| 摘要 |
PROBLEM TO BE SOLVED: To power up an electronic circuit from a telephone line without generating a lock-up state by applying the electric power of telephone lines to the electronic circuit according to the characteristics of the detected voltage between the telephone lines. SOLUTION: After the voltage level from a telephone office 16 rises to a voltage which is high enough for the accurate operation of an on-hook electronic circuit 18, the electric power of the telephone lines is only connected to the electronic circuit 18 to solve the trouble generated by the dynamic resistance and terminating resistance R1 of the electronic circuit 18. Concretely, a voltage detector 12 controls a switch 14. The buildup trip point of the voltage detector 12 is set to a point which guarantees a voltage VDD which is high enough for the accurate operation of the on-hook electronic circuit 18. When the voltage detector 12 trips, a reset signal turns on the switch 14 and the electric power of the telephone lines is supplied to the electronic circuit 18 at the voltage level guaranteeing the accurate operation of the on-hook electric circuit. |
