You may be able to devise a simple circuit which will work for your
particular site.  To come up with a simple circuit which will work on
generally any line is a different story.

Varibles to consider:

1. The DC on-hook voltage is typically 48V.  However, some telephone
exchanges in rural areas may go as high as 90V. Some, exchanges use 24
VDC.  During ringing, the HV ring signal may be superimposed on 48 volts,
or in some systems, it is superimposed on 1/2 the normal DC voltage.

2. The ring voltage and frequency will be different depending on what
equipment your telephone exchange uses.

3.  Most importantly, the different designs of telephones you have
connected to the phone line probably trigger at different ring voltage
levels.  A well-designed ringer circuit will reject noise caused by
pulse-dialers, otherwise you'll get what's known as "bell tapping".
Some phones I've worked with trigger with 7 Volts AC ( a typical ringer
voltage is 90 volts).

If you do use a back-to-back zener clamp, I sugest that you AC couple it
with a capacitor.  Use a capacitor of sufficient size and voltage rating
since the ring frequency can be as low as 16 Hz.

FCC part 68 as mentioned in an earlier post is a good source describing
the range of on-hook/ringing voltages a telephone is expected to work with
in the US.

#---------------------------------------------------------------------------
# Geoff Parker                           geoff@qdt.com

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On Thu, 18 Dec 1997, SHAWN ELLIS wrote:

> >         One should be careful about the type of load presented to a
> > telephone line pair.  The equipment at the switch to which your phone is
> > connected may range in complexity from rotary steppers and mechanical relays
>
> So basically, your saying that there is no practical, recommended way
> to cut off ringing from one extension on the inside wiring?  I
> thought this might be the case, and I would have to set up the ring
> blocker on the enterence point to the house.  Any other suggestions?
>