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From: Nagle@cup.portal.com (John - Nagle)
Newsgroups: sci.electronics
Subject: Re: power from phone line
Message-ID: <25271@cup.portal.com>
Date: 22 Dec 89 08:24:46 GMT
References: <14230@jumbo.dec.com> <318@marvin.moncam.co.uk>
Organization: The Portal System (TM)
Lines: 26


     You can indeed get a little power from the phone line, and there
are standards on how much you can draw.  When off-hook, you can 
draw over 100 MA, in fact.  All sorts of telephone-related devices
run off phone line power nowadays.  

     Bear in mind that all sorts of interesting surges and spikes
appear on phone lines.  Ring is nominally 88VAC, 20Hz.  Much larger
spikes can occur during electrical storms, and are considered part
of the normal operating environment.

     On the other hand, nothing particularly bad will happen if
you short the phone line for brief periods.

     Equipment that attaches to the phone line is supposed to be
FCC registered.  Automatic testing from the central office will
detect as faults lines which present unreasonable loads.  The
telco will probably think that they have a cable fault, and if
they spend time and effort chasing one down and discover that
your equipment is causing a false fault indication, you may
be billed for the repair costs.  This is especially true for
something that presents a DC load during on-hook periods insufficient
to take the circuit off-hook.  That, to the central office, looks
like water in a cable, and tends to be investigated.

				        John Nagle