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information about power supply of telephone switch!

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cotel

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Looking for any information on power supply of telephone switch. Schematics, appnotes, articles, e-books, related URL:s

Thanks
 

polarized

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powering devices from the phone line

Hi,


I am not sure do I understand you. Do you plan to power your device from the telephone pair of wires and this interests you, or you want to find out about the power system of the telephone switch(POTS) itself. If you plan to power your device from the telephone lines then you can search elektroda itself, because there is an application regarding PIC processors in which the question of powering the electronics from the telephone pair is explained and many other things related to the analog phone line.



Regards.
 

ted

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In any case you should be aware that the phone line "loop feed" is of quite high voltage and with substantial internal resistance.

The open loop voltage is typically 48V (might be 24V.. over 60V) and the positive side is nearly always the "ground side". This is to reduce corrosion issues in case a leakage current to earth. However the line is very symmetrical so you must not disturb it's balance in relation to earth--for AC it is needed to reduce crosstalk and compensate against main frequency hum.

The closed loop current is typically 20...50 mA, depending on system and on length of the phone line (and thus the line resistance). A phone may present around 10V while off-hook, and drawing the loop current.

The exact standards are varying by national whims and traditions, and are a real mess regarding details. However, the basic characteristics are close enough so one can construct a line circuit which is valid for nearly all countries without modifications or with minimal adjustments. (At least DC-wise it is quite feasible, but the real headache starts with the speech levels, ringing voltages, AC impedances, over voltage protection requirements and additional signalling such as caller ID and billing impulses.)

The power supply in a telephone switch is thus a DC source of typically -48V, and the line is fed through a feeding circuit balancing the AC functionality and limiting the current to maybe 40-50 mA or thereabout.
 

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