who has ventured onto your hallowed turf, apparently
  8. Phil Allison

    Phil Allison Guest

    ** No.

    Create your own.

    Do NOT come back and ask ME how to do that.

    Use your brain.

    ...... Phil
  9. Guest

    Thanks so much for that, Mike.
    So much more helpful than sending me away to "create" a DMM that uses
    a higher test current. jack

    ps I have a nice digital bench PS, so I will use that method to check
    my transformer winding resistances.
  10. Phil Allison

    Phil Allison Guest


    ** Why don't you just.......

    **** OFF TROLL !!

    Before you electrocute your stupid self.

    ......... Phil
  11. Mike

    Mike Guest

    Here's an better description of Kelvin sense resistance measurment.

    You don't really need a lot of current to get reasonably accurate measurments.
    You'll find that the winding resistance reading will creep upward as the current
    you send through the winding heats up the wire, so it's best to use a little
    current as you can and still get a decent voltage drop. You might start with 0.1A
    so that your volt meter will read .1V/ohm. That way you have no math to do except
    moving the decimal point. Hopefully your power supply will allow you to set
    the current and not just the voltage. If so just set the current to 0.1A and the
    voltage as low as you can and still maintain the 0.1A. Just be sure to measure the
    voltage drop directly on the windings. I used to build and sell milliohm meters
    to guys who race slot cars so they could check the windings on the little armatures.
    I used 0.01A constant current and still that would warm the wire up enough to see
    the readings creep up a bit. The Kelvin clips they show on the link above really
    do make things a lot more convenient especially if you will doing a lot of measurments.

    FWIW, phil allison and rest of his ilk are best totaly ignored. Those fools
    troll around doing little more than trying to stir up trouble.

    Your's was very reasonable question that deserves a reasonable response.
    I'm no engineer, but I would be more than happy to lend any assistance that
    I can to help you.


    "I think, therefore GOD is."
    J.P. Moreland
  12. Eeyore

    Eeyore Guest

    I do it this way. I have a current limited PSU with an adjustable current limit
    that I set to 1A. I then connect the item to be measured across the output
    terminals and measure the volts across the item in question. Be sure to use a
    Kelvin connection when measuring.

  13. Eeyore

    Eeyore Guest

    That's asking a lot these days. Kids today are spoon fed their education mostly.

  14. Guest

    Wonderful. Thanks. I've filed that as a keeper.
    I imagine that the test current used on my DMM is somewhat less than
    100mA, and yes, my PS can set current. Problem solved. Thanks.
    He apparently doesn't want me asking him certain questions, but I
    understood that usenet allowed you to answer what you wanted and to
    ignore what you were not interested in. Phil seems to have a
    compulsion to answer everything and object when some of this
    "everything" includes stuff that he doesn't want to answer. He needs
    to understand just what he can change and what he can't. If he does
    not like a question, then he would do himself a favour to just ignore
    it, like every other sane member of this group has learned to do.
    But perhaps he has a very empty life and usenet is a big part of it,
    and so he wants it his way. Yep, your advice is sound. Thanks.
    Thanks again, Mike, rgds... jack
  15. Guest

    The same sentiments of 2000 years ago :)

  16. Guest

    Great advice, thanks, Graham.

  17. Jamie

    Jamie Guest

    If a megger is what you want so be it how ever, in case you
    didn't already know. Most meggers generate HV in the average of
    500 Volts. Others will do more depending on the range of test
    you're doing.
    for doing G/T ohms, our units at work can go up to 2500 Volts
    to perform the test.

    I guess if you were just doing test on transformers and things
    of that nature, a basic meter of 500 volts will do fine.

    I've seen some newer one's use lower voltages for test.

    The general idea is to use a voltage on the fixture to which
    the item under test was designed to handle. Also the HV makes it
    easier to get High R readings.
  18. Phil Allison

    Phil Allison Guest


    ** Why don't you just.......

    **** OFF TROLL !!

    Before you electrocute your stupid self.

    OTOH, that might be a positive outcome for humanity.

    ......... Phil
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