Why we consider ground or reference point in electronic circuit.

Discussion in 'General Electronics Chat' started by sabbi, Feb 2, 2010.

  1. sabbi

    Thread Starter New Member

    Feb 2, 2010
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    Why we consider ground or reference point in electronic circuit.
     
  2. sabbi

    Thread Starter New Member

    Feb 2, 2010
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    no reply. why?
     
  3. bluebrakes

    Active Member

    Oct 17, 2009
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    how do you mean?

    having something like a battery in a circuit and there is still a ground symbol present?
     
  4. Wendy

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  5. SgtWookie

    Expert

    Jul 17, 2007
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    These forums are not interactive. Sometimes it can take days between a post and a reply.

    Even though a circuit may be completely isolated from a "real" earth ground, it is customary to declare one node in the circuit as being ground, or the 0v reference point. This makes documenting the other voltages in the circuit much easier to do, and much easier to understand, than if some arbitrary voltage were selected for the reference point.
     
  6. ELECTRONERD

    Senior Member

    May 26, 2009
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    Remember that there are different grounds; earth being the best.

    I think it works like this, someone correct me if I'm wrong:

    All electrons have a negative charge, but what defines them as "negative" or "positive" is the fact that one side has more electrons flowing than another side. So if one side has more electrons than the opposite side, it is more negative.

    Again, earth provides the best negative ground.

    Austin
     
  7. studiot

    AAC Fanatic!

    Nov 9, 2007
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    So what provides the best positive ground?


    One post and 7 minutes later one grouch by the OP then nothing more . No thanks or response to the four replies.

    Why do we bother?
     
  8. ShockBoy

    Active Member

    Oct 27, 2009
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    I would think whatever the clouds do in a thunderstorm. Maybe someone will put that in a battery.

    At least others might read the post and learn something from the responses.
     
  9. PackratKing

    Well-Known Member

    Jul 13, 2008
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    AC panel : for each breaker, neutral goes to a large bus bar, which is earth grounded [ at least so far as I have seen ]

    So if a given projects chassis, downstream from the transformer is connected to the green wire, paying due attention to L1 or hotwire polarity into the main switch [[ as do "double insulated" power tools ]] one always has an optional earth ground on board............. if the neutral is not being used as a ground.

    My main question; Old timers, please humor a relative noob :rolleyes:

    is it necessary to put any kind of diode [ huge ] or a fuse, on the earth ground strap from the AC panel, that will protect / prevent the ground plane conducting close-quarters lightning strikes in the back door...................????? I believe it was a strike of this nature that blew $ 5000 worth of camera service test equipment out for me some years back..........
     
  10. cumesoftware

    Senior Member

    Apr 27, 2007
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    Since the theory here is a bit fuzzy, I'll explain. As you know, all objects have electrons, and therefore, charge. But it is impossible to know the absolute charge of an object, and as so you cannot reference the voltage on the absolute charge (or potential) as you would for absolute quantities such as mass or volume. However, since electrons flow from the most negative charged object to the less negative (most positive) charged object, you can actually measure the difference between charges. That difference is called EMF, potential difference or simply voltage.

    In theory, you can assume that a reference terminal or pole has 0 charge, and that is your ground terminal. The other terminals are referenced from there, so their voltage is actually the potential difference to your ground reference. In practice, it works, because you don't know the absolute potential of any of the nodes or terminals.

    The ground is considered to be neither positive nor negative. Earth ground makes a good reference point, for example, but you can have a floating ground disconnected from the earth and it is a valid reference as well.

    Hope it helps! :)

    It seems to be that way in the US, for what I've been told, but here neutral is different from ground. Although neutral is referenced at 0V, it comes from a three-phase transformer. Earth is local, and it is not connected to ground.
     
    Last edited: Feb 2, 2010
  11. russ_hensel

    Well-Known Member

    Jan 11, 2009
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    It is possible to know the absolute charge of an object. Consider an electron +1, a hydrogen atom 0 = 2 -2 etc. Of course these are a bit unusual items. In any case the meaning of potential is an abstraction that comes from the nature of the electric field which is a bit less abstract. Since the field is conservative it can be derived from a scalar function of position. Del dot this function is then the electric field. The function is not unique, but all such functions differ only by a constant so potential differences are unique. Where 0 is is entirely up to you, but once chosen can not be moved around without adjustment.

    Ground is a conductor driven into the ground. Since the ground conducts ( more or less ) all such "grounds" are at the same potential ( more or less ). Since anyone can drive a stake in the ground it may be convientient to name is 0 but there is really not much significance in this. The same for any circuit. The most common point may convientiently called 0 v or ground.
     
  12. thyristor

    Active Member

    Dec 27, 2009
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    I suspect that what you may mean by your question is "why" is there a ground or reference point in a circuit. Well consider this, when we say a mountain is 10,000 feet high, what do we mean? 10,000 feet higher than what? We don't mean 10,000 feet above its base because its base may be higher or lower than another mountain's base so it would then be very difficult to get a relative comparison of mountain heights.

    We always need a reference point and, for mountains, it is usually sea level, so sea level becomes our ground or reference point and all measurements of the height of mountains and towns is referenced from sea level.

    However, if we consider the world's tallest building, the Burj Khalifa, in Dubai at over 800 metres (2625 feet), we don't mean 2625 feet above sea level, we mean above its base on the ground.

    Likewise, in a circuit, when we say point X is at +10 volts, we mean +10v above our reference point; and that reference point can be any point we choose. We can even choose point X as our reference point and now its voltage will be 0v instead of 10v although physically nothing has changed. So all voltage measurements are relative to some point; the point you choose to put the black lead of your multimeter in simple terms.
     
  13. studiot

    AAC Fanatic!

    Nov 9, 2007
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    A ground is more than simply any point (voltage) randomly chosen in a system or circuit and called ground.

    In addition to voltage there are both current (charge) and time aspects to consider.

    A ground acts differently from any other circuit element - it has unique properties.

    There may even be more than one ground in a circuit.

    Let us suppose we have a giant circuit with ground stakes driven in on Earth, Jupiter and Saturn. It is quite conceivable that all three planets will act as 'grounds', but be at (measurably) different potentials.
     
    Last edited: Feb 3, 2010
  14. studiot

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    Nov 9, 2007
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    Sophia, ask yourself what happens if you choose the middle wire and its 'voltage' switches between -25 and -25 once every 10 seconds.Meanwhile The voltage on the other two wires interchange 10 times every second.

    What is the ground voltage now?

    You need to take time into account.

    Alternatively suppose I connect a very low resistance between your centre wire and the upper 50 volt one and a very high resistance between the centre and the lower wire. Let us further suppose the upper and lower wire are connected to a powerful supply, but the centre one is just a small wire.

    Large currents flow. What then is the ground voltage in your centre wire?

    You need to take current flow into account.

    The main characteristic of a ground is that it does not change potential whatever the current flow during the timescale of interest. This is true of reference grounds, screening grounds and protective grounds.

    We have been over this before I advise a forum search.
     
  15. thyristor

    Active Member

    Dec 27, 2009
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    But if the middle wire is the reference, then its voltage won't change. It will appear that the voltage on the other two wires is changing relative to our 0v reference.

    I think there is confusion creeping in as to what may be a good "earth" point and the definition of a reference point. They are not necessarily the same animal.
     
  16. studiot

    AAC Fanatic!

    Nov 9, 2007
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    If you really think this perhaps you would like to explain why we ground protective earths to a well buried heavy duty metal stake rather than a one inch nail, dangled out of the window.

    By your reasoning it would make no difference to the person holding the exposed metal part of apparatus when a line to ground short occurs.
     
  17. thyristor

    Active Member

    Dec 27, 2009
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    If you read what I wrote carefully, you will see that I covered the point you have tried to make. If you need me to explain it in words of one syllable, I am of course happy to do so.
     
  18. studiot

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    Humour me. The stage is yours.
     
  19. GetDeviceInfo

    Senior Member

    Jun 7, 2009
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    Maybe you could explain the concerns of 'step voltage' surrounding substations.
     
  20. ShockBoy

    Active Member

    Oct 27, 2009
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    time is irrelevant, a man made unit of measurement! if you really want an equivical answer- leave time out of the equation. Maybe change 60 seconds to 75 seconds per min. Total man made---total B.S.
     
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