Abstract: An overvoltage protection circuit for directing a surge current on an input/output line, in particular, a telephone line, to a ground terminal when the surge current exceeds a predetermined level.

Abstract: A solid state insertion circuit comprising an opto-coupler triac driver is activated by a solid state switch to insert a ringing signal onto a subscriber telephone path in order to ring, for instance, a telephone. The opto-coupler driver functions as a zero voltage crossing triac to apply the ringing signal onto the telephone line only at a zero voltage cross-over thereof to thereby inhibit noise transients from being generated.

Abstract: A high impedance open circuit type of overvoltage protection circuit useful in conjunction with a SLIC is provided. Diodes are used to prevent current flow back into the SLIC. In addition, voltage-sensitive clamps are provided to clamp the SLIC current source and SLIC current sink into nonconductive states in the presence of an overvoltage condition.

Abstract: A programmable signal generator and method for generating frequencies that are subharmonics of a rectified 60 hertz input signal. Opto-coupled triacs systematically switch and rectify the input signal frequency to provide, for example, a ringing signal for a telephone system.

Abstract: Field effect transistors are used in a series current limiter circuit. The field effect transistors are depletion type transistors and are interconnected in a manner to decrease conduction when an overvoltage or overcurrent condition is sensed. Biasing resistors are used to maintain the transistors in a non-conductive state until the overvoltage or overcurrent condition is removed.

Abstract: A pair of PNPN thyristors are used to provide a solid state fuze for circuit protection. One of the pair of thyristors is self-biased on and provides normal current flow. The other thyristor turns the first thyristor off during a surge condition. A current sensing resistor is placed in series with the first thyristor to provide bias to the second transistor. The circuit will automatically reset itself when the surge condition is no longer present.

Abstract: A circuit for use with telephony systems or the like for protecting these systems, which may be comprised of integrated circuits, from high transient voltages. The circuit includes a silicon controller rectifier (SCR) having the cathode and anode coupled between an input of the telephony system and earth potential. The gate of the SCR is coupled via a resistor to the cathode thereof and is also returned through a reversed-biased diode to a source of bias potential whereby negative voltage transients are shorted to earth potential such that the integrated circuits as well as the bias potential source are not damaged.

Abstract: A loop sensing circuit is coupled between a solid state hybrid circuit that provides signal conversion between a pair of unidirectional transmission paths and a bidirectional subscriber loop. The loop sensing circuit includes placing respective protection resistors coupled in parallel with first and second sensing resistors between the hybrid and the subscriber loop terminals to provide a dc current path between the bidirectional subscriber loop and the hybrid circuit.

Abstract: The maximum current flow in a circuit is limited to a predetermined maximum value by dividing the load current into low and high current paths with the use of interconnected current mirror circuits. By limiting the current flow in the low current path by means of a low current constant current source, the maximum value of a much larger current flow through the load is limited to a predetermined value.