PURPOSE: To automate reset operation by coupling a three-terminal thyristor, which has a cathode and a gate arranged alternately, to an external current sensing resistor. CONSTITUTION: When a chip line 32 of a transmission line 30 pulls a current out of a chip line 22 of an SLIC 20, a voltage is generated at a current sensing resistor 12. Since parasitic serial resistance R2 is considerably lower than branching resistance R1, the voltage to appear at both the terminals of the current sensing resistor 12 appears at the base/emitter joint of Q1. The value of the current sensing resistor 12 is selected so as to guarantee the generation of a voltage enough for an overcurrent to turn on the transistor Q1 at both the terminals. The transistor Q1 turns on a transistor Q2 and the overcurrent is supplied from the ground to the chip line 32 of the transmission line 30 by a protector 10. Thus, a line interface at a long-distance communication exchange can be guaranteed.
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