Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!ukma!rutgers!att!tellab5!zantow
From: zantow@tellab5.tellabs.CHI.IL.US (Al Zantow)
Newsgroups: sci.electronics
Subject: Re: Using "Free" telephone power
Message-ID: <1325@tellab5.tellabs.CHI.IL.US>
Date: 2 May 89 01:35:52 GMT
References: <636@serene.UUCP>
Reply-To: zantow@tellab5.UUCP (Al Zantow)
Organization: Tellabs, Inc., Lisle, IL
Lines: 45

In article <636@serene.UUCP> gbell@pnet12.cts.com (Greg Bell) writes:
>
>This is in response to the person who was talking about lighting an LED from
>the 50 VDC phone line voltage as an indicator for hook status:
>
>
>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!).  
>
>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?
>

	There are some problems with trying to use phone power to monitior
the phone line.  The phone company equipment "watches" the current in you 
phone line, and if it goes above a few mA's it assumes you are off hook and
want dial tone.  When you realy are off hook the current in the line is limited
to about 50 mA's.  This is partly for your protection so you don't cook 
if you get across the phone line, but mostly since bipolar transistors are 
current operated devices.  A "regulated" current makes them happier than a
regulated voltage source, which allowed the earlier touch tone phones to be
more stable.

	The dial lights and such are probably powered by the yellow and black
wires that have about 12 VAC on then from a transformer wired into your
house (or aprtment) AC system.  This is so the phone company does not need
to provide all the power for the "goodies", just for the actual phone circuits.
The red and green wires are the ones that have the connection to the phone
company, and normaly are not dependent on comercial AC power to work.  This 
is so that you can use the phone after power is knocked out to yell at the
electric company for being such clods and letting the lights go out ;-)

	Not all houses have the "extra" AC power circuit wired up, you can
always check this with a meter though.  The power availabe from the
seconday AC is abount a few hundred mA's, so you can power just about
anything from it.


"tell me, tell me about Subscriber Trunk Dialing !!!" from Time Bandits

(the Devil was asking one of his minions for really EVIL technology like
fast breeder reactors, comercial television, ect)