Current limited subscriber interface circuit

An electronic interface circuit between a telephone subscriber line and a main exchange, comprising:

• first and second supply terminals adapted to be connected to the poles of a main exchange battery;
• first and second output terminals adapted to be connected to the wires of a telephone subscriber line;
• circuit means effective to provide, between the first supply terminal and the first output terminal, a first equivalent resistance, and between the second supply terminal and the second output terminal, a second equivalent resistance, said equivalent resistances having substantially the same predetermined values;
• detector means effective to detect the current flowing through the telephone line and extract a transverse component therefrom;
• means for generating a threshold current having a predetermined value;
• comparator means effective to compare the transverse component of the detected line current to the threshold current, and to produce a threshold signal where the two compared currents are equal to each other;
• and
• limiter means effective to limit the line current to the value of the threshold current;
• characterized in that
• the first equivalent resistance is connected directly to one of the exchange battery poles with the battery connected between the first and the second supply terminals;
• and
• the limiter means comprise a normally closed electronic switch connected serially to the second equivalent resistance and having a control terminal connected to the output of the comparator means, thereby said electronic switch is open whenever said threshold signal appears on its control terminal.

An electronic interface circuit connectable between a telephone subscriber line and a main exchange and having a threshold current with a predetermined value, the circuit comprising:

• first and second supply terminals adapted to be connected to poles of a main exchange battery:
• first and second output terminals adapted to be connected to wires of the telephone subscriber line:
• a first resistance circuit coupled between said first supply terminal and said first output terminal and having a first resistance value;
• a second resistance circuit having a substantially equal resistance value to said first resistance value and connectable between said second supply terminal and said second output terminal;
• a detector coupled to the subscriber lines and detecting a current flowing therethrough, said detector extracting a transverse component of said current from the subscriber lines;
• and
• a current limiter for limiting current, said current limiter using said first resistance and said second resistance when the value of said transverse component is less than the predetermined value of the threshold current, said current limiter using only said first resistance when the value of said transverse component is equal to the predetermined value of the threshold current causing the circuit to operate in a balanced configuration when the value of said transverse component is less than the predetermined value of the threshold current and in an unbalanced configuration when the value of said transverse component is equal to the predetermined value of the threshold current.