| 摘要 |
PURPOSE:To always execute optimum feeding without causing the inconvenience of heating in a single telephone set regardless of the length of distance between the single telephone set and a main device. CONSTITUTION:The feeding power of -48V is impressed on the cathode-side of a diode D1, and the feeding power of -24V, which is dropped by a DC/DC converter 114, is impressed on the cathode-side of a diode D2. When the single telephone set 2 is at a remote place, the line resistance of telephone lines 17 and 18 is high and voltage drop due to high resistance is large, voltage given to a single telephone interface circuit 111 becomes low, the diode D1 is turned on and the diode D2 is turned off. The power of -48V is supplied to the single telephone set 2. When the single telephone set 2 is at a nearby place, the voltage drop of the telephone lines 17 and 18 becomes small. Thus, voltage given to the single telephone interface circuit 111 becomes high, the diode D2 is turned on, the diode D1 is turned off and the power of -24V is supplied to the single telephone set 2. |
