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From: rmrin@inuxh.UUCP
Newsgroups: sci.electronics
Subject: Re: I need a telephone ring detector chip
Message-ID: <565@inuxh.UUCP>
Date: Thu, 23-Apr-87 11:26:23 EST
Article-I.D.: inuxh.565
Posted: Thu Apr 23 11:26:23 1987
Date-Received: Sat, 25-Apr-87 07:11:40 EST
References: <228@phoenix.PRINCETON.EDU> <7940@utzoo.UUCP> <441@haddock.UUCP>
Organization: AT&T Consumer Products, Indianapolis
Lines: 19

> >You have two wires coming in from the phone line.  First, each goes through
> >a 100k resistor.  Then there is a 10M resistor between them.  Then one of
> >them goes through a 0.02 uF capacitor, which should be rated for something
> >like 200V.  Then there are two diodes in parallel between the lines, with
> >opposite polarities (one with anode to top line, the other with cathode
> >to top line):  one a 1N4148 and one the input LED of a 6N139 opto-isolator.
> 
> One minor point: The 10 meg 'bleeder' resistor could cause problems.
> Some telephone line integrity tests involve measuring minute leakage
> currants. Leakages in that range (<100 meg) are usually indicative of
> brittle insulation on the line pairs. This could cause some repair
> folks to investigate the source. (I'm not sure why the apps note
> engineer felt he needed a bleeder. The capacitor has 200k in series
> with it, and is only charged to 50 volts max. Not what you'd ever call
> a shock hazard.)
> 
Ten megs will not cause any problems.  I would suggest builiding a circuit
such as this in a metal box to guard against component overheating in the
event funny voltages get on the phone lines.