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newbie DC question

Discussion in 'Electronic Basics' started by Rob Snyder, Dec 28, 2004.

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  1. Rob Snyder

    Rob Snyder Guest

    Greetings.

    I've been studying electronics on my own for a little while now, and
    there's one basic concept I can't seem to get my head around. I
    apologize in advance for the really basic question, but I just can't
    find the answer and it's keeping me up at night.

    I thought I understood DC, at least at a primitive level. In circuits
    with a battery, things make sense - there is a positive and a negative,
    current flows from the negative through the circuit to the positive...
    makes sense.

    Where I get lost is when I'm looking at a circuit with an AC to DC
    transformer. Typically, the output of this, after the rectifier, is a
    positive DC source, a negative DC source, and ground.

    What *is* this ground thing? Does current flow from the negative output
    to ground? From ground to the positive? Both? Neither? What they heck am
    I missing?

    Most explanations I've read just say something to the effect of "ground
    being a reference, with the current being more negative or more positive
    than ground". Needless to say, I can't turn that into something I
    understand.

    I appreciate any guidance anyone has to offer.

    Thanks!

    Rob Snyder
     
  2. Bob

    Bob Guest

    Rob,

    Great question.

    The term "ground", in electronics, is often misused. Technically, if you
    have a ground connection it means that some part of a circuit is connected
    to earth (or the ground).

    People often use the word "ground" when they really mean "common". A
    circuit's common is merely any node in the circuit that has been given the
    name "common" (or "ground") by some human. Typically a circuit's common is
    another name for one its power supply connections, and it's often also
    connected to a piece of equipment's chassis (metal). Sometimes this "common"
    is also connected to earth (ground) -- usually via the third wire (green) of
    a three wire electrical outlet.

    As you probably know, if you have a battery and you connect a passive device
    (e.g., a resistor) across the two terminals of that battery, the current
    (conventional) will flow from the more positive lead of the battery to its
    more negative lead. The current can be made to flow from negative to
    positive, but this would require the inclusion of some type of
    energy-storing device like a battery, capacitor, or inductor in the circuit.

    It's very typical to have the output of a transformer hooked up (to diodes
    and capacitors) such that you end up with three nodes -- a "common", a
    positive supply (+), and a negative supply (-). This will act the same as if
    you've connected to batteries in series (plus of one to the minus of the
    other), with the middle node being what you're calling "common". No current
    will flow into the earth (ground) with this hookup because it's not
    connected to ground. If you connect a resistor between your (+) and common
    then some current will flow. The same amount of current will flow if you
    connect that resistor between common and (-). More current will flow if you
    connect that resistor between (+) and (-).

    I haven't discussed the (very real) capacitance that is always present
    between any conductor and earth. This will allow currents of that circuit to
    flow to earth, for time-varying signals of that circuit, even though there's
    no apparent connection between your circuit and earth. For now, however,
    don't worry about this.

    Hope this helps.

    Bob
     
  3. It is a convenient reference point with which to measure many voltages
    in the circuit. It may or may not also be connected to the Earth with
    a grounding wire.
    Once you pick a convention (electrons travel around the circuit one
    way, holes and 'conventional current' the other way) just be
    consistent it its application. If you want to think of current as a
    flow of electrons, then current will take all paths that connect a
    more negative voltage to a more positive one. Ground, by definition,
    is a zero volt reference point.
    You have to separate in your mind voltages from currents. Nodes can
    be have a more positive or negative voltage than ground. Pick any two
    nodes, and one may be the more positive and the other the more
    negative, or vice versa. This potential difference is what drives
    current through the things connected between those nodes. Currents do
    not have the same sort of positive and negative polarities that
    voltages do. Current polarities just refer to whether an actual
    current goes the assumed way (positive current) or the opposite way
    (negative current) so that you can add them up correctly for things
    like Kirchoff's law (the sum of all currents into a node is zero).
    Have I helped or added to your confusion?
     
  4. Terry

    Terry Guest

    ROB: Just to see if I can add to your confusion by using some examples. :)

    My car has a 12 volt battery. The negative terminal is connected to the
    chassis of the vehicle.
    So within the vehicle that is a 'common' point for all the negative
    connections. I suppose in a way it is the vehicle 'ground' although clearly
    it is not connected to the ground since the vehicle rolls on rubber tyres!

    Many years ago my father had a car in which the positive end of the battery
    was connected to the frame/chassis of the car so the positive potential was
    'common' in that car!

    In telephone exchanges they used very large 48 volt battery installations
    charged by large rectifiers. In almost all cases the positive side of the
    battery was connected to the ground of the building. This -48 volt power was
    used to operate electromechanical telephone equipment. But there were often
    other 130 volt power supplies inside the same building that had their
    negative end connected to the same ground; this +130 volt power was mainly
    used to operate tube type equipment. So that 'ground' was a common point
    reference point for both supplies.

    Occasionally you will get circuits where neither positive or negative is, or
    needs to be grounded, these often referred to as 'floating'.

    So the term 'ground' is best thought of as a common reference point, for a
    particular piece of equipment or situation.

    Just to confuse some more? Most telephone circuits require a pair of wires!
    But there were and are very simple telephone and signalling systems which
    use one wire as a connection and the ground as the other connection. Often
    called "One wire and ground/earth systems".

    Email if you'd like to discuss some more. Terry.
     
  5. Active8

    Active8 Guest

    That's what .basics is for :)
    You got good answers here but let me nitpick since you're new.

    *Charge* flows. *Current* is *charge flow* So *charge* flows from A
    to B. "Current flow" is an expression sometimes hard to avoid. "The
    potential difference effects/causes a current in the resistor."

    <snip>
     
  6. John smith

    John smith Guest

    I remember when I was a kid...
    Behind my grandfather's TV was a bottle of Coca Cola with dirt in it and a
    piece of wire
    from the TV in the dirt.
    I asked my father WTF? (not in those words.)
    It wasn't until I started reading about "toobs" and other stuff that I
    realized...
    In Spanish literal translation, Ground = Earth = dirt (tierra)
    So, there was a bottle of dirt connected to the TV.

    Rob, you are in the right place. You got some good questins and good
    answers. Keep'em comming to try to
    get these kids in the NG straight and narrow. Otherwise they turn into black
    matter.
     
  7. Rob Snyder

    Rob Snyder Guest

    Wow... all of these responses were extremely helpful to me, and I'm
    grateful for each person's time in responding. Thanks!
     
  8. Darmok

    Darmok Guest

    All good responses you are getting here, one thing you are likely to see in
    books or in formal classes that can initially be confusing is the convention
    of current flow. Most texts even today define current to flow from positive
    to negative as the standard "convention". This makes sense intuitively as
    flowing from higher potential (or pressure, think water), to lower. But in
    reality we know that current flow is the movment of charge (electrons) which
    is the opposite, for the most part (I don't want to talk about hole current
    right now).

    But as already mentioned for the purposes of solving circuits it doesn't
    matter what convention you choose as long as you stick with it and stay
    consistent in the circuit analysis. I just wanted to bring it up in case you
    decide to formally study electronics or engineering, then you WILL see this.

    Best regards,

    Darmok
     
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