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

On a sunny day (Thu, 12 Apr 2007 18:20:22 +0200) it happened OBones
<obones_gfd_@xxxxxxxxxxxxxxx> wrote in
<461e5c35$0$25709$426a34cc@xxxxxxxxxxxx>:

Ross Herbert wrote:
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.

Well, a regular phone line has a DC component, if I'm correct.


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 trust you on this, but how do I access this for a one off series, when
I generally take the parts from the local hobbyist shop?
I'd rather use the optocoupler isolation as it uses readily available
parts that even come up in SMT packages, which are definitely smaller
and easier on the layout than a through hole part.

I have used optocoupler for audio from phone lines.
Firts quastion you must ask then is : Do you have a second isolated supply?
In my case I had thta, so I could drive the LED of the opto cooupler
with an opamp current source (linearity).
Just picked the voltage of the line.


Uin opamp
-||-- +
out ---------
|
--- - |
| \ / / LED
| ---
---------------------|
|
[ ]
|
----

In case a switchmode is present the extra suply is just some extra turns on
a potcore.


At the reception side (different opamp!!) I had the photo transistor
drive into the inverting input ... low impedance, good high frequency response,
good linearity.

In the example the OP (you?) gave the LED already draws quite a bit of current.
I am not sure about the linearity of that circuit.

.

Relevant Pages

  • Re: Phone line interface: Replacing a 1:1 transformer with an optocoupler
    ... I'm designing a system that uses a MT8870 DTMF decoder to detect ... 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 ... transformer they don't need to be big and clumsy. ...
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