Two-level telephone current supply - has high and low parallel voltage supply routes, with lower isolated by forward biassed diode

摘要

The circuit supplies current over several kilometres of line to a subscriber phone (Pa) for operation and charging of a local buttery (BL), and avoids the problem of passing charging current through the relay (Re1) which signals the line state to the exchange. The circuit comprises two parallel power supply routes. The first comprising a high constant voltage supply (V2) in series with a high resistance (R2). The second supply has a low constant voltage supply (V1) in series with a low resistance (R1) and the relay (Re1) and followed by a diode (D1) biassed for current flow from the low voltage supply along the phone line in the same sense as current flow from the high voltage supply. The high voltage source can be formed by addition of a voltage sour source in series with the low voltage source. The positioning of the relay allows its actuation only when a high current consumption indicates telephone use.