| 摘要 |
PURPOSE:To prevent burning of a scanning matrix and the like, even if a continual failure current flows, by inserting a current protecting element showing a high resistance with a current being a prescribed value or over in series between the scanning matrix and a cut-off contact. CONSTITUTION:When a subscriber's telephone set 1 hooks off, a part of current flows in the path of ground-resistor 12-cut-off contact 18b-protecting circuit 3- subscriber's line 21b-telephone set 1-subscriber's line 21a-protecting circuit 3- cut-off contact 18a-current protecting element 24- resistor 14- resistor 15- battery, and the other part of the current flows separately in a drive circuit 10 or a reading circuit 11 via diodes 16, 17. In this case, if a high voltage 23 is impressed continually and a current being the operating current or below of the protecting circuit 3 flows continually, because of the contact of a power line on the subscriber's line 21a or induction, the resistance of the current protecting element 24 is increased to suppress the flowing current, allowing to prevent the burning of the resistors 14 and 15. |
