piclist 2001\01\02\104146a >
Thread: telephone line interface
www.piclist.com/techref/pots/dtmf.htm?key=telephone
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face BY : David VanHorn email (remove spam text)



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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