Direct access coupler

A direct access coupler capable of withstanding lightning surge transients comprising:

• isolation transformer means including a primary winding having a first end and a second end, and a secondary winding;
• first capacitor means including a first end which is connected to said first end of said primary winding and a second end;
• current source means including
• an input lead connected to said second end of said first capacitor means,
• an output lead connected to said second end of said primary winding,
• second capacitor means having a first end connected to said output lead and a second end,
• first resistor means having a first end connected to said second end of said second capacitor means and a second end connected to said input lead, said first resistor for developing a bias current,
• second resistor means having a first end connected to said input lead, and a second end,
• first transistor means having a base connected to said first end of said first resistor for receiving at least a portion of said bias current, a collector coupled to said second end of said second resistor means, and an emitter for developing an amplified bias current,
• third resistor means having a first end connected to said output lead and a second end,
• fourth resistor means having a first end connected to said input lead, and a second end,
• fifth resistor means having a first end connected to the emitter of said first transistor means, and a second end,
• second transistor means having a base coupled to said second end of said fifth resistor means, a collector coupled to said second end of said fourth resistor means, and an emitter connected to said second end of said third resistor means, said second transistor means for causing said off-hook current to flow through said third resistor means,
• sixth resistor means having a first end connected to the emitter of said second transistor means, and a second end,
• diode means having an anode connected to said emitter of said first transistor means, and a cathode,
• third transistor means having a base coupled to said second end of said sixth resistor means, an emitter connected to said output lead and a collector connected to said cathode and to said first end of said first resistor means, said third transistor means being responsive to the level of the potential developed across said third resistor means by said off-hook current and operative to divert a portion of said bias current to maintain said off-hook current at a predetermined magnitude, and
• third capacitor means having a first end connected to said output lead, and a second end connected to the base of said second transistor means,
• said second and fifth resistor means and said diode means being operative to provide transient energy protection for said first transistor means, said second and third capacitor means being operative to prevent transient energy from driving said second transistor means into saturation, said fourth resistor means being operative to limit said off-hook current, and said sixth resistor means being operative to limit the base current to said third transistor means to a safe level;
• a first terminal and a second terminal for connection across a telephone line pair;
• first circuit means for coupling signals between said first terminal and said output lead of said current source means;
• and
• second circuit means for coupling signals between said second terminal and said input lead of said current source means, said second circuit means including switching means having an on-hook state wherein said switching means is nonconductive and an off-hook state wherein said switching means is conductive, said switching means being responsive to a control signal and operative to change from said on-hook state to said off-hook state,
• whereby an off-hook current of said predetermined magnitude which is voltage independent and of a value at least large enough to apprise the telephone company of the off-hook state of said switching means is conducted through said current source means, whereby AC signals are coupled between said first and second terminals and said primary winding, and whereby DC signals are blocked from flowing into said primary winding by said first capacitor means.