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beginner's 'ground' question

Discussion in 'Electronic Basics' started by Robbs, Jul 18, 2003.

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  1. Robbs

    Robbs Guest

    A very basic thing about grounds I'm embarassed I don't know:


    *********** (***********: some circuit
    | | | with a positive terminal &
    | | | multiple ground connections)
    | | |
    = = =

    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
    anotherpart of the circuit? Or is just

    | | |
    | | |
    | | |
    +----+----+------- V 0 (ground


    I have several books on electronics/electricity, but a basic, practical
    explanation of the concept of 'ground' can't be found in any of them.
    'Reference voltage' turned up a few things, but nothing i've been able to
    apply in my head to building real-life circuits.

    Also, I've seen the word 'rail' used in many posts in this group,
    referring (I think) to the negative and positive terminals(?). Is that
    correct? I suspect there's a subtle difference in how they're used.

    Thank you for any help.

  2. Just connecting them all together is the normal way. When you are
    dealing with high gain or other noise sensitive circuits, the details
    of where all the currents flow in the ground connections gets
    important, because no connection is zero ohms, so every current
    through every trace and wire drops a little voltage. But in general,
    just connect them all together.
    Yes. Rails are just connections to the supply. They are called
    rails, I think, because if they are drawn on the schematic, one is
    often drawn across the top and one across the bottom, with the circuit
    laying between the tracks.. or rails. The drawing convention most
    common is positive rail across the top, negative rail across the
    bottom, and signal flow generally left to right.
  3. No diodes. They are all tied together. In some applications,
    the circuits diagrams don't tell you everything you need to
    know, for example "starring" a connection may not be shown,
    under the assumption you'll know what needs it. But I don't
    expect you to know about that or other things.

    Normally, all grounds are just tied together somehow.
    Hmm. You will often hear it as "voltage rail" as "rail" has
    several meanings (including a "rail of ICs.") A voltage rail is
    just a very commonly used voltage level. For example, there may
    be 5 or 6 connections to the positive terminal of a battery to
    gain access to that voltage, directly. It might then be called
    the "positive rail" then. You might also hear "ground rail" as
    ground is just another voltage.

    It's like a "hitching post" or "hitching rail" for horse leads,
    sort of. Haven't you ever rode up to a rail and tied your horse
    to one?

  4. The term "ground" is widely misused and abused when dealing with
    electricity and electronics.

    Sometimes, particularly in AC power wiring and some radio antenna
    systems, "ground" really does mean "a connection to the earth".

    However, most times it would be better to use the term "common" or
    "reference point" instead of "ground". In these 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 voltages elsewhere in the circuit. There is nothing
    mysterious or magic about this "ground" - it is just one connection in
    the circuit.

    Most often, "ground" or common is the most negative terminal of the
    power supply (and everything connected to it), but it may sometimes be
    the most positive terminal. In many analog circuits (audio
    amplifiers, etc.), "ground" is the midpoint of the power supplies, so
    you have both positive and negative power supplies for the circuit.
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