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Reading an Analog Meter, given ranges

  1. May 8, 2016 #1
    1. The problem statement, all variables and given/known data
    1. Let’s say you know that the input signal will be about -0.25 V. What range should you select to give maximum deflection without going off scale? Draw a line, with a straight-edge, on the picture above, showing how the pointer would point. Below is the picture of the analog meter.

    2. Relevant equations


    3. The attempt at a solution
    The range selected would be 10V, then divide 10V by 10 (the range) giving you 1V. After this, -.25V is divided by the 1V, giving you -.25V. I am not sure if doing it this way is correct? Screen Shot 2016-05-08 at 2.46.30 PM.png
     
  2. jcsd
  3. May 8, 2016 #2

    berkeman

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    Staff: Mentor

    Welcome to the PF.

    Your solution drawing does not look correct. You see that the meter display can show either + or - values, right? And 0.25 is closest to (without going over) which of the available ranges?
     
  4. May 8, 2016 #3
    .25 would be closest to the 0.3 V range.
     
  5. May 8, 2016 #4

    berkeman

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    Staff: Mentor

    Correct! So if you set the meter to the +/- 0.3V full-scale range, where will the needle point when you put in the 0.25V test signal? :smile:
     
  6. May 8, 2016 #5
    Great! Would I have to do a calculation to figure out where the needle will point?
     
  7. May 8, 2016 #6

    berkeman

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    Staff: Mentor

    Nope. Just draw where it would be on the meter. Remember, you are using the lower scale, with negative full scale as -0.30V and positive full scale as +0.30V. Where does the needle point to show -0.25V?
     
  8. May 8, 2016 #7
    I would say -0.30V, but for deflection would it be the +0.30 V?
     
  9. May 8, 2016 #8

    berkeman

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    Staff: Mentor

    No. The needle will point on the meter face so that you will be able to read the voltage as -0.25V. So the needle can't deflect all the way to -0.30V, or you would mis-read the voltage.

    Think of those scales as just number lines. If you have a number line from 0 to 10, and the value you want to indicate is 8, where would you point your finger along the number line? :smile:
     
  10. May 8, 2016 #9
    You would point your finger to the line 8. So the needle will be pointed at -0.25V on the lower scale (somewhere in between the 0 and 0.5)?
     
  11. May 9, 2016 #10

    berkeman

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    Yes! Sorry it took me so long to respond. :smile:
     
  12. May 9, 2016 #11
    Umm... If the range setting is 0.3V full scale, how close to full scale would 0.3V be and how close to full scale would 0.25V be?
     
  13. May 10, 2016 #12
    ADDENDUM TO POST #11:
    Where did the 0.5 come from?
     
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