Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cs.utexas.edu!samsung!usc!ucsd!brian
From: brian@ucsd.Edu (Brian Kantor)
Newsgroups: sci.electronics
Subject: Re: power from phone line
Keywords: phone, power, remote
Message-ID: <10345@ucsd.Edu>
Date: 7 Dec 89 14:59:03 GMT
References: <14230@jumbo.dec.com> <3542@kitty.UUCP> <6300@lindy.Stanford.EDU> <5651@internal.Apple.COM>
Reply-To: brian@ucsd.edu (Brian Kantor)
Organization: The Avant-Garde of the Now, Ltd.
Lines: 32

Typical phone lines in the USA have around 48 to 52 volts DC on them
when the phone is on-hook (hung up, eh?).  That drops to "talking
battery" of about 4 to 10 volts when the phone is off-hook.  During
ringing, about 90 to 120 volts of 20 Hz alternating current is applied
to the line.

To power something off the phone line, you have to determine whether
the device needs constant power or only needs power when the phone is
off-hook.  You see, if you draw any significant amount of current (a few
milliamps or so), the central office equipment senses that as a phone
being off-hook.

Devices that require constant power generally derive it from the phone
line by using extremely low-power circuitry (almost always CMOS stuff),
and feeding the incoming line through some high-value resistors,
rectifiers, regulators, and filtering components to keep the ringing
voltage and lightning surges from frying the device.

Devices that only need power when the phone is to be off-hook can grab
more power, but they need to do it through a coupling circuit to allow
the audio interface to the phone line to survive.  The same sort of
protective devices and isolators are needed.

You can buy complete modules to interface to the phone line, but I've
not seen any marketed which also provide device power.  Probably you
could twist one into doing that, though.

A few minutes spent with a voltmeter and a telephone line is a rewarding
experience, and educational too.  A scope will let you see some of the
timing and frequencies used as well.  Try not to zap your ass on the
ringing voltage.
	- Brian