🚀 go-pugleaf

In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes:
>
>I'm still amazed at how phones run entirely off the phone line power.  They
>have lit dials, and audio amplifiers... and I had trouble lighting an LED!
>(trouble means the phone company equipment gets loaded down enough that it
>shuts off your line for a while!).  

An off-hook telephone is *required* to draw a minimum amount of
current.  I believe that it is permitted to draw about 20 milliamps
from the line in an *off-hook* condition.  The Central Office's
switching equipment takes current draw above a certain limit as an
indication of an off-hook condition.  If you were trying to light that
LED with the phone on-hook, you were actually taking the line
off-hook.  After 15 seconds or so, the dial tone generator would time
out and you would have an open line.

Please note that the (Western Electric) telephones (e. g. "Princess")
with lighted dials are supplied 6 VAC by a small line transformer
which is typically connected to the black and yellow "unused" wires in
the phone jack.  Multi-line phones are a whole 'nother ballgame, with
a closet full of electronics and power supplies to support them.

>Can anybody offer insite on the secrets of running audio amps and chips off
>the meagre phone line power?  How 'bout the ammount of current the phone line
>can source?

I've used line-powered modems.  They use all CMOS parts to keep
current consumption down, and as I said, since an off-hook device is
required to draw a certain amount of current, why not use it to power
the modem?  Don't expect to be able to use phone line current and keep
the circuit on-hook, however.  Even if you could, you would have to
cope with the possibility of an incoming ring signal, which is
typically 60-90 VAC!  (Yes, Virginia, you can ring a phone with house
current.)


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