Portable telephone subscriber's line polarity tester - uses opto-couplers in diode bridge for line interference immunity

摘要

A portable test-set checks whether the wires of a subscriber's line have been crossed over during installation. A diode bridge (D) has opto-couplers (La, Lb) in two of its arms and the AC diagonal is plugged into the line. Another opto-coupler (L3) has its switch (Ke connected across the DC diagonal to simulate the subscriber's hook switch. Pressing the test button (DK) closes this circuit and loop current flows through one bridge opto-coupler (La) or the other (Lb) and operates indicators (Aa or Ab). A battery/mains unit provides power for the indicators and hook-switch opto-coupler. Opto-couplers are used to make the test-set immune to interference voltages induced in the line. A cut-out (s) opens if excessive loop current flows and the opto-couplers are protected by non-linear circuits such as Zener diodes.