From: Floyd Davidson <floyd@ptialaska.net>
Newsgroups: comp.dcom.telecom.tech
Subject: Re: Phantoms on phone lines
Date: 08 Jul 2000 00:22:56 -0800

"Don Mansfield" <rmansfield@worldnet.att.net> wrote:
>This is something I know has been done in years past and I am hoping there
>are some folks out there who can help me with this.
>
>I have the need to remotely control a relay over about .25 miles (that's
>one-quarter of a mile) of a dry, metallic pair from PacBell. I also need to
>pass audio over the pair at the same time. In my scheme, I would phantom
>+12v on the pair via a centered tapped 600/600 ohm transformer with the
>negative end going to ground/earth. On the far end, a 12v relay between the
>center tap of another 600/600 ohm transformer and ground.
>
>Does this appear to be workable? Are the voltages right? How much current
>will this sucker draw and will that cause problems with a standard 600/600
>ohm transformer (repeat coil like the old 111-C?)
>
>Any help on this is greatly appreciated.

Sounds almost perfect.  (I just can't remember exactly what a
111C coil is, as opposed to a 120C, which would be a 600 split
winding coil that can handle loop current.)

You might want to consider at least 24 volts though, to avoid
any trouble with ground potentials and to give your circuit a
little more current, and you do not want to operate the relay
from the center tap to ground if possible.


 T<-)||(-----------------//----------------------------)||(->T
    )||(                                               )||(
    )||(----------+                         +----------)||(
    )||(---+      |                         |      +---)||(
    )||(   |      |                         |      |   )||(
 R<-)||(---|------|------------//-----------|------|---)||(->R
           |      |                         |      |
           +--||--+ 1 mfd cacpacitor        +--||--+  1 mfd capacitor
           |      | (non-polarized)         |      |  (non-polarized)
           \      \                         \      \
           / 100  / 100                     / 100  / 100
           \ Ohms \ Ohms                    \ Ohms \ Ohms
           /      /                         /      /
           \      \                         \      \
           |      |                         |      |
           |      o                      +------------+
           |       \                     | RELAY COIL |
           |      o \                    +------------+
           |      |
           V      V
           +      -


The resistors can be adjusted to match the requirements for the
relay and the voltage used.  Five percent tolerance is fine, but
do keep each pair the same value.

The capacitors can be almost anything.  Just be sure they are
non-polarized.  You can use any value from 1 mfd to 4 mfd or
even larger.

  Floyd

--
Floyd L. Davidson                          floyd@barrow.com
Ukpeagvik (Barrow, Alaska)