Land Line Telephone Interface Circuit

[Craig Maier]

Here is an Application Note that shows how to build a Land Line Interface Circuit. It provides direct coupling between a telephone land line and an analog input:

http://www.diamondcut.com/AppNotes/AN9b.pdf

Craig

[ric29]

Craig,
It should be a resistor between transformer and diodes. Just my cents.

[Craig Maier]

No - the 20 KOhm Resistor on the primary side"reflects" through the transformer which limits the current through the diodes and limits the ring current from the interface circuit. It does so by the mutual inductance term in the audio transformer transfer function which carries through the 20 KOhm resistance to the secondary. So, the circuit works as shown.

If that resistor was transposed in position to the secondary side, the transformer would saturate on the ring condition and drag down the phone line during the ring process.

Craig