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analog telephone voltages

Discussion in 'Electronic Design' started by [email protected], Dec 13, 2004.

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  1. Guest

    Hi all,

    I am trying to gain some knowledge on POTS phone systems. I need to
    know what the minimum voltages for ring and supply if I have a private
    point to point connection using standard telecom interfaces? I really
    don't care what the standard is... I am just interested in what it
    would take to get a point to point system working (reliably of
    course....). I am sure that I mis-worded this question so please ask
    for clarification if I messed it up too bad.
    Thanks in advance for any help.

    Frodo
     
  2. I read in sci.electronics.design that
    It depends on which instruments you are using. Some ringers will work on
    much lower voltages than others.
     
  3. Dbowey

    Dbowey Guest

    jeff posted:

    << I am trying to gain some knowledge on POTS phone systems. I need to
    know what the minimum voltages for ring and supply if I have a private
    point to point connection using standard telecom interfaces? I really
    don't care what the standard is... I am just interested in what it
    would take to get a point to point system working (reliably of
    course....). I am sure that I mis-worded this question so please ask
    for clarification if I messed it up too bad.
    Thanks in advance for any help.
    POTS is one thing, and point-to-point is another.

    What channel type did you order? Some include network ring voltage, and some
    do not. Please describe in the terms you and the telco discussed.

    Don
     
  4. Guest

    Actually, I do not intend to connect the private line to the telco. It
    is just between two devices that work on a standard POTS line. I want
    to be able to generate off-hook, ring, and on hook so that I can use a
    low end modem between two points that expect to be on a telco line. I
    hope I have been able to clarify this...if not...give me another shot
    at it please...
     
  5. Guest

    John,

    I have seen the specs for one part that seems to indicate it will sense
    a ring voltage as low as 17Vac rms. Can I generally depend on that?
     
  6. Joop

    Joop Guest

    Not interested in the specifications, but do want to know minimum
    requirements???

    Oh well, do some digging here:
    http://www.discovercircuits.com/T/telephone.htm
     
  7. Dbowey

    Dbowey Guest

    Jeff posted:

    << Actually, I do not intend to connect the private line to the telco. It
    is just between two devices that work on a standard POTS line. I want
    to be able to generate off-hook, ring, and on hook so that I can use a
    low end modem between two points that expect to be on a telco line. I
    hope I have been able to clarify this...if not...give me another shot
    at it please...
    Over what distance?

    If you are within a few hundred feet you can probably ring a phone with 12V, 60
    Hz.

    12VDC will give you at least 20mA loop current thru the two phones, so that is
    adequate.

    If the computers are "real close" you could just use No Modem connection,
    a.k.a. Null Modem.

    Don
     
  8. I read in sci.electronics.design that
    No.
     
  9. Guest

    Quick specs from memory:

    On-hook: -48 VDC
    Off-hook: less, down (up?) to maybe -5 VDC
    Ring: 90 V, 20 Hz at the phone company, maybe down as far
    as 40 V by the time you get it

    I have run two phones using a 9 V battery. I'm not sure what the
    lower limit is. If you have an AC-powered supply, it has to be
    filtered pretty well to keep hum off the phones.

    For more details, see pages 935-936 of "The Art of Electronics".

    Matt Roberds
     
  10. ryan wiehle

    ryan wiehle Guest

    what POTS are you talking about? USA-Canada?
     
  11. Dave VanHorn

    Dave VanHorn Guest

    Telephone systems really don't work on voltage.

    For US-Centric systems, you'll find that the CO will see you off-hook if you
    draw at least 20mA.

    There's a book, called "Understanding Telephone Electronics" that is a very
    good reference.
     
  12. Mike Fields

    Mike Fields Guest

    However voltage is a very important part ... just try having your little
    fingers across the ring and tip wires when the telemarketer calls
    and you get -48v with a 90 volt 20 hz ringer signal ... your ears
    will wiggle at 20 hz !!! ;-)
     
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