Digital Multimeters volts measurement

Discussion in 'General Electronics Chat' started by Chacabucogod, Jul 10, 2015.

  1. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    How do digital multi meters measure voltages that can range from 0 to 200 DC volts or more. Is there a circuits that demaplifies it? or do they have simple resistor voltage dividers?

    Thank you.
     
  2. #12

    Expert

    Nov 30, 2010
    16,247
    6,744
    It has always been voltage dividers as far as I know. Why spend money for an active de-amplifier chip with several parts attached when a 3 cent resistor will do the job?
     
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  3. WBahn

    Moderator

    Mar 31, 2012
    17,715
    4,788
    You wouldn't want to use a "deamplifier" because that circuit would have to be able to tolerate 200V (or commonly 1000V). So you use a voltage dropping resistor, commonly in the form of a voltage divider configuration.

    Many old analog meters use a meter movement that required a certain current to get a full-scale deflection. All you did to make it a voltmeter was to put in a series resistor of the right size so that a full-scale deflection mapped to a particular full-scale current. You can do the same thing with an ADC circuit using a fixed current sense resistor with a series voltage drop resistor.
     
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  4. Chacabucogod

    Thread Starter New Member

    Nov 16, 2013
    26
    1
    All right! Thank you!
     
  5. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
    1,135
    200
    Those old meters had specs like 50K ohms per volt, so the input was dependent on the voltage meassured. Some test equipment like tube testers the voltage used for calibration took this variable input Z into account, so a modern 10 M meter won't work.

    Modern general purpose multimeter have a constant 10 Meg-ohm input Z which is the direct result of voltage divider.

    Oscilloscopes are either 50 ohms|| xx pf or 1 Meg ohms xx pf where xx is usually about 20 pf. This allows the use of impeadance divider and a 10 M input when using a probe.

    Outside of the multipurpose meters, you can find higher input impedances and impedances that are dependent on range/
     
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