[Gllug] OT: What is the REN of a modern powered phone?

[Christopher Hunter]

On Sun, 2010-11-07 at 10:26 +0000, Chris Bell wrote:
> Hello,
>    Old analogue phones with line-break dialling could sit across the
> incoming line as long as only one was actually in use and there were not
> more than six 1Kohm impedance ringers in series. The specification was
> changed so that up to 4 higher impedance ringers could be connected in
> parallel ("4 REN").
>    Many modern phones do not require the "ringer" wire because they use tone
> coding, so what is their actual REN? And how many can actually sit on the
> line but with their handset on the rest without excessive line loading? I do
> use my landline, and it is much more convenient to have phones available
> where they are needed.

Chris - they all still have the third "ring" wire. The ringing
(nominally 80V AC) is stripped off the incoming line by a 1µ8 160V
working capacitor.  The "bell" circuit inside the phone is fed through a
pair of back-to-back 3V9 zener diodes, so that the speech doesn't get
shunted by the bell circuit.  Each "bell" has a nominal impedance of
4000 ohms for a REN of 1.  Subsequent bells are wired in parallel, so
four telephones should give a REN of 4 and an impedance of 1000 ohms.
Each phone is equipped with a "bell shunt" which connects a 100 ohm
resistor (effectively) across the bell to prevent the bell sounding off
if using pulse dialling, and to prevent the spurious "ting" when a
handset is replaced after use.

You can connect as many telephones as you like across the line, as long
as you don't exceed the REN ("Ringer Equivalence Number") of 4.  The
telephones and other telephonic kit I designed for Panasonic and other
manufacturers usually had a REN of 0.5 (or 0.2 in the case of answering
machines and cordless phones), so you could connect more of them!

It really DOES matter.  If you exceed a REN of 4, you will get strange
things happening, and BT will charge you money to tell you that you're
overloading their ringing circuit!

Chris



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