voltmeter reading accuracy

Discussion in 'The Projects Forum' started by mah, Jan 22, 2014.

  1. mah

    Thread Starter Active Member

    Mar 15, 2010
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    I am doing simple ohm's low using sinometer digital power supply and potentiometer 50 watt when i begin to increase the current the voltage from the power supply decreased that's okay but when i measured the voltage on the potentiometer using my voltmeter it give a value lower than that displayed on the power supply and this value decreased more and more when increasing the current , so what is the reason for this accuracy error ? is it my voltmeter needs the battery to be changed ?could the battery weakness do that?or the error is in the power supply
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    How much voltage and current are you supplying from the power supply?
    Did you take into account the resistance of your connecting cables?
     
  3. mah

    Thread Starter Active Member

    Mar 15, 2010
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    I am feeding more than 2 Amps and 25 volts , if wires resistance were taken into account that should affect also on the power supply digital reading but that didn't happen .
     
  4. mcgyvr

    AAC Fanatic!

    Oct 15, 2009
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    Are you simple measuring the voltage drop across the potentiometer?
    So you had a 25V supply.. What "reading" did you get when you attempted to measure the voltage on the pot?
     
  5. MrChips

    Moderator

    Oct 2, 2009
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    Measure the voltage across the terminals of the power supply.
    Then measure the voltage across the pot.
     
  6. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    when i began to increase the current the voltage on power supply display drops but slowly may be .1 or .3v and also on the voltmeter but when increasing the current after 1 Amp the vlotmeter reads on the potentiomter 29.5 v but it is still 30.2 on the power supply display .
     
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    just because the power supply meter reads 30 volte does not mean your getting 30 volts to your load. the resistance of the wires from the supply to the load will drop the load voltage some. measusr as close to the load as possible.
     
  8. bwilliams60

    Active Member

    Nov 18, 2012
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    What type of meter are you using and what is the rated accuracy of the meter? Do you have any bad connections?
     
  9. shteii01

    AAC Fanatic!

    Feb 19, 2010
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  10. crutschow

    Expert

    Mar 14, 2008
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    Not true. The wire resistance affects only the voltage at the load. The power supply meter measures the voltage before the drop caused by the wire.
     
  11. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    Fine. Now try measuring from the + terminal of the power supply to the + terminal of potentiometer. Then do the same measurement between the - terminal pairs.

    See if these measurements plus the potentiometer voltage equal the power supply voltage. It *must* be equal, or your measurements are in error.
     
  12. mah

    Thread Starter Active Member

    Mar 15, 2010
    276
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    thank you all . it was the wires resistance because it have small area . i used thicker one .
     
  13. ErnieM

    AAC Fanatic!

    Apr 24, 2011
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    There's still a voltage drop in those "thicker" wires. ;-)
     
  14. BillB3857

    Senior Member

    Feb 28, 2009
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    2 amps and 25 volts is 50 watts. What king of pot are you using to adjust your experimental current?
     
  15. spinnaker

    AAC Fanatic!

    Oct 29, 2009
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