Path: utzoo!dptcdc!jarvis.csri.toronto.edu!mailrus!husc6!cs.utexas.edu!uunet!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Conferencing two phone lines
Summary: Transformer saturation from DC line current...
Message-ID: <3102@kitty.UUCP>
Date: 19 Apr 89 04:02:29 GMT
References: <6612@cbmvax.UUCP>
Distribution: usa
Organization: Recognition Research Corp., Clarence, NY
Lines: 41

In article <6612@cbmvax.UUCP>, adam@cbmvax.UUCP (Adam Levin CATS) writes:
> I have two phone lines that I want to be able to "conference"
> from time to time.  We learned that you shouldn't just connect
> tip-to-tip and ring-to-ring because of voltage differences between
> your location and the phone company (not to mention if a call
> should come in on one line while a call is in progress on the other).
> 
> It was advised that one needed to use an isolation transformer
> between the two lines.  As I recall, this was a 600 Ohm 1:1
> transformer.  I bought such a beast at Radio Shack, but connecting it
> to the phone lines seems to load them too much; there is an
> obvious degradation of the signal.

	The reason that you have degradation is that the transformer
windings are saturating from the DC line current, which can vary from
30 to 120 mA, with 60 mA being a typical value on a typical telephone
loop for a transformer whose windings probably have a DC resistance of
less than 100 ohms.

	You need a transformer that is specifically designed to pass
at least 50 mA of DC without significant saturation.  These transformers
are readily available - but not from Radio Shack.  Manufacturers of
such transformers include Stancor (Essex) and Triad-Utrad, and these
are available from industrial electronic distributors.

	There is another technique which involves an inductor placed
across each telephone line to handle the DC current flow with minimum
voice frequency insertion loss.  The isolation transformer is also
connected across the telephone line, but with a series non-polarized
capacitor (typically 1 to 2 uF).  In this arrangement there is no DC
current flow through the transformer windings.

	An ideal low-cost solution would be to find a surplus telephone
apparatus transformer, such as a Western Electric 120C Repeat Coil
or a 202A Repeat Coil.  These devices will handle DC currents of
100 mA with little insertion loss.

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