Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sun-barr!newstop!eastapps!vergil!gsteckel
From: gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR)
Newsgroups: sci.electronics
Subject: Re: Drawing DC phone current
Summary: microamperes at most
Keywords: SK2086, phones
Message-ID: <6487@eastapps.East.Sun.COM>
Date: 31 May 91 21:45:31 GMT
References: <8053@nst> <1084@eplunix.UUCP> <1991May29.084859.5324@specialix.co.uk> <8085@nst> <107170@sgi.sgi.com> <1991May31.135017.13702@specialix.co.uk>
Sender: news@East.Sun.COM
Reply-To: gsteckel@east.sun.com (Geoff Steckel - Sun BOS Hardware CONTRACTOR)
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Organization: Omnivore Technology, Newton, Mass. (617)969-3448
Lines: 27

>>In article <8085@nst> grayt@Software.Mitel.COM (Tom Gray) writes:
>>| No problem will result if you draw less than the 7mA from the loop.
>
>>No *immediate* operational problem... However, in the nightly automatic test
>>scans [forgot the Telco acronym], your line will be flagged as "leaky", and
>>may eventually spit out a trouble ticket. When they figure out the "leak" is
>>inside your house, they'll send you the "repair bill" for a "false service
>>call due to customer premises equipment".

Everyone interested in this thread should look at (Usenet) Comp.dcom.telecom,
which (among other things) discusses the nuts and bolts of telephony.

(foggy memory alert...) I think the leakage resistance of a (legal) on-hook
unit with a REN (ringer equivalence) of 1.0 is 22 Megohms.  That means a leakage
current of only a couple of >micro< amperes.  There's an FCC spec & everything.
This requirement makes a line-powered off-hook light much more interesting.

I have seen a phone alleged to have been FCC registered which has a line
powered off-hook light; it seems that they cheat a little.  When another
instrument goes off-hook, what looks like a CMOS comparator turns on the
light.  At this point the lighted instrument is off-hook.  The tricky part
is shutdown when all the other instruments go on-hook.

	geoff steckel (gwes@wjh12.harvard.EDU)
			(...!husc6!wjh12!omnivore!gws)
Disclaimer: I am not affiliated with Sun Microsystems, despite the From: line.
This posting is entirely the author's responsibility.