Re: What does Rail Mean ?

From: Chretien (Cretian_at_123.com)
Date: 02/16/05 

Date: Wed, 16 Feb 2005 17:03:09 -0600

"Chris" <cfoley1064@yahoo.com> wrote in message
news:1108586599.307135.293930@c13g2000cwb.googlegroups.com...

Chretien wrote:
> Im sorry Im a unclear on your explanation. Are you saying that Rail
is
> either the full amount of current that the circuit is provided with
or 0
> (presumably - or ground). (Im sorry I dont know what vdd is. Im
believe Vcc
> is basically the full current put into a circuit before you start
doing
> something with it. )
>

Hi, Chretien. Back in days of yore, we used to draw schematic diagrams
with the power supplies shown, not assumed, like this:

 +---------o--------o---------o--------o-------------->
           | | | |
           | | | |
           | .-. | .-.
          .-. | | .-. | |
          | | | | | | | |
          | | '-' | | '-'
          '-' | '-' |
           | o--. | o--.
           | | | | | |
        || | |/ | || | |/ |
>-||-o------| '---||-o------| '---->
        || | |\ || | |\
           | | | |
           | o---. | o---.
          .-. | | | | |
          | | .-. | .-. .-. |
          | | | | --- | | | | ---
          '-' | | --- | | | | ---
           | '-' | '-' '-' |
           | | | | | |
 - | | | | | |
  ---------o--------o---o-----o--------o---'---------->
created by Andy´s ASCII-Circuit v1.24.140803 Beta www.tech-chat.de

Whether the supply node was positive or negative with respect to GND,
or GND/Common itself, it was represented as a straight line running
left to right across the page. Also, generally circuit action is
assumed to be running from left to right, too. Well, if you have any
imagination you can start to see the above as a railroad track type of
thing, which I believe is where the term came from. (Actually, this
was also common when tubes/valves ruled the earth, but Andy's ASCII
doesn't have any tube symbols).

Generally the more positive node of the supply is above the more
negative one on the paper, which doesn't necessarily mean it's a
positive supply (in the days when PNP Germanium transistors were
common, with a single supply the more positive node of the supply was
usually GND or the positive rail, and the other node was the negative
supply and the negative rail).

Sometimes you wanted to draw "split supplies", with V+ or Vcc being the
top or positive rail, and V- or Vee being the negative or bottom rail.
GND was then assumed with the standard GND symbol. So the term "rail"
simply refers to any of the power supply lines, +, -, or COM. Most of
the time it refers to the most positive and most negative nodes of the
supplies available.

Single supply low-voltage op amps whose outputs are optimized to go
from the positive node of the supply (positive rail) to the negative
node of the supply (negative rail) are called "rail-to-rail op amps".
These are frequently made to operate on logic power supplies. So a
3.3V "rail-to-rail" op amp will be one designed to operate on 3.3VDC
supply, and have an output that can get within mV of 3.3VDC and within
mV of GND.

Is this a little more clear?

Oh yes, and by the way, Vcc usually refers to a power source/supply
which is positive with respect to GND, and Vee usually means a power
supply which is negative with respect to GND. This is the normal (NPN)
transistor orientation on transistor circuits which had split supplies.
 And Vdd is more positive, and Vss is more negative (FETs). Vdd means
the positive node of the power supply and Vss means GND or the negative
node of the power supply.

Good luck
Chris 

--
Yes that does help clarify things.
Thanks.

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