1. Field of the Invention
The present invention relates to a power and signal transmission system which makes signal transmission via lines and permits transmission of power by using lines such as those used in a public telephone network.
2. Description of the Related Art
In general, in exchanges, etc., power transmission is made via signal-transmitting lines. In a typical exchange, first and second communication devices are connected to each other via communication lines. A digital signal is applied to the input terminal of the transmitting circuit of the first communication device and is converted by a transmitting circuit, which is powered from a power source, to a signal having no direct current component. The converted signal is, in turn, transmitted on the lines via a signal-source resistor and a transformer. The receiving circuit of the second communication device receives the signal transmitted over the lines. In the first communication device, a high-voltage source delivers a high-voltage from the center tap of the secondary winding of a transformer, connected by a signal-short-circuiting capacitor, to the lines. The high voltage supplied to the lines is lowered by line resistance and then applied to a DC-to-DC conversion circuit in the second communication device. The conversion circuit converts the input high voltage to a power supply voltage required with a receiving circuit in the second communication device.
However, the use of transformers and choke coils for power transmission will increase the cost and make the system large because they are costly and large. In addition, there are disadvantages in that the assembly of the transformers and the choke coils takes a lot of time and labor, and the assembly cost also increases.