Adaptor for MODEM for switched telephone network - has continuous current regulator coupling line rectifier bridge through overvoltage protection circuit to MODEM

摘要

The device has a continuous current regulating circuit (1) coupled through a bridge rectifier circuit (4-7) to the line wires (10,11), of which one has a series resistance. The regulating circuit is coupled through an over voltage protection circuit (20) and coupling transformer (17) to a modem (14). The regulating circuit time constant is several milliseconds. The arrangement enables an electrolytic capacitor (15) coupling the protection circuit to the modem and the transformer to be reduced in size and the low time constant of the regulating circuit allows a current control circuit for an electronic dialling control to be added. The regulating circuit has a Darlington transistor pair connected across the bridge output by resistors and zener diodes, the Darlington base being connected to a transistor (31) of the protection circuit biassed by a resistor and zener diode chain.