PURPOSE: To reduce the cost of installation for a doorphone, a camera, etc., by inserting a constant current regulator having a high output resistance between a high impedance regulator connected to the supply terminal of an interphone cable and a DC power supply and using the constant current source so as to supply power to the high impedance regulator.
CONSTITUTION: The power supply section consists of a DC power supply 18, the constant current regulator 19 and a high output impedance regulator 20. A DC output resistance of the constant current regulator 19 of a master equipment 10 is sufficiently larger than the DC output/input resistance of the impedance regulators 20, 37 connected respectively to the power supply and power receiving ends of the interphone cable 40. Thus, the DC voltage drop by the cable 40 is imposed almost on the input/output voltage of the regulator 19. As a result, the sum of the power loss of the cable 40 and the power loss of the regulator 19 is always nearly constant, and the operating point of the high output/input impedance regulators 20, 37 is kept nearly to an optimum value regardless of the length of the cable 40.
SUGA MITSUHIRO
HENMI TADASHI
