What does Rail Mean ?

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.

Reply to
Chretien
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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.

--
John Popelish
Reply to
John Popelish

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.

Reply to
Jamie

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believe Vcc

doing

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

formatting link

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

Reply to
Chris

it

Negative or

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

formatting link

Reply to
Chretien

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.

Reply to
Chretien

is

or 0

believe Vcc

doing

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

formatting link

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.

Reply to
Chretien

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.

--
Roger J.
Reply to
Roger Johansson

does

Vcc

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

Reply to
Lord Garth

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.
Reply to
Jamie

and i didn't know this?

Reply to
Jamie

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! :)

Reply to
Jamie

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

Reply to
Lord Garth

message

0

believe

doing

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

Reply to
Lord Garth

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?

Reply to
Terry

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!"

Reply to
Terry

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