Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!usc!zaphod.mps.ohio-state.edu!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: power from phone line
Summary: Stealing power from telephone lines...
Keywords: phone, power, remote
Message-ID: <3542@kitty.UUCP>
Date: 7 Dec 89 00:48:41 GMT
References: <14230@jumbo.dec.com>
Organization: Recognition Research Corp., Clarence, NY
Lines: 46

In article <14230@jumbo.dec.com>, murray@jumbo.dec.com (Hal Murray) writes:
> Suppose you have some remote instrumentation, like a seismometer, 
> or weather station, and you are transmitting that data back to a 
> central collection point via a phone line.
> 
> Can you scrounge any power from the phone line? I'm not thinking of 
> many watts, just enough to run a few slow CMOS chips.

	The answer is yes.  Since you did not specifify where you are
referring to a dial subscriber line or to a leased line, I will briefly
cover both issues.

	If you are referring to a conventional subscriber telephone line
at the remote site which you dial using the public switched telephone
network, then you can obtain between 1 and 2 mA from the line when it
is on-hook.  Some simple Ohm's law says this is about 100 mW.  Using a
carefully designed DC-DC converter, one can convert this voltage to say,
7.0 volts to trickle-charge a battery.  The battery can then store this
energy to provide a much higher current to power a modem, microprocessor
and related circuitry during the intermittent intervals when the telephone
line is automatically answered and data is exchanged.

	The precedent for the above circuit design using a DC-DC converter
is in telephone subscriber line carrier circuits, such as the Continental
AML.

	If you are referring to a leased line, then you can superimpose
a certain amount of DC power provided that you use dual-winding transformers 
or inductors to provide DC/signal separation.  In this day and age,
telephone companies will generally not provide leased line circuits having
DC continuity beyond the serving central office.  If you are able to
obtain a leased line circuit, you are generally limited to 100 mA of
current and no more than +/- 135 volts from either conductor to ground
(i.e., 270 volts conductor-to-conductor).  This type of circuit will give
you LOTS of power - even with a long loop resistance of say, 3,000 ohms.

> Is there an obvious book I should know about that answers things like 
> this?

	Not that I am aware of.  I'm afraid that experience is the only
teacher here.

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