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

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

Hi,

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?

Confusedly yours,

(I completely see this post being used as ammunition against me later
in life for being clueless of EE)

2. ### John PopelishGuest

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 AutonGuest

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
stops.

Tim

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