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Resistor placement, +/- rails, and DC current direction

Discussion in 'Electronic Basics' started by mask, Sep 3, 2004.

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  1. mask

    mask Guest


    I have two questions.. and they will both make apparent that I'm just
    now getting into EE.

    I've been reading a soldering book which covers the basics of
    soldering, reading/drawing schematics, and very basic EE overall.

    There are some things that the author assumes the reader would know..
    and unfortuantely, I'm not one of those readers. So, the questions:

    The author says several times "The positive and ground rails run from
    left to right".. Well.. it seems to me that they don't really "run"
    either way.. they're just horizontal lines. As far as I've seen on
    most schematics so far, you read the top one from right to left, and
    the bottom from left to right. (Typically because the battery is shown
    in the center/right with positive going up and ground going down)..

    So.. Is that assumption true? That the DC current flows out of the
    positive, to the circuit, and then basically makes a CW loop back
    around to ground?

    Second.. He always starts his schematics off with a switch (upper
    right hand corner) and then a LED/resistor. However.. the LED is
    always BEFORE the resistor in the flow. How does the resistor resist
    anything if the current is hitting the LED first? I guess I don't
    understand that part.

    And third.. Everywhere I read, says that current flows from Negative
    TO positive.. They say that the "Convention is to say that it flows
    from positive, because in the start people couldn't see that the
    electronics flow from negative to positive" or "Since DC electricity
    is the flow of electrons from the negative (-) terminal of a battery
    or generator, through a conducting material such as a metal wire, to
    the positive (+) terminal, the electricity goes in a complete circuit
    from the source and back again"

    So.. If it actually flows from negative to positive.. How exactly
    does placing a switch right between the circuit and the positive
    terminal actually do anything? Shouldn't the circuit be designed as if
    the flow was coming from the negative terminal?

    Thanks in advanced,

    Confusedly yours,


    (I completely see this post being used as ammunition against me later
    in life for being clueless of EE)
  2. Right. Normally, I build schematics with signal flow from left to
    right and more positive voltages above more negative ones. These are
    just conventions that make it easier to parse a schematic into
    recognizable functional blocks.
    The schematic shows what is connected to what. It is your job or
    figure out what these connections cause to happen in the actual
    circuit. Many important details like how various components are
    connected together (single point grounds, guard under layers, shield
    traces, etc.) are left out on the schematic, though they may be
    important to the function of the circuit. Designers who are worried
    about communicating these functional details find ways to include them
    on the schematic, even if it is just with notes.
    Current does not start some where and travel. It occurs the whole way
    around a loop, simultaneously. So a resistor anywhere in a conducting
    loop will drop voltage within that loop, leaving less to drive the
    total current through the other parts. Kirchoff's voltage law says
    that the sum of all the individual voltages around and loop must add
    up to zero. The supply provides the motive voltage that must be
    consumed by all the other parts that complete the loop.
    Electrons are negative charges, so they leave nodes that are more
    crowded with electrons (negative voltages) and are attracted to nodes
    that are less crowded with electrons (more positive voltages). Though
    electrons are the most common mobile charge carriers, they are not the
    only ones. Positive charge carriers go the other way. Current is the
    net rate of movement of positive charge, including the sum of both
    positive and negative charge carriers. Electrons being negative
    charge carriers go opposite to positive current.
    It makes no different in most cases. When you open a switch it drops
    a voltage equal to the supply voltage and there is no voltage left in
    the loop to move charge (produce current) through any of the other
    parts in the loop.
  3. Tim Auton

    Tim Auton Guest

    Which way the electrons are going is largely irrelevant (for designing
    stuff anyway). The convention says it goes from +ve to -ve and you
    really should try to think that way, or you'll get confused over the
    directions of "arrows" (like the diode symbol) in schematics, which
    are all based on "conventional current".
    The operative word is "circuit". Break the circuit anywhere and it all

  4. mask

    mask Guest

    Current does not start some where and travel. It occurs the whole way
    Thanks, I appreciate your responses on everything. I picked up 'The
    Art of Electronics' and the student guide to go with it.. Within
    reading just the first few pages things have already become much
  5. Excellent! If you have other questions, don't hesitate to ask.
    There are many who lurk and learn from the questions others pose.
    And they go into the archive for people to search through, later.
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