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What does Rail Mean ?

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

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  1. 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. Chretien

    Chretien Guest

    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. Jamie

    Jamie Guest

    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. Chretien

    Chretien Guest

    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. Chris

    Chris Guest

    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. 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 Garth

    Lord Garth Guest

    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. Chretien

    Chretien Guest

    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. Chretien

    Chretien Guest

    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 Garth

    Lord Garth Guest

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

    Lord Garth Guest

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

    Jamie Guest

    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. Jamie

    Jamie Guest

    and i didn't know this?
     
  14. Jamie

    Jamie Guest

    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. Terry

    Terry Guest

    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. Terry

    Terry Guest

    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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