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What is ethernet isolation tansformer

Discussion in 'Electronic Basics' started by Jack// ani, Nov 9, 2005.

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  1. Jack// ani

    Jack// ani Guest

    Hi there,

    Why are they used.....the one which you can find in almost all ethernet
    cards!!

    Thanks
     
  2. Andrew Holme

    Andrew Holme Guest

    They remove common-mode signals. They also help with EMI supression.

    You can get large voltages between mains earth in different parts of a
    building (or opposite sides of a room even). Without the transformer, the
    sensitive input circuitry would need to deal with the common-mode signal.

    If you're playing around with ethernet like this guy
    http://www.fpga4fun.com/10BASE-T.html you can get away without the
    transformers for the purpose of a table-top FPGA experiment, but not
    otherwise.
     
  3. w_tom

    w_tom Guest

    Your question was answered accurately by Andrew. Galvanic
    isolation is also part of the computer's internal protection.
    Any protection effective adjacent to the computer is already
    inside the computer. That includes this galvanic isolation
    provided by the transformer since typically destructive
    transients are common-mode.

    Just another reason why effective computer protection that
    is distant from the machine and close to earth ground are to
    keep destructive transients from overwhelming this internal
    computer protection.
     
  4. Pooh Bear

    Pooh Bear Guest

    Doesn't the the transformer also provide a differential connection ?

    Graham
     
  5. Jack// ani

    Jack// ani Guest

    Thanks Andrew and Tom.....BTW what is Galvanic isolation??
     
  6. It simply means that there is no path for DC current, i.e. a metallic
    connection.
     
  7. Hi,

    not really: it means that there is NO DIRECT PATH for any current
    at all!

    Direct path means any kind of conductive (or semiconductive)
    material. By this definition, the magnetic coupling of a
    transformer provides galvanic insulation.

    With regards to saftey standards (like ISO 60065 or UL1650 ...) the
    term galvanic insulation is mostly coupled to a clearly specified
    value of withstand voltage, that the insulation must keep without
    any wrong behaviour:

    For a mains transformer (PRI to SEC) this is:

    1500Vacrms or 2120Vdcpeak for basic insulation

    3000Vacrms or 4240Fdcpeak for reinforced insulation

    By this you can see, that also AC currents are not allowed to run
    directly from one side to the other. These tests are made with one
    probe connected to the (shorted) primary windings and the other
    probe connected to the (shorted) secondary windings.

    Regards
    Michael
     
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