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How to measure DMM's current circuit resistance?

Discussion in 'General Electronics Discussion' started by Carl Ellis, Feb 5, 2016.

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  1. Carl Ellis

    Carl Ellis

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    Feb 5, 2016
    To meet a minimum accuracy spec, a device specifies that the 4-20mA DC current measurement from its 'test' terminals requires a milliamp meter with an impedance of less than 10 ohms. None of the meters in the shop have a current circuit 'resistance' value in the spec sheet.

    I assume that a DMM meter's current circuit has a relatively low resistance across which the voltage drop is measured and then converted to a current value.

    To measure the resistance of the 20mA milliamp current scale of digital multimeter I was considering using a 1.5V dry cell in series with a 'precision' (0.1%) 100 ohm resistor to get an approximate 15mA current source (not a particularly heavy load on a dry cell).

    [​IMG]

    Measuring the voltage drop across the meter's input terminals (A to B) meter should show the voltage drop across the internal resistor in the DMM under test. That IR drop divided by the 15mA current should tell me the DVM's resistance.

    From a physics class 45 years ago, I seem to recall that a battery/cell has an internal resistance. Is that internal resistance large enough to be a consideration for a measurement like this?

    Or is there some other (relatively) simple measurement that is I can make?
     
  2. TedA

    TedA

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    16
    Sep 26, 2011
    Do you have a second meter with a resistance range? Just measure it!

    The meter under test need not be turned on, but the current range of interest must be selected.

    The most common voltage burden for DMM current ranges is 200mV full scale, for all current ranges. So higher current ranges use lower value shunt resistors. Meter specifications are more likely to list this voltage, rather than the shunt resistance.

    The 200mV value works out to require a 10 ohm shunt for a 20mA range. Throw-in some switch contacts and leads and you will have a bit "over 10 ohms".

    A 200mA range is likely to use a 1 ohm shunt, so that might work for your application, if you don't need the full accuracy of your meter.

    Ted
     
    Carl Ellis likes this.
  3. Harald Kapp

    Harald Kapp Moderator Moderator

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    2,443
    Nov 17, 2011
    but the voltage drop when measuring amps should be specified. From R=V/I (max. voltage drop at max. current in range) you get the resistance. The results should match what you get using Ted's method, also a good one.
     
    Carl Ellis likes this.
  4. Carl Ellis

    Carl Ellis

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    Feb 5, 2016
    Gentlemen, thank you for your succinct replies.

    I read Ted's reply and went, "Duh". Why should I create a milliamp meter from components when I can use a commercial unit to do the same thing? I'm guess I'm showing my age. And thanks for the 'typical' values used for ranges.

    I'll re-check spec sheets for the IR drop, Harald.

    Thanks again,

    Carl
     
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