adam@cbmvax.UUCP (Adam Levin CATS) (04/18/89)
This topic was pretty well hashed out on the net about half
a year ago, but the exact method wasn't covered.
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. I made the connection like so:
Tip 1___ ___Tip 2
| |
\ \
/ /
\ \ 600 Ohm 1:1 transformer
/ /
Ring 1___| |___Ring 2
Has anyone successfully built a device like this?
I'd really appreciate hearing about it! Adam
--
Adam Keith Levin -- CATS Commodore-Amiga Technical Support
1200 Wilson Drive / West Chester, PA 19380 (215) 431-9180
BIX: aklevin UUCP: ...{amiga|rutgers|uunet}!cbmvax!adamlarry@kitty.UUCP (Larry Lippman) (04/19/89)
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. <> UUCP: {allegra|ames|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> VOICE: 716/688-1231, 716/773-1700 {att|hplabs|utzoo}!/ <> FAX: 716/741-9635, 716/773-2488 "Have you hugged your cat today?"