JPH01231624A - Surge protective circuit for electronic line switch - Google Patents

Abstract

PURPOSE: To reduce increment of DC voltage due to surge voltage, by providing a current limiting circuit for controlling a line switch which suppresses line current below a specific level.

CONSTITUTION: When line current higher than 150mA flows through a loop, voltage is produced across a resistor 22 to turn a transistor 14 ON thus causing line current flow through a resistor 21. When the voltage across the resistor 21 increases, the base current of a transistor 13 decreases thus decreasing the collector current, and thereby the line current drops below 150mA to turn the transistor 14 OFF. Current does not flow from the collector of the transistor 14 to the resistor 21 in this state, and thereby the base voltage of a transistor 12 drops to feed current to the base of a transistor 13 thus feeding collector current. When the collector current increases, the voltage across the resistor 22 increases to turn the transistor 14 ON again. When such condition is repeated, the load of the transistor 13 is relieved.

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