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From: Gerry Schneider
X-Mailer: Mozilla 4.73 [en]C-SYMPA (Win98; U)
Subject: Re: Telephone Ring Suppressor
References: <email@example.com> <3D8FCA72.DB9DB70C@sympatico.ca> <firstname.lastname@example.org>
Date: Tue, 24 Sep 2002 01:44:07 -0700
NNTP-Posting-Date: Tue, 24 Sep 2002 02:06:30 EDT
Organization: Bell Sympatico
Allan Herriman wrote:
> Here's an example of a ring detector in a DAA that is DC coupled. It
> works by sensing the peak voltage:
> According to the web site, this DAA has been specifically designed for
> North American operation.
> It needs the addition of an RC network to model a ringer, so that the
> exchange can tell that there's a "phone" on the other end of the line.
> But this is just a dummy ringer. The real ring detector is DC coupled
> (and very high impedance).
Thanks for the info, Allan. This device is merely sneaking in under the limits for
allowable on-hook DC leakage current. It specs maxes of about 10 Mohms DC (pretty
sloppy - should be much higher) and about 140 kohms AC from tip or ring to circuit
ground, so in effect it is AC-coupled - just done by internal impedance trickery
rather than a real capacitor. For N. American legal devices, the DC component of
on-hook impedance is ideally meant to be infinite - any non-zero value of leakage
current merely degrades the performance and leaves the user open to loss of service
if the total current of multiple paralleled devices exceeds the telco's watchdog
detection threshold (usually more than 100 uA). It's up to manufacturers to ensure
that their 21st Century products are compatible with the original specs set in the
very early 20th Century! :)
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