[SML] Tele-Q & Practical Phones

[Nigel Worsley]

Frank Wood wrote:
> Telephones still meet very old-fashioned standards, and sometimes
> expect to see big relays across the line. I would think that 100mA
> would be typical.

Electronic exchanges typically feed the line with a constant current
of 25mA, and because it is considered a reasonable scenario that two
handsets will be lifted at the same time ( eg. for transferring a call
by shouting at someone else in the house to pick up the other phone )
it is a requirement for phones to function correctly on 12.5mA in the
UK, and similar specifications exist in most other countries.

Much older telephones have similar characteristics, as the 50V feed
voltage and much of the sometimes very long wiring hasn't changed over
the years.

Nigel Worsley