Ground as a reference point for measuring voltages

Discussion in 'Homework Help' started by webplodder, May 11, 2012.

  1. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Hello. I have been reading about using some arbitrary point as ground in a circuit in order to measure voltages with respect to it but am still perplexed because the text I am reading does not seem to make clear why this is useful in electronics. Any help appreciated. Thank you.
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    Voltage is measured as the difference in potential between two points.

    A voltmeter has two leads which must be connected to the two points whose potential difference is to be measured. Any two points may be chosen.

    It is customary and convenient to use a COMMON point in the circuit as a reference potential. This is not necessarily GROUND.

    It is common practice to use one of the two terminals of the POWER SUPPLY as the reference.

    Furthermore, it is common practice to use the terminal labeled as -V as the reference but this is not a strict rule. If the circuit uses a split power supply where there are +V, COMMON and -V connections, then it is convenient to use the COMMON terminal as the reference point.
     
  3. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Thank you, MrChips, but what I was really getting at was what purpose does this serve? Does it make it easier to find faults in a circuit or has to do with obtaining more precise measurements than otherwise, or what? It may seem a naive question but when you are a beginner these things are not always obvious.
     
  4. MrChips

    Moderator

    Oct 2, 2009
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    Selecting the reference point is a matter of convenience.
    This has nothing to do with precision of measurements.

    One is interested in the voltage difference between two points, i.e. the voltage drop across a certain device.

    For example, one is interested in the voltage drop across a resistor, diode or the base-emitter junction of a transistor. If you are going to write down the voltages of all the nodes in a circuit diagram it is best to use a COMMON reference point.

    Check this out if you do not understand the concept of voltage drop:

    http://forum.allaboutcircuits.com/showthread.php?t=56372
     
  5. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Ok, I see. Isn't is possible though to simply work out voltage drops mathematically?
     
  6. MrChips

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    Oct 2, 2009
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    What do you mean? Can you give an example?
     
  7. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Sorry, yes, all I meant was if you have a couple of resistors in series, to take a simple example, can't you just work out the voltage drops using ohm's law?
     
  8. BSomer

    Member

    Dec 28, 2011
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    Yes, you can do it through the math. However, you are relying on the parts being the exact value the are supposed to be. Components like resistors have a tolerance, a range in which they will actually be. If you are using 5% resistors the actual value may be 95% - 105% of the rated value. Capacitors can have a tolerance of 20%.

    Taking this into consideration your measured voltage most likely will be off of the calculated measurement. If your circuit requires a certain voltage at a certain node to operate correctly and it isn't working properly you can measure it with the meter using the above mentioned methods.
     
  9. MrChips

    Moderator

    Oct 2, 2009
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    You can use Ohm's Law if the components are linear, i.e. the relationship between voltage and current is a straight line.

    Diodes and transistors have non-linear current-voltage characteristics and hence Ohm's Law does not apply.
     
    webplodder likes this.
  10. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Ah....the penny drops! It's a pity my textbook does not spell this out, but now I understand fully. Much obliged for your assistance, BSomer. Thank you. Thanks to MrChips also.
     
  11. #12

    Expert

    Nov 30, 2010
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    "Convenience" is the most important part of the answer. You can declare any place you want as "common". Whether it is convenient is the most important criteria for where you select to be the common point.

    Of course there are other ways to do it. Which is most convenient is your choice.
     
  12. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Yes, I see. So you can't really be sure of voltage drops without measurement. Makes sense. Thanks again.
     
  13. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Yes, I get that now. Allows for flexibility I guess. Thank you.
     
  14. Jony130

    AAC Fanatic!

    Feb 17, 2009
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    To measure the voltage we need two point in the space. One of this point is treat as a reference point. We have a very similarity situation when we try to measure a height of an object. We need a reference point. The most common reference pint is "above mean sea level". But when you measure the height of the table in your house the floor now becomes your reference point.

    [​IMG]
     
  15. webplodder

    Thread Starter Member

    Mar 23, 2010
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    Thanks Jony130, that illustrates it nicely. :)
     
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