Another question on telephone circuits...

Discussion in 'General Electronics Chat' started by Externet, Mar 1, 2008.

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  1. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Hi all.
    Seems there is several members that know about telephone circuits, so I will ask suggestions.
    I do know the workings of telephones and central office supply circuits well, years ago before internet I designed half a dozen different electronic gadgets powered only by the telco line; a microcontroller handling LCD and modem, a portable emergency radio receiver, a beacon, battery charger, substitute battery supply, line lenght to the central office calculator plus some line status indicators.

    I would like to learn other power supply circuits / regulators / techniques / approaches for operating electronic circuits from telco power. How would you do it ? I did not find much on the net.
    Miguel :)

    (Software circuit simulators may vomit when you specify no ground):(
     
  2. Audioguru

    New Member

    Dec 20, 2007
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    It is illegal to steal power from a telephone line.
    If you draw too much then the telephone line will be off-hook and will be busy.
     
  3. cumesoftware

    Senior Member

    Apr 27, 2007
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    It depends on the country. In Portugal, for example, it isn't. But then again the power is limited to about 100mA here.
     
  4. Audioguru

    New Member

    Dec 20, 2007
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    Of course the power from a phone line is limited. A phone needs only about 30mA and must work if it is far and has a lot of resistance in the long wires.
    The American minimum leakage resistance is 5M so the max leakage current is 50V/5M= 10uA.
     
  5. Externet

    Thread Starter AAC Fanatic!

    Nov 29, 2005
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    Thanks

    This is not about stealing anything. It is about operating circuits from telco power.
    Draw too much and the line will be seized ? Sure, to seize the line is necessary to operate the circuit into the line. It is not expected to power anything with the line on-hook.:)

    Sure, it is limited to about 0.25W off hook and to about 1W at ringing, depending on the distance to the telco. Leakage has nothing to do with circuits to supply regulated power.:)

    I have used Schottky bridges and low drop regulators as LP2950, but I was expecting some responses using DC-DC converters or any clever alternatives.
    Any interesting approaches ?

    Miguel :)
     
  6. rherber1

    Member

    Jan 6, 2008
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    No matter which country you are in, I am fairly certain that you will never find any telco line powered equipment which uses a switching converter powered from the line. It is extremely difficult to filter switching transients from being fed back into the line, and if there is one thing which telco's will not tolerate it is external injection of noise. Switching converters are a no-no on line powered devices.
     
  7. beenthere

    Retired Moderator

    Apr 20, 2004
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    This is not a productive thread.
     
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