# What does Rail Mean ?

Discussion in 'Electronic Basics' started by John Popelish, Feb 16, 2005.

1. ### John PopelishGuest

Any of those. A rail is a voltage distributed to several parts of a
circuit. If you connect a 9 volt battery to something, one of those
connections is the positive rail and one is the negative rail. I
don't know if this came from electric trains that got their power
between the grounded outside and hot third rail, or if it came from
the convention of drawing schematics with the power distribution lines
drawn horizontally across the diagram or from something else. One
place this reference to supply voltages as rails occurs in opamps that
are designed to work with input voltages over the full range from the
positive ot the negative supply voltage and able to swing their output
voltage over the same range. They are called rail to rail amplifiers.

2. ### ChretienGuest

I know that rail tends to be a slang term in electronics, but what does it
mean and where does it come from. Im not sure if its Positive or Negative or
ground.

Thanks.

3. ### JamieGuest

simply means that inputs and outputs can swing to either the highest
Vcc/Vdd or to the lowest ground point/0
for example.
LM741 does not have Rail output, this means that a 741 can not have
it's output reach the Vcc+ (Supply voltage). nor does it reach it's
lowest point .
there are Op-amps and logic chips that have Rail operations.
a simple application comes to mind.
lets assume you have a Vcc of 5+ and you need an op-amp to have a
range output of 0..5+ using a single ended supply. in this case, you
would need an op-amp that could reach the rails of the supply voltages.

4. ### ChretienGuest

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. )

In other words if you have a wall wart and you provide 12 volts to a circuit
and you have + and - traces going around the circuit for little devices to
draw their power from, rail is the 12 Volts, + or - . Now if you have a
resistor connected to this 12 Volts that draw currnet from it and drops the
current and goes to some other electronic parts this voltage is NOT rail.

Maybe the concept is over my head to understand ?? Here is a diagram of what
I think you said. The wire between the resistors and light would not be
considered Rail. or am I all wrong.

___ ___
o------Rail-----|___|--|___|--.
|
,---.
| X |Light
'---'
|
|
o-----Rail--------------------'
(created by AACircuit v1.28.4 beta 13/12/04 www.tech-chat.de)

5. ### ChrisGuest

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---'---------->
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

6. ### Roger JohanssonGuest

It is more likely that it goes under your head, this is a very simple
concept.

When we need to distribute power to all circuits on a pcb we often make
long straight "rails" of metal, or a copper trace along one border of
the pcb, or in a snakelike pattern.

(really new pcb's are laminated, and the rails are now complete planes of
copper inside the pcb)

When we draw circuits on paper we usually draw a horisontal line across
the top of the circuit and we use that as the positive voltage from the
power supply. There is often also a horisontal line at the bottom of the
circuit, to show the ground connections.

(Another alternative is to draw a grounding symbol everywhere where
something is connected to ground. All these ground symbols are in reality
connected to each other, in a car it is the chassi which is ground, or
the neutral rail)

This is sometimes also how we lay out circuits in reality, on pcb's.

So we have come to refer to these power supply rails as rails. It means
just that, a long piece of metal, or a long, wide, copper trace which
carries electrical power to many circuits on a pcb.
The lower rail is a real rail.
The upper dotted line is not a rail along all of its length, because the
wire/rail is interrupted by resistors. The upper line is thus only a rail
until it comes to the first resistor. The points after the resistor are
not in direkt contact with the plus pole of the battery.

A real rail carries the same voltage as the power supply, along all its
length. It is a direct contact to the power source.

7. ### Lord GarthGuest

Vcc - voltage at the (transistor) COLLECTOR, the positive supply
Vdd - voltage at the (MOSFET) DRAIN, the positive supply
Vss - voltage at the (MOSFET) SOURCE, ground

8. ### ChretienGuest

Roger thanks I get this. And its what I ment (perhaps not properly said)
when I said anything between the components is not rail. Your clarification
would be everything starting with the first component to the last is not
rail. I think thats what you said anyway.

Thankyou.

9. ### ChretienGuest

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---'---------->
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

10. ### Lord GarthGuest

It for the OP...but if you didn't, now you do!

11. ### Lord GarthGuest

For me, it was when I got my first "skull burn"!
Damn, those hurt!

12. ### JamieGuest

when you see the term used here "RAIL" , it mostly reflects the
ability for an component like an Op-AMP to be able to have on its
output what is on the supply line.
Rail being the supply line in voltage.
seeing the term Rail to Rail means that a device is able to
swing fully to one side or the other.
many logic chips and op-amps are not able to output fully to the
+ / - rail levels.
Look at the difference between a MosFet and a Bipolar transistor
being used as a switch.
The bipolar would have a problem due to the average 0.6 volt drop that
is natural in the emitter where is, the MosFet is like a resistor.

13. ### JamieGuest

and i didn't know this?

14. ### JamieGuest

well thanks, i know i am getting old for example when
i was looking in the mirror this morning, the glare
from the forhead almost blinded me!

15. ### TerryGuest

The word rail.

As a verb it means to complain loudly.

However as a noun in electrial circuit use;

Have generally seen it used to mean a point in a circuit 'common' to many
components.

For example someone looking at a circuit with transistors might observe that
they are all fed from a common 16 volt Vcc supply 'rail'.

Or someone installing an audio amplifier installation in the trunk of car
may arrange a fused 12 volt feed from the car battery to the trunk area;
reaching the trunk may then refer to that 12 supply as "The 12 volt rail
(common)", feeding several items of equipment.

In older telephone exchanges there were/are 48 volt DC 'bus bars' from the
battery/power supply room, overhead. The various aisles of equipment were
tapped off of these major supply 'bus bars' through large fuses. While we
never called them anything but 'bus-bars' I guess the term "48 volt DC
supply rail" would have been apprporiate?

Haven't AFIK seen the 'bottom end', that is the ground, chassis, common zero
voltage point in circuits frequently referred to as 'a rail"; although one
could consider it to be, say, the 'ground rail'.

So generally it means something that is electrically common to several/many
parts of a circuit. Certainly an 'individual' input point for audio or
triggering a circuit would be the opposite meaning of a rail. Also an
'individual' output of a circuit would generally not be called a rail,
unless it happened to feed many items; whereupon I supposeit could be called
'An output rail'?

Any help or more confusing?

16. ### TerryGuest

Someone wrote.
Wear a hat!
Still got me hair on top but uncertain of quality of skull contents!!!
"Err; now what was I going to do next?"
Ah yes! I think it was;
"Try to find my glasses!"

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