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embarrassing questions (long)

Discussion in 'Electronic Basics' started by Stephen Ramsay, Jul 16, 2003.

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  1. I'm going to reveal the extreme depths of my ignorance here, but I just
    can't get my head around schematic diagrams and I'm hoping someone can
    help.

    I'm an absolute newbie. I've read Forrest Mimms and several other books
    designed for beginners. I know what the various circuit symbols mean,
    and I understand (at least in a preliminary way) what the various
    components do. I also think I understand the ohm's law, and the various
    rules governing series and parallel circuits. It suspect, however, that
    I don't understand these things fully, and that this is part of the
    problem.

    All the books I have start out by describing lots of closed circuits.
    In each case, the circuit has a battery. The lines in the diagram lead
    from the positive terminal of the battery, through several components
    (resistors, diodes, lamps, whatever), and return to the negative
    terminal. All of this seems to make total sense.

    However, each book, at a certain point in the explanation, abandons this
    format and moves (without warning) to a schematic like the one I've
    crudely described below -- that is, there's a voltage in, a voltage
    out, and a number of ground symbols. In other words, the circuit
    isn't "connected" in the way the earlier ones were. I'm having a lot
    of trouble understanding how to read even these simple circuits, which
    means, of course, that the more complicated ones in magazines are
    entirely illegible.

    +V
    |
    /
    \
    /
    \
    |
    ------------------------->V out
    | |
    v |
    - \
    | /
    v \
    - /
    | |
    v |
    - |
    | |
    = =

    [I'm sure I'm not doing this right. The little "v"s are represent three
    diodes in series. It's the first circuit example in Mimms on p. 101]

    Now, let me try to walk through my muddled thinking on what's going on
    here, and see if someone can tell me where I'm making the big conceptual
    errors.

    1. What do those ground symbols mean? Well, they're supposed to be
    connected, so let me imagine that they're connected to each other, and
    that they're further connected to the negative terminal of the battery.
    Now, ground, if I understand it correctly, is the point at zero voltage.

    2. But what is "zero voltage?" If voltage is analogous to pressure,
    then zero pressure might mean that electrons are flowing in an unimpeded
    manner. No, that's got to be wrong, because that sounds like
    resistance. Perhaps it means that the voltage drop across the entire
    circuit goes from 12 volts at the positive terminal of the battery to
    zero volts at the point indicated as ground. I'm sure I'm confused,
    because voltage doesn't "go" anywhere. Perhaps now I'm confusing it
    with current.

    3. What if there was a wire connecting the positive terminal to the
    negative terminal and nothing else. Where would the "zero voltage"
    point be in that case? Perhaps it would be the case that the wire
    itself is zero voltage and that the voltage rating (say, 9v) is the drop
    *across" the battery itself. Hmm.

    4. What to do with V out? Well, it has to be connected in some way to
    the rest of the circuit (or, presumably, to another circuit that's
    connected to this one). Mimms indicates that the voltage at that point
    is n x 0.6 volts. Okay, I think I get that. The diodes are adding
    voltage in 0.6 increments. But what is that wire connected to? If we
    connect it to ground, it seems like we'll have a situation in which the
    n x 0.6 voltage is connected to this mysterious "zero voltage."

    5. Perhaps we could imagine that the n x 0.6 voltage is connected to
    some load that is in series with the ground point. The load "uses it
    up" so that the voltage returns to zero. Okay, that has just got to be
    wrong. I'm still thinking of voltage as if it's resistance.

    As you can see, I'm utterly confused. And I have a sense that my
    confusion is borne of fundamental misunderstanding. I would really
    appreciate it if some kind soul would take the time to show me the error
    of my ways. I feel like I'm not going to get anywhere until I sort this
    out.

    Steve
     
  2. Stephen Ramsay wrote:
    (snip)
    Looks fine to me.
    A volt meter has two leads. Voltage is always measured between two
    nodes. The ground symbol in this diagram just means that one of your
    voltmeter leads connects to that node, and since the supply point is
    listed with a positive sign, it makes some sense to connect the
    negative voltmeter lead ot ground, so that all measured voltages will
    be positive numbers. This way, conceptually, you can imagine
    voltages at various nodes, without having to add that each of those
    voltages is with respect ot this arbitrary ground node. You don't
    have to say it, but you have to think it. In addition to all other
    nodes having some voltage with respect to the ground node, there are
    many other pairs of nodes that you might want ot measure between, for
    some reason. For instance the voltage across each resistor should be
    proportional to the current passing through that resistor (with the
    resistance being the proportionality factor). Diodes have a very
    different rule that relates voltage across to current through that is
    logarithmic. Capacitors and inductors have time involved in their
    voltage to current relationships, etc.
    It is a reference point that other voltages are measured with respect
    to. If the circuit is not actually connected to earth in any way,
    there can be an arbitrary voltage (such as static electricity) on this
    ground with respect to earth. That makes no difference to any of the
    parts in the circuit, until some node in the circuit contacts
    something that has a path ot earth, or is influenced by the electric
    field related to that earth voltage difference.
    You get to assign the zero voltage point, since it is just the node
    you hook the negative meter lead to. If you wire across the battery,
    the side you have chosen as ground (common is a better word, if it is
    not actually connected to earth) is zero volts, by definition, and the
    other side of the battery is very close to zero volts. Its voltage is
    related to the resistance of the wire (very low) times the current the
    battery can push through it, which is limited by the internal
    resistance of the battery.

    So if you chose the negative battery terminal as voltage common, and
    the wire has a milliohm of resistance and the battery can deliver no
    more than an ampere of current (say, a 9 volt battery with 9 ohms of
    internal resistance) than the positive terminal will briefly show a
    voltage of .001 ohm * 1 ampere = 1 millivolt positive, with respect to
    voltage common. Then the battery will get hot (that same ampere
    passing through the batteries resistance will produce 9 watts of heat)
    and then the battery will give out, and the voltage will approach
    zero.
    A wire can be thought of as approximately a node having a single
    voltage all along its length, only is you are willing to ignore its
    small resistance and current. This is often a fine approximation,
    until you get to a high gain circuit that happens to amplify the
    voltage drop across some bit of wire or circuit trace, contaminating
    the desired signal with an unwanted one.
    You should be able to answer that by now. A volt meter has two
    leads. Where are the two implied connections when measuring Vout?
    Measuring the voltage between some node and ground implies that some
    other circuit component (or group of components) will be connected
    between the same two nodes, so that that voltage has some point. If
    the load were connected between Vout and some other voltage in the
    circuit, measuring Vout with respect to voltage common (ground) is of
    only indirect utility.
    Has this helped?
     
  3. For drawing convenience, it is common practice to show connections to
    power sources and "ground" as symbols scattered around the drawing,
    rather than drawing all the connections.


    "Ground" is a much misused and abused term in electricity and
    electronics.

    Sometimes (in AC power wiring and some radio antenna systems),
    "ground" really does mean "a connection to the earth". However, in
    most cases, "ground" is just the point in the circuit that the
    designer decided to call "zero volts" - it is where he puts the black
    lead of his voltmeter when measuring voltage elsewhere in the circuit.
    When used this way, it might be better to say "reference point" or
    "common", instead of "ground", but common practice seems to be to use
    "ground".

    There is nothing mysterious about "zero voltage". It is just the
    point in the circuit we use as a reference. You cannot measure
    voltage "at" a single point - you must always measure the voltage
    between two points, and one of them is usually "zero volts".

    The "zero volt" point in a circuit is often the most negative terminal
    of the power supply, but in some circuits could be the most positive
    terminal. In many analog circuits, "zero volts" is the mid-point of
    the power supplies - you have both positive and negative power
    supplies.

    In your circuit above, you just have to imagine that a battery is
    connected between the V+ and ground points.


    The circuit acts as a simple voltage regulator. The Vout terminal
    would, presumably, go to some other circuit that wants this regulated
    voltage.
     
  4. Robbs

    Robbs Guest

    Would you need to place diodes or something similar to isolate each ground
    connection from another? i.e.:

    Would

    | | |
    | | |
    | | |
    | | |
    | | |
    | | |
    = = =

    indicate that you'd need to build the following in reality?

    | | |
    | | |
    | | |
    | | |
    | | |
    | | |
    | | |
    \--->|---`--->|---`-------- V 0 (ground)

    to prevent current from one ground connection travelling back up to another
    part of the circuit? Or is it just

    | | |
    | | |
    \ | /
    \ | /
    \ | /
    \ | /
    \|/
    |
    |

    V 0 (ground)

    I have the ARRL's 2003 handbook, and several other books, but not a word on
    it. Too basic, I suppose. And what does one refer to when one uses the word
    'rail'?

    Thank you for replying.

    Robbs.
     
  5. All points having the same symbol or signal name are connected
    together directly - no isolation diodes or anything.

    The reason for using this convention is simply to avoid having to draw
    lines representing power and ground all over the schematic. In
    digital schematics particularly, it is quite common to use "net names"
    on short wires to indicate connections.

    For example, you may see
    clock
    --------

    at the output of the clock signal source, with no apparent connection,
    then see
    clock
    ----------

    at the input of several other devices on the schematic - again, this
    avoids drawing lines that may be hard to follow all over the schematic
    "rail" generally refers to a common power or ground connection. "+12V
    rail" is the +12 volt source, and anything connected to it.
     
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