Fax to Modem interface

Posted on May 1, 2012

The circuit can be easily built to a small plastic box with telephone connectors in both ends. The circuit takes about 10-20 mA current, so if the circuit is not in use all the time it is best to use batteries. You can also use small wall transformer to power the circuit. Remeber to take a wall transformer which gives good quality output (regulated model preferred), because if the wall transformer gives out noise, it can be hard on the line (usully you hear 50/60 Hz buzzing).

Fax to Modem interface
Click here to download the full size of the above Circuit.

The 680 ohm resistor (used for limiting the power from 12V source to safe value for telephone electornics) causes some problems because it causes imedance mismatch to the telephone line. If this causes problems, you can add a capacitor to the circuit which makes sure that the circuit apperas to be like piece of wire in voice frequencies you can avoid mismatch caused by that 680 ohm resistor. I added a 22 ohm resistor in series with the capacitor to limit the surge current which appears when the equipments go on-line. For normal FAX to modem usage this modification should not be necessary because equipments should be able to cope with those impedance mismatches the original circuit causes.

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