Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!samsung!ctrsol!lll-winken!scooter!neoucom!wtm
From: wtm@neoucom.UUCP (Bill Mayhew)
Newsgroups: sci.electronics
Subject: Re: power from phone line
Summary: saving the ringer current for re-use
Keywords: phone, power, remote
Message-ID: <1853@neoucom.UUCP>
Date: 10 Dec 89 03:27:09 GMT
References: <14230@jumbo.dec.com> <5255@cbnewsc.ATT.COM>
Sender: wfd@neoucom.UUCP
Organization: Northeastern Ohio Universities College of Medicine
Lines: 41


The on-hook allowable leakage for a telephone instument is in the
low microamp region for obvious reasons.  I know that in the past,
and maybe the present, that line sharing in rural areas that gives
private service to two parties on the same line used leakage
current.  One party got regular baseband service.  The second party
operated on a carrier system AM modulated somewhere around 50KHz -
100KHz.  The carrier system interface at the subscriber's drop
contained a nicad battery to supply ringing power, as baseband
ringing is impossible (without a gas tube frequency-selective
ringer in this case).  The nicad also supplied talk power for the
local loop.  When the set was on-hook a small leakage current was
used to recharge the nicad pack.

A couple years ago, I read about a neat system developed in Italy.
They used a fiberoptic connection from the CO to the subscriber
residence.  The subscriber's set was recharged from the laser power
received from the CO.  Obviously, this system wasn't real effective
for subscribers more than a few Km distant from the CO.

One method of getting free power from the phone that I've yet to
see exploited is to save the power from the ring signal.  I've seen
a number of instruments that draw 0.25B or more power when ringing.
It would be a farily simple matter to attach an isolation
transformer to the line for squirreling the ring power into a
nicad.  As long as the remote seismograph were called on a regular
basis, the ringing could keep the battery fresh, if the device
permitted the line to ring a number of times before going off hook.

I have an old UDS manually operated modem that entirely phone line
powered.  It contains 60 some 4000 series CMOS ICs, and even
generates RS-232C compliant output voltage for the computer
interface by using it's off-hook loading to power itself.  Pretty
neat piece of work.  Too bad that it couldn't go off hook or dial
automatically.  All of the modem's voltages were derived from
clever resistor divider networks; only the -6 volts for the RS232
output was derived from a switched capacitor.  At least, that's my
recollection.


Bill