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Scoping phone line interface circuits

Discussion in 'Electronic Design' started by Bill, Jul 27, 2006.

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  1. Bill

    Bill Guest

    I want to use a scope to check out a circuit that is connected to
    the phone line tip & ring (a plain old USA analog home phone line).
    A continuity check shows that the scope probe ground clip is tied
    to the green-wire ground pin of the power chord.
    Am I right in thinking that I need to float the scope for this (ie. power
    the scope with a 2-wire extension chord)?

  2. "Bill"
  3. Rich Grise

    Rich Grise Guest

    Well, it's 'cord', but it might be cheaper to just get a cheater plug.
    It's a two-prong plug and a flying green ground lead with a lug, and
    a 3-prong socket. We used to float scopes with these all the time -
    just cut off or securely tape up the ground lead.

    Good Luck!
  4. Tim Shoppa

    Tim Shoppa Guest

    That's one way to do it.

    More preferable is to use a differential probe, or two channels in
    difference mode (gotta be within the common-mode voltage limits
    though), or a battery-powered scope.

    Tim .
  5. Bill

    Bill Guest

    the scope with a 2-wire extension chord)?
    Yes, 'cord', :eek:), that struck me after I posted.
    (I'm involved with music more than electronics lately...can you tell?)

    Thanks to all for responding.
  6. Rich Grise

    Rich Grise Guest

    And, of course, you know all the safety rules arout working around a
    floating scope, right?

    Good Luck!
  7. The only caveat is the internal mains filter which connects the scope ground
    to both mains lines though a few capacitors. A better way would be to use an
    isolation transformer.

  8. Bill

    Bill Guest

    Well, maybe?... it is one of the reasons I decided to post. I mostly do
    embedded controller programming, and my hardware design experience has
    been primarily digital, low voltage. This is my first experience with
    telephone related circuits.

    Let me list what I've considered:
    1) Since the ground clip of the scope probe is tied to the chassis,
    the chassis of a floating scope will be at whatever potential the
    probe ground clip is attached to. So, make sure the scope chassis
    isn't touching anything else (other than the wood bench top :).
    2) Even though the scope controls are mostly plastic, it'd be wise
    to go by the "keep one hand in your pocket" rule when making
    scope adjustments.
    3) Don't connect the telephone line to the circuit until the probe
    is securely connected to the test point of interest, and hands
    are free, ...then plug the phone line jack in.
    4) The phone will be ~50VDC (on-hook), and could be 130Vrms when

    Now, I did just see the response posted regarding the caps on the
    scope's transformer and use of an isolation transformer..., so after
    I digest that a bit, the above list may grow. But if there is
    something I didn't list that I should be aware of, I'd certainly
    appreciate any and all advice.
  9. Bill

    Bill Guest

    Thanks for pointing that out. I do have the tech manual & schematics
    for the scope, packed away in some box due to a recent move. I'll
    dig that out and get a better picture of things.

    As to the isolation transformer, could a Superior Electric Powerstat
    Variable Transformer be used? I just happen to have one..., purchased
    ~15 years ago for an engineer I was assisting who decided he didn't
    need it after I'd ordered it, and I decided to just keep it instead
    of returning it. It's never been'd be nice to finally
    put it to good use after lugging it around all these years. :eek:)
    I'll have to dig that out too and see if there is a schematic inside,
    because I do remember it having a power switch and a 3-prong receptacle.

  10. no_one

    no_one Guest

    Many variable transformers are auto transformers and therfore not isolating.
  11. Rich Grise

    Rich Grise Guest

    No. Powerstats are Variacs, which are autotransformers with a movable

    It won't give you any isolation, although it could be useful if you were
    troubleshooting the scope power supply. ;-)

    Just float the scope with a cheater, and ground your signal ground to
    the + side - you're talking POTS, right? Usually, the positive side of
    the POTS line is closer to earth ground potential - it has something to
    do with galvanic corrosion, or something.

    And, of course, remember that a floating scope can electrocute you.

    Have Fun!
  12. Rich Grise

    Rich Grise Guest

    Your safety list looks pretty OK for me, especially if all you're doing
    is poking around with POTS (plain old telephone service) lines. You might
    even try the scope grounded, the probe grounded to some "earth ground",
    and see what potential you see on each line relative to earth ground.

    If you have a dual-trace scope, you can watch them simultaneously, and
    if it has an "A-B" function, you've got your differential scope right
    there. :)

    Good Luck!
  13. jasen

    jasen Guest

    That can be done. At worst there'll be 100V on the phone line (barring acts
    of god like thunder storms). your call.
  14. Bill

    Bill Guest

    I had doubts about it, but since I've yet to open it I wasn't sure.
    I keep thinking a project might come my way someday that will need
    it, but as such things often go, that will probably happen a week
    after I decide to sell it on Ebay :).
    Thanks for the 'comforting' warning :). Actually it's the mains
    filtering thing that bothers me the most, 'cause that would mean
    the chassis would have, say, 1/2 the power mains voltage on it
    at all times. The service manual schematic for my scope doesn't
    show any such caps, and the quality of the manual doesn't give me
    any reason to beleive it isn't complete, so it probably doesn't
    apply for this scope. But I think if I do decide to float the
    scope, I'll see if I can borrow an isolation transformer just to
    be safe(r). But first I'm going to try your suggestion of leaving
    the scope grounded and just probing the circuit without the ground
    clips. It is a dual channel scope, has an ADD button, and one
    channel can be inverted. And yes, this is a plain old home phone
    line. Also, after digging out the manuals, I was reminded that
    this scope has a connection for being powered from a 24V battery,
    so that provides another option to ponder if needed.

    Thanks again for your help,
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