Re: Phone line interface: Replacing a 1:1 transformer with an optocoupler

On Wed, 11 Apr 2007 22:17:35 +0200, OBones
<obones_gfd_@xxxxxxxxxxxxxxx> wrote:

Hi all,

I'm designing a system that uses a MT8870 DTMF decoder to detect
tones
on a phone line and the typical phone line interface is done via a
1:1
600ohm "telephony" transformer, like this :



-. ,--------o------ To MT8870
Line )|( |
)|( z
-' '-. A
| |
Gnd |
V
z
|
Gnd

The two zeners are here to ensure the voltage does not go above Vcc
applied to MT8870.
My problem is that the transformers for these applications are bulky
and
take a lot of space. Hence I was wondering if I could replace the
transformer by a diode bridge and optocoupler pair like this:

Vcc
|
___ .----------. |
+-+----|___|---|-. .-|-'
A A | | |/ |
Line -+ | | V -> | |
-(-+ | - |> |
A A | | | |
+-+------------|-' '-|-- To MT8870
'----------'

My main concern is the frequency that the optocoupler can support. I
have a bunch of 4N25 here and considering the max rise time and fall
time with a security margin, I'd say it can transfer signals up to at
least 30kHz which to me is enough for this application as DTMF
signals
only go up to 2kHz.
Can anyone confirm that my calculations are right, and that the
arrangement of the transistor side of the 4N25 is fine?

Thanks a lot for your help.
Cheers
Olivier

Olivier,
Provided that you don't have any DC current flowing in a 600:600
transformer they don't need to be big and clumsy. Since you say you
only want to "detect dtmf" then you should be able to arrange your
transformer with a blocking capacitor in series with the primary
across the line.

A 220nF 250Vdc film cap and a Tamura MG-21 (600:600) miniature audio
transformer which has a 3dB freq range from 100Hz - 100kHz would be
suitable. The transformer measures only 8mmx10mmx11mm.
http://www.tamura-ss.co.jp/en/electronics/trance03/pdf/mg_mx.pdf

I dug up a circuit for a dtmf monitor here
http://www.glolab.com/freeinfo/kdial.pdf and it uses a 100nF cap in
series with primary across the line.
.

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