Reading an Analog Meter, given ranges

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Homework Help Overview

The discussion revolves around selecting the appropriate range for an analog meter to measure an input signal of approximately -0.25 V, ensuring maximum deflection without exceeding the scale. Participants are examining the characteristics of the meter and how to interpret its readings based on the selected range.

Discussion Character

  • Exploratory, Assumption checking, Problem interpretation

Approaches and Questions Raised

  • Participants discuss the selection of a 10V range and its implications for measuring -0.25 V. Some question whether this approach is correct. Others suggest that a range of +/- 0.3V would be more appropriate, prompting further exploration of where the needle would point for the given input signal.

Discussion Status

There is an ongoing exploration of the correct range setting for the analog meter. Some participants have provided guidance on interpreting the meter's display and the implications of the selected range, while others are still questioning the details of the readings and calculations involved.

Contextual Notes

Participants are navigating the constraints of the analog meter's scale, including the need to avoid misreading the voltage and the implications of using negative and positive full-scale values. There is also a discussion about the significance of the 0.5 V mark in relation to the readings.

Sarah Dabaja
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Homework Statement


  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.

Homework Equations




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
 
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Sarah Dabaja said:

Homework Statement


  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.

Homework Equations




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? View attachment 100459
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?
 
berkeman said:
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?
.25 would be closest to the 0.3 V range.
 
Sarah Dabaja said:
.25 would be closest to the 0.3 V range.
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:
 
berkeman said:
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:
Great! Would I have to do a calculation to figure out where the needle will point?
 
Sarah Dabaja said:
Great! Would I have to do a calculation to figure out where the needle will point?
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?
 
berkeman said:
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?
I would say -0.30V, but for deflection would it be the +0.30 V?
 
Sarah Dabaja said:
I would say -0.30V, but for deflection would it be the +0.30 V?
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:
 
berkeman said:
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:
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)?
 
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  • #10
Yes! Sorry it took me so long to respond. :smile:
 
  • #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?
 
  • #12
ADDENDUM TO POST #11:
Sarah Dabaja said:
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)?
Where did the 0.5 come from?
 

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