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'[EE]: telephone line interface'
2000\12\18@235933 by pang khong lin

picon face
hi and good afternoon. I am trying to interface to the telephone line using
an impedance matching transformer -  ETAL P3191. At the moment there are
only three part.

1. The transformer part that is suppose to bring the phone line to off
  hook
2. A ringing signal detector circuit
3. A double pole relay that is suppose to switch the telephone line to
  either Part 1 or    Part 2.

Based on the information i have, a typical telephone line will go off hook
when the impedance detected is around 600 ohm. Then the ringing signal will
be terminated. The problem is it does not goes off hook. I tried changing
different resistor on the secondary but no result. Any idea, please? Below
is the transformer circuit.

Tip                0.68uF
o----------------| |-------o--     --o------------------
                             S   S                    |
                             S   S                    |
                             S   S                 Resistor
                             S   S                    |
o--------------------------o--     --o------------------
Ring

I got this circuit from the datasheet. The original value of the cap is at
6.8uF. I replace it with 0.68uF because it is very expensive for a cap with
a rating 200Vac. at that value. Is that the reason for not being able to go
off hook?
Another question - the capacitor must be there to block dc and prevent the
transformer from being saturated. If there are no dc, how does it know that
the resistance is 600ohm?

thank you.

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2000\12\19@003850 by David Duffy

flavicon
face
pang khong lin wrote:
{Quote hidden}

You need to have a DC path on the telephone line side for the exchange to
detect the off-hook status. The circuit shown is for transferring audio to and
from the line. The resistor goes across the line to generate off-hook
condition.
There should be plenty of telephone interface circuits on the net.
Regards...

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2000\12\19@003908 by David VanHorn

flavicon
face
>1. The transformer part that is suppose to bring the phone line to off
>   hook

That series cap prevents you from going off hook, but beware, this
transformer may not be designed for a "wet" interface.

>2. A ringing signal detector circuit

Bridge rectify the line, on the output of the bridge, series a zener of
>60V, an optoisolator, and a current limiting resistor.

>3. A double pole relay that is suppose to switch the telephone line to
>   either Part 1 or    Part 2.

The ring detector can be left in circuit, it has no effect when the line is
looped.
You do want a relay, or opto, to loop the line.
Current is at least 20mA looped, can be greater than 100mA

{Quote hidden}

Don't mess with that cap value, or you're messing with your frequency
response and return loss.

You'll also need a hybrid circuit, unless you are only talking OR receiving.


>Another question - the capacitor must be there to block dc and prevent the
>transformer from being saturated. If there are no dc, how does it know that
>the resistance is 600ohm?

Impedance, not resistance.



It's not as simple as it looks when done right.
:)


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2000\12\19@071149 by Max Toole

picon face
In a message dated 12/19/00 12:41:05 AM Eastern Standard Time,
spam_OUTpiclistTakeThisOuTspamAUDIOVISUALDEVICES.COM.AU writes:

> >hi and good afternoon. I am trying to interface to the telephone line using
>  >an impedance matching transformer -  ETAL P3191. At the moment there are
>  >only three part.
>  >
>  >1. The transformer part that is suppose to bring the phone line to off
>  >   hook
>  >2. A ringing signal detector circuit
>  >3. A double pole relay that is suppose to switch the telephone line to
>  >   either Part 1 or    Part 2.
>  >
>  >Based on the information i have, a typical telephone line will go off hook
>  >when the impedance detected is around 600 ohm. Then the ringing signal
will
{Quote hidden}

with
>  >a rating 200Vac. at that value. Is that the reason for not being able to
go
>  >off hook?
>  >Another question - the capacitor must be there to block dc and prevent the
>  >transformer from being saturated. If there are no dc, how does it know
that
>  >the resistance is 600ohm?
>
>  You need to have a DC path on the telephone line side for the exchange to
>  detect the off-hook status. The circuit shown is for transferring audio to
> and
>  from the line. The resistor goes across the line to generate off-hook
>  condition.
>  There should be plenty of telephone interface circuits on the net.
>  Regards...
>
Motorola makes a couple of speech circuits for coupling to the phone line and
converting from 2-wire to 4-wire.  The one that we use is the MC34014P.
Works great.

Max

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2000\12\19@085446 by Olin Lathrop

face picon face
> The problem is it does not goes off hook. ...
>
> Tip                0.68uF
> o----------------| |-------o--     --o------------------
>                               S   S                    |
>                               S   S                    |
>                               S   S                 Resistor
>                               S   S                    |
> o--------------------------o--     --o------------------
> Ring

It's been a while since I messed with phone circuits, but I'm pretty sure
you need to be DC coupled (draw a steady current) to go off hook.  This
means you can't have a capacitor in series.


*****************************************************************
Olin Lathrop, embedded systems consultant in Devens Massachusetts
(978) 772-3129, .....olinKILLspamspam@spam@embedinc.com, http://www.embedinc.com

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2000\12\19@142240 by Don Hyde

flavicon
face
To go off hook, you need to make a DC connection that draws at least 20 mA.
Some telephone interface circuits use a transformer which is capable of
taking that DC current, and simultaneously performing the DC isolation and
impedence transformation needed between the line and your circuitry.  Such a
transformer does not need a capacitor between it and Tip and Ring.  It will
be physically rather large, on the order of 2 x 2 inches (nearly 5 x 5 cm).

If the transformer is more on the order of 3/4" x 3/4" (2 x 2 cm), then it
is only for DC isolation and impedence transformation, and something else is
required to complete the DC path.  This is often a toroidal inductor.  This
is not an inexpensive set of components, but is the smallest and lightest
that will meet the various safety requirements of US Part 68 FCC regulations
using only passive components.  Some mass-market designs use
active-component-based solutions which can be smaller and cheaper, but it is
a serious engineering challenge to make this work and meet all the legal
requirements.

BTW, if you look at a typical modem card, you will also notice that the
.68uF cap is big and carries a hefty voltage rating, there is a MOV or two,
thick PC traces, and frequently a spark gap or two in the PC layout to deal
with the lightning safety requirements.

> {Original Message removed}


'[EE]: telephone line interface'
2001\01\02@093506 by pang khong lin
picon face
Thanks for helping out David(s). So it seems i got all mess up. I did :-)
The transformer interface which i have seen aplenty on the net are meant for
receiving audio signals. All it does is providing the required 600 ohm
impedance so that the ac signals will be properly transfered. And as for
making the phone line off hook there should be a resistance of around 200
ohm across the line. ( or am i still wrong ?  :-(    )
Now my simple mind tells me that if i were to put the 200 ohm across the
line, the phone line should at least goes off hook. As for the 600 ohm
impedance, it will not be 600 ohm anymore. honestly i do not know how to
change the value on the transformer side. The datasheet for the transformer
gives a value of 6.8 uF and 430 ohm for the capacitor
and resistor. ( refer to the diagram below ). It will be helpful to know how
they got these values.

I did not managed to find any sites that offer some sort of method (
schematic ) to put a phone on hook and off hook. I have tried opening up
several house phones to see how they work, but it is very difficult to trace
the connection. Anyone has any schematic that shows how to bring the phone
off hook and on hook or maybe even the schematic
of a typical phone?
Thanks again and good day.


From: David Duffy <piclistspamKILLspamAUDIOVISUALDEVICES.COM.AU>
Subject: Re: [EE]: telephone line interface
Date: Tue, 19 Dec 2000 15:26:20 +1000

pang khong lin wrote:
   hi and good afternoon. I am trying to interface to the telephone line
using an impedance matching transformer -  ETAL P3191. At the moment there
are only three part.
1. The transformer part that is suppose to bring the phone line to off
  hook
2. A ringing signal detector circuit
3. A double pole relay that is suppose to switch the telephone line to
  either Part 1 or    Part 2.

Based on the information i have, a typical telephone line will go off
hook when the impedance detected is around 600 ohm. Then the ringing signal
will be terminated. The problem is it does not goes off hook. I tried
changing different resistor on the secondary but no result. Any idea,
please? Below is the transformer circuit.

   Tip                0.68uF
   o----------------| |-------o--     --o------------------
                                S   S                    |
                                S   S                    |
                                S   S                 Resistor
                                S   S                    |
   o--------------------------o--     --o------------------
   Ring

I got this circuit from the datasheet. The original value of the cap is at
6.8uF. I replace it with 0.68uF because it is very expensive for a cap with
a rating 200Vac. at that value. Is that the reason for not being able to go
off hook?
Another question - the capacitor must be there to block dc and prevent
the transformer from being saturated. If there are no dc, how does it know
that the resistance is 600ohm?


You need to have a DC path on the telephone line side for the exchange to
detect the off-hook status. The circuit shown is for transferring audio to
and
from the line. The resistor goes across the line to generate off-hook
condition.
There should be plenty of telephone interface circuits on the net.
Regards...


From: David VanHorn <.....dvanhornKILLspamspam.....CEDAR.NET>
Subject:  Re: [EE]: telephone line interface
Date:    Tue, 19 Dec 2000 00:22:38 -0500

>1. The transformer part that is suppose to bring the phone line to off>
>hook

That series cap prevents you from going off hook, but beware, this
transformer may not be designed for a "wet" interface.

>2. A ringing signal detector circuit

Bridge rectify the line, on the output of the bridge, series a zener of
>60V, an optoisolator, and a current limiting resistor.

>3. A double pole relay that is suppose to switch the telephone line to >
>either Part 1 or    Part 2.

The ring detector can be left in circuit, it has no effect when the line is
looped. You do want a relay, or opto, to loop the line.
Current is at least 20mA looped, can be greater than 100mA >Based on the
information i have, a typical telephone line will go off hook
{Quote hidden}

Don't mess with that cap value, or you're messing with your frequency
response and return loss.

You'll also need a hybrid circuit, unless you are only talking OR receiving.

>Another question - the capacitor must be there to block dc and prevent the
> >transformer from being saturated. If there are no dc, how does it know
>that >the resistance is 600ohm?

Impedance, not resistance.

It's not as simple as it looks when done right.
:)


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2001\01\02@104146 by David VanHorn

flavicon
face
At 09:35 AM 1/2/01 -0500, pang khong lin wrote:
>Thanks for helping out David(s). So it seems i got all mess up. I did :-)
>The transformer interface which i have seen aplenty on the net are meant for
>receiving audio signals. All it does is providing the required 600 ohm
>impedance so that the ac signals will be properly transfered. And as for
>making the phone line off hook there should be a resistance of around 200
>ohm across the line. ( or am i still wrong ?  :-(    )


You're right. These are two separate functions, though they can be combined
into the transformer at the expense of more iron in the transformer.

A 20mA constant current source across the line will work well to take you
off hook.
The CO needs to see 20mA to take you off hook for sure.
Why is this good? What's the impedance of a constant current source? :)


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2001\01\02@111329 by Dan Michaels

flavicon
face
pang khong lin wrote:
>
>I did not managed to find any sites that offer some sort of method (
>schematic ) to put a phone on hook and off hook. I have tried opening up
>several house phones to see how they work, but it is very difficult to trace
>the connection. Anyone has any schematic that shows how to bring the phone
>off hook and on hook or maybe even the schematic
>of a typical phone?


I have some links to telephone circuits/info on my page.
The first 2 links show lots of circuits:

http://www.oricomtech.com/teklink2.htm#Tele2

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