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Is ground the same thing as the black wire in DC?

Discussion in 'Electronic Basics' started by, Jul 22, 2004.

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  1. Some circuits talk about + and -, other talk about VCC and GROUND.
    One even talked about "Chassis ground" as opposed to rod-in-the-earth

    If I attach only the red wire from my power supply, and attach the other end
    to a metal table or something - the circuit wouldn't work would it? I would
    think you'd need to have the black wire (actually called "negative") as
    "ground" for that circuit.

    Am I correct? I know this is a VERY basic question... So many books can
    cause a bit of confusion though - the disadvantage of learning on your own.

  2. andy

    andy Guest

    + and - refers to the potential difference between the two wires. This
    isn't absolute - as long as the supply was designed right, you could have
    the -ve wire at ground+500V, and the +ve at ground+512 volts, say. But
    usually the -ve wire is connected to some kind of stable ground voltage,
    and then the positive wire is so many volts above or below that ground.
    You can do this the other way round - have the +ve wire at ground, and the
    -ve wire at ground minus x volts. or have a split rail supply which gives
    ground +/- x volts.
    not unless the table had a good connection to whatever ground your power
    supply is using - it might be at the right voltage to start with, but
    there would be no path for the current to follow to complete the circuit.
    so all you would do is charge up your table to ground+12V. (i.e. the table
    would be acting as a capacitor)
  3. Ban

    Ban Guest

    It is a basic question and should have been cast in the basic group. What
    every child already knows is: For a current to flow you need a closed loop
    eg. from positive to the consumer and back to the other polarity. If that
    table is connected to the other polarity via the earth wire(red) a current
    will flow. Many power supplies have their 0V output connected to the
    earth(red or yellow/green). Any metal enclosures(computer supply) must be
    connected by law to prevent built-up of dangerous voltages on the chassis,
    and very often also the secondary side ground is connected there. In a house
    with metal tubing the central heating or water supply is as well connected
    to earth, so a gadget will sometimes work.
    If the power supply is isolated (plastic casing, only two conductors to the
    plug), there is no way of a loop and no current will flow.
  4. Robert Baer

    Robert Baer Guest

    The color of a wire is merely a matter of convenience and one should
    never depend on a given color to have any particular meaning or use.
    "Ground" is merely a reference point; one might have a current
    monitoring circuit with schematic showing a "ground", but the device
    might be umpteen feet above the earth, monitoring current in a 200KV
    line. Where then would you say ground is??
  5. On Sat, 24 Jul 2004 09:50:37 GMT, Robert Baer

    I'm sure the electrons don't care, but in many applications there are
    *laws* regarding the colours of wire.
    Still, as you say, it is prudent not to make assumptions, particularly
    where safety is concerned.

  6. .... particularly in -48V DC systems, where red is sometimes used for
    ground and black is used for -48V. I've also seen the reverse: black
    used for ground and red for -48V. Check your local regulations, then
    measure it with a multimeter just to make sure.

  7. Bob Wilson

    Bob Wilson Guest

    There are no laws when it comes to DC wiring. In North America, it is fairly
    typical to use black as the colour of a negative wire and red as positive. In
    Germany, it is brown for ground and black for positive. And so on.

    Bottom line is that there is no absolute certainty with low voltage DC wiring.

    True ground wires in an AC circuit that is UL/CDA/CE/VDE/NEMKO/SEMKO/DEMKO
    (etc) approved, are either green, of more commonly green with a yellow stripe.
    But because these are related to high voltage appliances, they come under the
    regulations (NOT laws!) of the above mentioned regulatory agencies.

  8. I read in that Bob Wilson <[email protected]_no_spam.
    In Europe, the colours of mains conductors ARE legal requirements. And
    the -MKOs are now commercial test houses, not regulatory agencies.
  9. Mjolinor

    Mjolinor Guest

    Please elucidate.

    AFAIK there are no hard and fast laws for colours. I think maybe it depends
    on which parts of the power path you are talking about. (Generator ------
    lots of other bits----- TV)
  10. I read in that Mjolinor <>
    Premises wiring and mains flexibles.
  11. Bob Wilson

    Bob Wilson Guest

    Agreed. My point was simply that low voltage wiring colours are not regulated.

  12. For purposes of human "safety and harm"...

    GREEN is the ONLY ground!

    If y'all are lookin for RF ground... good luck!

    Indialantic-By-the-Sea, FL

  13. Bob Wilson wrote:

    No: red positive, blue negative in low voltage DC circuits.
    Thats safety ground.
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