| 摘要 |
PURPOSE:To decrease a terminal voltage and to reduce the number of parts and cost by using the LED of a photocoupler for detecting service current as a constant-voltage source in a constant-voltage circuit and providing a circuit having the function of detecting constant current and service current. CONSTITUTION:When DC voltage is being applied from trunk lines 1, 2, polarity is determined by a diode bridge consisting of diodes 3-6, and applied to a constant current circuit constituted of an NPN transistor 8, resistors 7, 9 and a photo-coupler 10. Voltage VREF between the base of the NPN transistor 8 and negative pole is the forward voltage of an LED of the photo-coupler 10 and the fluctuation of voltage due to the change of current value is small. Accordingly, the constant-current circuit has sufficiently a large impedance to a calling signal applied to the trunk lines 1, 2. Therefore, there is no loss between trunk lines 1, 2 and the calling signal terminals 14, 15 and the calling signal is transmitted and the condition of the DC terminating circuit is satisfied.
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