Digital and Analog Voltmeters-Differences

Discussion in 'Homework Help' started by raifteri, May 15, 2012.

  1. raifteri

    Thread Starter New Member

    May 15, 2012
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    I'm a beginner, so please bear with me.

    I read in one tutorial that voltmeters have an internal resistorin series with the probe leads. The author seemed to be describing analog voltmeters.

    Then I read in another that voltmeters have an internal resistor in parallel with the probe leads. It seemed this author was describing digital voltmeters.

    So is some of this information inaccurate or is this one of the differences between analog and digital voltmeters?
     
  2. StayatHomeElectronics

    Well-Known Member

    Sep 25, 2008
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  3. #12

    Expert

    Nov 30, 2010
    16,337
    6,821
    All voltmeters have internal resistors that adjust the measured voltage before it goes to the measuring parts so the voltage won't blow up their guts. The only thing that matters to you is that the meter will be a load on the circuit and thus might cause an error if you are measuring high impedance circuits.

    Most voltmeters have an internal resistance of 1 million ohms or 10 million ohms. Most of the time, the current through the internal resistance is not enough to throw your measurement off. You can "model" the voltmeter as a resistance in parallel with what you are measuring to calculate the error caused by the voltmeter.
     
  4. crutschow

    Expert

    Mar 14, 2008
    13,041
    3,243
    The typical analog voltmeter mechanical meter movement is basically a current operated device with an intrinsic low resistance. You change the resistance in series with the movement to change the sensitivity. Thus the input resistance varies with range (a typical value is 20kΩ/V as determined by the range full-scale).

    A digital voltmeter is a voltage operated device with a high intrinsic impedance so a variable resistor attenuator network is placed from the input to common to adjust the meter sensitivity. The attenuator is design to maintain a constant impedance with a change in gain (typically 10MΩ).

    Note that for high voltage ranges (above 500V for my example analog meter) an analog meter can have a higher input impedance than a digital meter.
     
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