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From: jon_sree@world.std.com (Jon Sreekanth)
Newsgroups: comp.dcom.telecom
Subject: Re: Why Does Device Cause Ringing to Stop?
Message-ID: <74331@bu.edu.bu.edu>
Date: 8 Feb 91 14:10:07 GMT
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X-Telecom-Digest: Volume 11, Issue 102, Message 4 of 11


In article <16771@accuvax.nwu.edu> atn@cory.berkeley.edu (Alan
Nishioka) writes:

> The problem enters when I add a 200ohm resistor and a 2n2222
> transistor across the line to implement the hold button. (Common
> emitter, with a 47K base resistor to a CMOS latch output)

I'm not familiar with Key Switch Units, but assuming yours just simply
distributes the incoming telco line, the specs on the DC battery and
AC ring voltage are 56.5 V DC, 40 to 150 V RMS at 15.3 to 68 Hz (so
called FCC B ring). 2N2222 has a Vceo of 40V, so it's almost certainly
breaking down, and shorting ring.

In a similar application, in one of my designs, I've used an MPSU10,
which is a 300V rated NPN transistor. It does not have too high an
hfe, so to drive it from an HC, I use an MPSA42 (also 300V) in a
Darlington arrangement, so MPSU10 is guaranteed to saturate.

Also, it's probably safe to preface this with a bridge rectifier (I
use 1N4004's). This way, it can hold the line offhook, even if
polarity is reversed. I've found several homes with tip and ring wired
with incorrect polarity. Don't know about KSU's.

> When I wire the base of the transistor low (off), the problem goes
> away.

This puzzles me, but try the high voltage transistor, anyway.

Regards, 

Jon Sreekanth

Assabet Valley Microsystems			Fax and PC products
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