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"zero out" ohms on DMM

Discussion in 'Electronic Design' started by Lee Carkenord, Jan 15, 2004.

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  1. I have a Micronta DMM by Radio Shack, circa 1985.

    When I short the probes together prior to resistance check, it shows
    approx. resistance of .8 ohms.

    The probes themselves nominally have zero resistance, determined with
    another, much more sophisticated DMM.

    Is there any kind of internal adjustment to make, to get shorted
    probes to read ZERO ohms?

    A trim cap or pot? Something to tweak..........?

    Any ideas or comments?

    Thank you.. Lee Carkenord
     
  2. mook johnson

    mook johnson Guest

    Not worth the trouble. Just subtract .8 ohms from the measurement and
    you're there. For most measurement thats close enough. Anything more
    precision than that requires a more sophisticated DVM.
     
  3. That is about the right resistance for the wire leads, and the
    resistance between the two probe surfaces when you touch them together.
    Anything you mess with in the meter will throw all the other ranges out
    of spec. Just remember to do a little mental math when reading low ohm
    resistance.

    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  4. Seriously, if your 'other much more sophisticated DMM', thinks that the
    leads have zero resistance, there is something wrong with _it_. Around
    0.4ohm, is typical for a multimeter lead set. The Micronta unit probably has
    some extra resistance internally in the sockets, and between these and the
    meter sense point.
    Subtract 0.8ohms from the readings. It is simpler than playing around. The
    odds are, that unless the meter is designed to do this (and has the ability
    to subtract a value just from the 'ohm' reading), any adjustment, will
    result in the opposite error being present in other readings.

    Best Wishes
     
  5. Jim Yanik

    Jim Yanik Guest

    Better DMMs use a 4-terminal measurement mode to eliminate the lead
    resistance as a factor. HP14401A is a good example.

    I wonder what the guy's DMM zero reads when using a shorting bar instead of
    meter leads?
     
  6. And of course, before we had digital meters, nobody (except those who
    were dealing with very small resistances and used special equipment) ever
    noticed the resistance of the leads. The scale simply couldn't show that
    small a resistance.

    Michael
     
  7. What is your problem?
    --
    We now return you to our normally scheduled programming.

    Take a look at this little cutie! ;-)
    http://home.earthlink.net/~mike.terrell/photos.html

    Michael A. Terrell
    Central Florida
     
  8. I read in sci.electronics.design that Michael Black
    Meet the shunt ohmmeter. Home constructed multimeters often used a 1
    mA/100 ohm meter. OK, put a 1.01 ohm shunt on the meter, so it reads
    full-scale with 100 mA. Now connect a battery and a pot and adjust to
    get full-scale reading. Now a 0.1 ohm resistor in parallel with the
    shunted meter gives a reading of nearly 10% of full-scale. A resistance
    of 0.005 ohms can be detected.
     
  9. Al

    Al Guest

    Hah, I had an engineer working temporarily in my lab who tried to read
    the values of 22 pF caps using 4 ft long leads. Needless to say, he
    didn't last long.

    Al
     
  10. You should have opted for the analog version; the one I bought in college
    (and still appears to work!) has a compensation thumbwheel; touch the probes
    together and turn the thumbwheel till it indicates 0...

    Regards
    Bob Monsen
     
  11. normanstrong

    normanstrong Guest

    What does the meter read in the most sensitive DC Volts range when you
    short the probes together? How about the most sensitive DC Ma range?

    Norm Strong
     
  12. I wonder what the guy's DMM zero reads when using a shorting bar instead of
    It reads .7 ohms with a short fat copper shorting bar placed across the terminals.

    Lee Carkenord
     
  13. What does the meter read in the most sensitive DC Volts range when you

    With switch in DC VOLTS position, it has a constantly changing value
    (of millivolts) displayed when leads are shorted together.

    With switch in DC MA position, readout says 00.0 whether leads are
    shorted together or not shorted together.

    Lee Carkenord
     
  14. normanstrong

    normanstrong Guest

    The fact that it wanders in volts and not in mA suggests that there's
    some leakage at the input. I'd open the unit up and clean out around
    the input divider and the input to the A/D converter.

    One other question. Can you put the instrument in the 1 volt range?
    If so, what does that range look like with nothing at the input?

    Norm Strong
     
  15. One other question. Can you put the instrument in the 1 volt range?

    Thank you for your response.

    Its an economy-type digital multimeter, with auto-ranging. It has
    switch positions for AC volts, and for DC volts, but there is no
    switch position for "1 volt".

    Lee Carkenord
     
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