Path: utzoo!utgpu!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!mcsun!ukc!slxsys!stevem
From: stevem@specialix.co.uk (Steven Murray)
Newsgroups: sci.electronics
Subject: Re: Drawing DC phone current
Keywords: SK2086, phones
Message-ID: <1991May29.084859.5324@specialix.co.uk>
Date: 29 May 91 08:48:59 GMT
References: <1081@eplunix.UUCP> <8053@nst> <1084@eplunix.UUCP>
Organization: Specialix International, London
Lines: 39

raoul@eplunix.UUCP (Nico Garcia) writes:

>In article <8053@nst>, grayt@Software.Mitel.COM (Tom Gray) writes:
>> Draw less than 4mA from the loop and you will not have any 
>> problems.

>Only works in the off-hook position, Tom. A standard phone is about 600
>ohms, and has about 10 Volts across it off-hook. That's *about* 15-20 mA:
>the standards are deliberately extremely broad. It's not clear at all to me
>that 4 mA will not maintain the "off-hook" status in the phone company's
>circuits. However, the phone company does *not* want DC paths available for
>the on-hook state. 

I'm sure it is true the phone company does not want the DC path - nevertheless
they do allow for some leakage across the circuit.  The original poster was
about right I think.

>In fact, let me go try this (putting a 2700 ohm resistor across my office
>phone line). Hmmm: goes off-hook as the voltage drops to 18 volts. Maybe my
>line is picky, but forcing it off-hook is not what *I* want from an
>in-use detector.

You used 2700 ohms - but the off-hook voltage was something like 18 volts -
thats not 4 milliamps.   Just leave the 4 milliamps bit for a moment, try
a resistor of value 22k.  Its still not 'proper' but I think you will find
it works.  I'm not suggesting you do this on anything but your own private
telephone exchange :-)
 

>-- 
>			Nico Garcia
>			Designs by Geniuses for use by Idiots
>			eplunix!cirl!raoul@eddie.mit.edu

-- 
Steven Murray
uunet!slxsys!stevem  stevem@specialix.co.uk
I am speaking, but  | If these are your opinions, then we are in agreement!!
not for my employer.| Flames, spelling errors, complaints > /dev/null