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In article <1325@tellab5.tellabs.CHI.IL.US>, zantow@tellab5.tellabs.CHI.IL.US (Al Zantow) writes:
> 	There are some problems with trying to use phone power to monitor
> the phone line.  The phone company equipment "watches" the current in you 
> phone line, and if it goes above a few mA's it assumes you are off hook and
> want dial tone.

	The minimum off-hook threshhold current ranges between 10 and 20
milliamperes, depending upon the type of central office apparatus.

> When you realy are off hook the current in the line is limited
> to about 50 mA's.

	I don't know about AT&T #5ESS, Northern Telcom DMS-series or
other current production electronic central office apparatus, but older
ESS and electromechanical central office apparatus had no active current
limiting whatsoever.

	Current limiting was achieved by a combination of a 200 ohm
resistance on the ring lead to central office battery, and a 200 ohm
resistance on the tip lead to central office ground.  Assuming a 50 volt
office battery, the maximum short circuit current in the central office
at the distributing frame is 125 mA.  Loop resistance of outside cable
plant further reduced this 125 mA to lower levels; however, if you
were close to the CO, you could easily get 100 mA of short-circuit
current across the telephone loop.

> This is partly for your protection so you don't cook 
> if you get across the phone line, but mostly since bipolar transistors are 
> current operated devices.  A "regulated" current makes them happier than a
> regulated voltage source, which allowed the earlier touch tone phones to be
> more stable.

	This is simply not true; the central office apparatus regulated
absolutely nothing.  Earlier touch-tone telephone sets used a _single_
germanium transistor to generate the DTMF tones.  This tone generator was
an _extremely_ clever design which would oscillate under virtually ANY
condition (provided the tip/ring polarity was correct; later telephone sets
were equipped with bridge rectifiers called "polarity guards" by WECO to
solve this problem).
 
> 	The dial lights and such are probably powered by the yellow and black
> wires that have about 12 VAC on then from a transformer wired into your
> house (or aprtment) AC system.

	The voltage is actually 6 to 8 volts AC for WECO sets using a
2012A transformer.

	Interestingly enough, in the early 1970's GTE/Automatic Electric
manufactured a "Trimline" lookalike telephone called "Styaline" which had a
dial illuminated by an electroluminescent (EL) panel.  The EL panel was
powered by a "plug" adapter which plugged into the AC power line and
contained two 1 megohm resistors in series with each leg.  NO TRANSFORMER -
just two series resistors.  Hard to believe, but true.

<>  Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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