Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!think.com!eplunix!raoul
From: raoul@eplunix.UUCP (Nico Garcia)
Newsgroups: sci.electronics
Subject: Re: Drawing DC phone current
Keywords: SK2086, phones
Message-ID: <1085@eplunix.UUCP>
Date: 29 May 91 18:34:40 GMT
References: <1081@eplunix.UUCP> <8053@nst> <1084@eplunix.UUCP> <1991May29.084859.5324@specialix.co.uk>
Organization: Eaton-Peabody Lab, Boston, MA
Lines: 35

In article <1991May29.084859.5324@specialix.co.uk>, stevem@specialix.co.uk (Steven Murray) writes:
> 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 :-)

Yeah, OK. I used 2700 ohms to mimic 10 Volts (what folks refer to as
"off-hook" voltage, although this is not correct) delivering only 4 mA, and
to see if it would force an off-hook condition. It did so. Voltage in the
off-hook state depends on voltage drop from your telco, and their line
voltage. 22 kohm might work at one home (2 mA at -48 Volts worst case in the
States' phone system), but not in another, due to a lower impedance from the
telco permitting more current. And what if you put one on every phone, and
have ~5 phones? All of a sudden you`re drawing 10 mA. Enough for off-hook
detection, I'm certain.

Typical operating current, according to my copy of "Understanding Telephone
Electronics", is 20 to 120 mA. That doesn not mean that 4 mA or less won't
trigger off-hook or maintain it. Doesn't mean it will: I believe the original
poster that it didn't happen for 4 mA, on *his* line. Also, the telco has to
provide that current. I can only assume they won't notice half a dozen users
drawing it.  Widespread use, however, and I'll *bet* they notice such a
constant drain of a few percent of the active load. And they could start
pulling phone service because of it, until it stops.

It's kind of like operating things beyond spec: I don't like relying on such
behavior, or designing circuits that do so. It feels like really bad design
to me. This is a common example of every new gadgetmeister solving the
problem, and most of them doing it *badly* because of subtleties they don't
think about.
-- 
			Nico Garcia
			Designs by Geniuses for use by Idiots
			eplunix!cirl!raoul@eddie.mit.edu