Solid state switching circuit and method for simulating a relay

摘要

A switching control signal switches a pair of emitter coupled transistors via a drive circuit jointly connected to the base electrodes of the transistors so as to drive the base-to-emitter junctions between an "on" condition in which both base-to-emitter junctions are forward biased and an "off" condition in which both base-to-emitter junctions are reverse biased. In the "on" condition, the collectors of the transistors simulate closed relay contacts capable of conducting substantial current flow and in the "off" condition the collectors simulate open relay contacts capable of accomodating substantial standoff voltages. The drive circuit which controls the "on" and "off" conditions of the transistors includes separate turn "on" and turn "off" circuit paths for producing fast and uniform switching characteristics at the collectors. An electro-optical isolator circuit couples the source of the switching control signal to the drive circuit so as to maintain isolation between the switching control signal and the current or voltage switched by the transistors. Overload protection circuitry automatically turns the transistors "off" in the event of an overload condition and a Zener diode bridge network connected across the collectors of the transistor pair serves to protect the transistors from excessive voltages inadvertently applied to the collectors.