From: (John Lundgren)

 > The way the SCR and zener works is that the first phone that goes
 > off-hook gets the full 48 volts DC which makes the 21 V zener conduct
 > and fires the SCR for the rest of the time the phone is off hook.
 > Any other phone then can't get the 21V to fire the SCR, and they all
 > remain open.  Simple and effective.

 Some circuits add another zener in series with the SCR allowing an
 unprotected line to disconnect the protected lines when it is picked
 up, since the voltage will fall below 8.2 volts and turn off the
 SCR's. The other diode passes the negative ring voltage.

                     Anode     Cathode
     + <---+-------------  SCR ---------|<----------+------->
           |               Gate      8.5 volt       |
   To      |   12 volt      |          zener        |
  line     +------|<--------+                       |  To phone
           |                                        |

     - <---------------------------------------------------->


Date: 23 Feb 1996 02:59:39 -0500

Original Subject: Re: Telephone blocking circuit?

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