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Pulse transformers, why?

Discussion in 'Electronic Basics' started by ErikBaluba, Apr 6, 2006.

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  1. ErikBaluba

    ErikBaluba Guest


    I'm putting together my own SPI based LAN interface using a cheap Microchip
    ethernet chip.

    Why are circuits interfaced to Ethernet have pulse transformers? What make
    such transformers special? I saw one example schematic for a
    pulsetransformer were it was grounded via 2000V capacitors. Is data
    transmitted on an Ethernet's twisted-pair cables by generating high voltage
    pulses to facilitate longer distance and higher speed because of increased
    S/N or something? I tried to find voltage specs for Ethernet signals on
    google but couldn't find any.

    And then what about such transformers used on the phone lines? I figure they
    are necessary to step down the "high" phone line voltages?

  2. DJ Delorie

    DJ Delorie Guest

    Lightning strikes.
  3. Andrew Holme

    Andrew Holme Guest

    Ethernet uses low-voltage differential signalling. The transformer is
    required to remove common-mode voltages which may result from pick-up on the
    cable, or from differences in mains earth potential around the building.
    The common-mode signal could easily overload or even destroy the receiver.
    Ethernet pulse transformers have a 1:1 ratio.
  4. DJ Delorie

    DJ Delorie Guest

    Not all of them. Some are 1:1.414, 1:2, or 1:2.5. I suppose other
    ratios are possible too.
  5. No idea why they use the phrase "pulse" transformer (I guess they could
    have just as well used "data" transformer), but its simply a transformer
    to float the battery and ground, just like in a telephone system. In a
    network card, none of the data in/out pairs are directly connected to a
    power supply or ground.
    telephone ) || (
    audio ) || (-----> -48 volt power supply "battery"
    signals ) ||
    ) || (-----> Earth ground
    ) || (
    No, see above.
  6. Joerg

    Joerg Guest

    DecaturTxCowboy wrote:

    True. It's just a customary term since there are no different words for
    small and large transformers in English. In other languages they call
    them by what they are really for, like 'Trenn-Uebertrager' in German.
    The first part of that means 'separation', as in isolate.
    'Transformator' would be their word for the big iron that is connected
    to mains. In English a pulse transformer is considered something small,

    Erik: If these transformers weren't there all it would take is one large
    inductive spike and the Ethernet card would be gone. For example when
    the big air conditioner motor kicks in and the CAT-5 cable happens to
    run near its supply line.

    Regards, Joerg
  7. Rich Grise

    Rich Grise Guest

    They're called "pulse transformers" to distinguish them from all of the
    other varieties of transformers - here's an interesting read:

    From my own learnings/experience, I'm speculating that they use pulse
    transformers because of their excellent rise-time characteristics, which
    would translate into frequency response, and they also can handle enough
    power to put a good, solid, reliable signal on the line, plus I intuit
    that they have good noise immunity on the receive side, so to speak.

    Hope This Helps! (Feh - hope it's accurate! :) )
  8. Rich Grise

    Rich Grise Guest

    Any ratio at all that's a ratio of two integers - how do they get 1:1.414?
    1000:1414 turns?

    Since this is .basics, it should be explained that the turns ratio is
    equal to the voltage ratio, and inversely proportional to the current
    ratio, and something gets squared to work with impedances, but I slept
    or drank my way through that class. ;-)

  9. DJ Delorie

    DJ Delorie Guest

    I have no idea. They're labelled as "1 : sqrt(2)". I assume they
    don't have trancendental turns in there.

    Perhaps they use 5:7 and hope it's close enough? ;-)
  10. Tim Williams

    Tim Williams Guest

    (sqrt(2) ratio)
    Bah, just end the last turn a few degrees shy of 7 turns. :p

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