Ground And AC

Discussion in 'General Electronics Chat' started by zaidqais, Aug 24, 2009.

  1. zaidqais

    Thread Starter Member

    Feb 14, 2009
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    Hi Guys
    I have a question about AC and ground
    when the polarity change in the AC voltage source ( like the following figure )
    [​IMG]
    does the ground area become positive or stay 0 Volt ? What will happen to the ground ... :cool:

    Thank you
     
  2. alphacat

    Active Member

    Jun 6, 2009
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    The earth is considered to be 0V, so if the ground of your circuit is earthed, then its 0V at any given time.

    I'm also intersted to know how does the ground behave when its floating (not earthed).
     
  3. zaidqais

    Thread Starter Member

    Feb 14, 2009
    16
    0
    I am really confuse about ground and how we deal with it in AC
    i mean in the figure above does the voltage in the positive polar is zero ? and what is the voltage in the negative polar ??
     
  4. alphacat

    Active Member

    Jun 6, 2009
    186
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    I only have experience in measuring voltages related to Earth.
    If your ground is earthed, then your ground is constantly equal 0V, and the voltage of the node V_source - R1 will alternate between Vp and -Vp.
     
  5. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    any voltage measurement is taken from some point to a reference point. The reference point is that only, a point of reference. If you ask, does ground take on some charge?, that can only be answered if you specify in reference to what?
     
  6. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    The actual Earth usually will absorb just about any charges, so an electrical connection to Earth will generally be uncharged. A circuit ground is named that because it traditionally was connected to actual ground as in earth.

    Nowadays, a circuit may not connect to the actual Earth, but it still is helpful to call a common point "ground". When that terminology is used, voltages are measured with respect to that point.

    So your circuit diagram is not quite right where there are "+" and "-" signs beside the AC generator symbol. The side of the AC generator that connects to ground will always be taken as zero volts, and the other side will alternate between + and - volts.
     
  7. vindicate

    Active Member

    Jul 9, 2009
    158
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    I've been struggling with this concept also. More so with AC from the wall. The best I could come up with is there isn't actually a 0V in an AC circuit.

    If you use a 12V center-tapped transformer even though they state it is 6-0-6, I find the to be false. In reality it is 0-6-12. The center tap is not actually 0V but 6V. So in the case of using only 2 diodes and using the center as ground, it is not ground as in 0V but it's ground as 6V.

    I could be wrong but that's what I've found.
     
  8. davebee

    Well-Known Member

    Oct 22, 2008
    539
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    vindicate, the measurements are kind of arbitrary. Say your transformer has a 12v secondary winding with ends A and B, and a centertap C. If you measure between A and B, you'll measure 12 volts. If you measure between A and C, or B and C, you'll measure 6 volts.

    If you call C the common, you could say that with respect to C, ends A and B each measure 6 volts. So this is a 6-0-6 transformer.

    If you call A the common, you could say that with respect to A, C measures 6 volts and B measures 12 volts. So this is now a 0-6-12 transformer.

    Neither way is really wrong; they're just different ways of looking at it.
     
  9. Jony130

    AAC Fanatic!

    Feb 17, 2009
    3,963
    1,098
    It's stay 0V because we assume that it's 0V all the time, but voltage on "A" node is equal -0.7V
     
  10. vindicate

    Active Member

    Jul 9, 2009
    158
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    Right, that is how I was explaining there technically is no 0V for a ground. You can say that the reference point is 0V when in reality it may not be.
     
  11. davebee

    Well-Known Member

    Oct 22, 2008
    539
    46
    In zaidqais's example, one side of the circuit is marked as grounded, so it is at 0 volts.

    In yours and my transformer example, the secondary is not necessarily connected to any ground at all, so all we can say for sure about the voltage between the terminals are what we measure between the terminals with a voltmeter.

    If you mean by "reality" what is the voltage that exists between an isolated transformer winding and ground, it's anybody's guess. It would depend on the leakage of all the various insulated paths that lie between the winding and ground.

    But if you ground one side of a circuit, and call it a reference point, then it is at 0 volts. It doesn't matter whether the circuit carries AC, or DC, or both. What other "reality" is there?
     
  12. GetDeviceInfo

    Senior Member

    Jun 7, 2009
    1,571
    230
    In other words, voltage, or an electrical potential, exsists between two points. It does not exsist at a single point alone. Ground then, has no charge, when compared to itself, even though magnetic flux and eddy currents surround us.
    If you scope between two isolated systems, all you'll see is ambient or common noise.
     
  13. vindicate

    Active Member

    Jul 9, 2009
    158
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    By grounding do you mean physically connected it to a source of ground(a water pipe, the ground wire on an outlet)? If that's what you mean, at least 90% of the linear wallwart transformers I've seen have had no source to ground.

    Or by grounding to you mean that you "ground" it by saying it is the reference point for the circuit, be it 6V, 1V or 15V.
     
  14. Computer Engineer

    New Member

    Aug 25, 2009
    7
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    Great attachments :cool:
    I understand it , when the polarity changed to be positive in the grounded area , the grounded area still 0 v but the negative area changed to be negative Volt so 0 - (-Vin) =Vin volt and we have and different in voltage
    hopefully I understood it right :rolleyes:
     
  15. davebee

    Well-Known Member

    Oct 22, 2008
    539
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    It is confusing when terms like "ground", "reference", "earth" are used interchangabily without being clear on how they're being.

    I was using the term "ground" as in attach to a water pipe, but it doesn't make any difference for the voltage discussion.

    You're right, many wallwarts are not grounded. They're cheaper that way, and usually the equipment they operate doesn't need grounding to run.

    But with a transformer secondary, even if no terminal is actually grounded, you're still free to call one of the terminals your reference point. Then you could say that there is 6 volts from the reference point to one remote terminal, 12 volts from the reference point to the other remote terminal, or zero volts from the reference point to itself.

    Or by choosing another terminal as the reference point, the other two terminals might both measure 6 volts.

    You can pick any reference point you want because voltage is always measured between two points, like distance. You could say that the distance between your house and a train station is a mile, but it wouldn't make any sense to say that the distance of your house is a mile. Same with voltage - you could say that the voltage between two points is 6 volts, or that the voltage from a terminal to ground is 6 volts, but it makes no sense to say that a terminal measures 6 volts, unless it is clear what the reference point is.

    In many circuits, there is a set of connections that are an unspoken reference point, often called ground even when it isn't actually grounded to Earth. This is so common that it is common practice to label individual points in a circuit with a voltage value. But in every case, those voltages are referred to the common point unless explicitly labeled.
     
  16. vindicate

    Active Member

    Jul 9, 2009
    158
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    Thats the exact answer I was looking for. Thanks for that.
     
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