| 摘要 |
A current-limiting circuit of a battery-feed circuit for a digital telecommunications line provides a low start-up current limit when power is initially applied to the digital line. It automatically and gradually changes to a high current limit as the load on the line is charged. When the load is fully charged, the circuit provides a low impedance to the line. Upon occurrence of a fault, the circuit switches back to the initial low current-limit state. The circuit comprises a feed transistor (Q1), a control transistor (Q2), and a high-impedance resistive circuit (R1-R4). The feed transistor limits (403) current flowing to the R lead of the line to the high limit when the voltage across the feed transistor is low and the feed transistor is saturated. The control transistor partially turns off the feed transistor when the voltage is intermediate causing the feed transistor to provide an intermediate current that is inversely proportional to the voltage across the feed transistor. When the voltage across the feed transistor is high, the control transistor turns the feed transistor off and the resistive circuit supplies the low current to the R lead. <IMAGE> |
