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Is it possible to limit voltage going to a telephone?

Discussion in 'General Electronics Discussion' started by Murkeyside Exile, Aug 19, 2016.

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  1. Murkeyside Exile

    Murkeyside Exile

    Aug 19, 2016
    Hi everyone, I'm new to the forum and not an electrical whiz by any stretch of the imagination!

    I understand - happy to be corrected on any point - that the standard voltage on a BT telephone line in the UK is around 50v but this goes up to 75v to ring the connected telephone. I also understand that if the telephone line is behind a switchboard it is possible to prevent the telephone from ringing and effectively stopping 75v from ever hitting the telephone.

    Is it possible to do something similar with a standard BT telephone line and guarantee that 75v never hits the telephone?
  2. Externet


    Aug 24, 2009
    I suspect that UK telephone lines have/had more than two wires, as I may have read somewhere.

    If the wire carrying the (75V+?) AC ringer signal stops its route at a certain switchboard, it can clearly prevent from reaching the telephone on a single pair.
    So first, confirm; is there more than a single pair reaching your telephone ?
    - If yes, find such ringer-dedicated and disconnect.
    - If there is no more than a single pair; a circuit would have to be implemented that senses the ringer signal and opens the line, isolating the telephone from it. Could be a comparator over 50V, or other things.

    Check the net for explanations on BT typical wiring and ringer workings/disabling. I have not.
  3. Gryd3


    Jun 25, 2014
    Don't most phones have the ability to switch off the ringer anyway?
    This project sounds like one of those things that perhaps you should not be working with...

    Why would you want to prevent the ringing signal to pass through the line?
    Mongrel Shark likes this.
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