| 摘要 |
PURPOSE: An exchange subscriber circuit having a harmful voltage preventing circuit is provided to easily implement and protect an internal circuit from a harmful voltage by controlling an operation voltage simply with a control resistor. CONSTITUTION: Channel slicks(401,501) are subscriber line circuits in an exchange. Current limiting PTCs(404,504) perform a current buffering first on a positive or negative harmful voltage applied to a tip/ring stage of the channel slicks(401,501). First surge protectors(403,503) limit a harmful voltage first which has passed through the current limiting PTCs(404,504) according to an operation voltage. Bridge diodes(402,502) induce the positive harmful voltage to ground. A common second protection circuit(405) has a reverse current protecting diode and a Zener diode maintaining a fixed voltage value. In the common second protection circuit(405), a first transistor where negative voltages of several channel meet is operated if a base-emitter voltage is greater than a battery voltage of a reference board and a second transistor is operated accordingly to make the harmful voltage and current flow to ground to thereby protect an internal circuit. A control register is added between the diode and a base of the first transistor.
|
