Intrinsically safe current supply to telephone receiver - has transistors and resistors between one line carrying additional supply voltage and storage battery

摘要

The current supply is suitable for the transmit/receive equipment of a telecommunications, esp. telephone system situated in mines or other areas where explosions may occur. The terminals (1, 2) of a storage battery (3) are connected to a telephone line carrying an additional supply voltage. A first resistor (R3) in parallel with a series circuit consisting of a second resistor (R4) and the emitter-collector path of a transistor (T3) is inserted into the line between one terminal (1) and the storage battery. The resistance of the first resistor (R3) is high in comparison with that of the second (R4). A second (control) transistor (T1) is provided, whose base-emitter path is in parallel with a third resistor (r2) in series with the first resistor (R3).