Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!uunet!pilchuck!amc-gw!richm
From: richm@amc-gw.amc.com (Rich Moran)
Newsgroups: sci.electronics
Subject: Re: power from phone line
Keywords: phone, power, remote
Message-ID: <1052@amc-gw.amc.com>
Date: 7 Dec 89 17:43:57 GMT
References: <14230@jumbo.dec.com> <3542@kitty.UUCP>
Reply-To: richm@the-end.amc.com (Rich Moran)
Organization: Applied Microsystems, Redmond, WA
Lines: 22

}
} 	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.
} 

This is fine if you aren't worried about FCC regs.  Part 68 specifies a minimum
5M-ohm input impedance for DC on tip-ring at voltages below 100V.  Even with
a 56V battery at the CO, that's less than a milliwatt.

rich
-- 
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Rich Moran
richm@amc.com
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