Protective circuit for thyristor-switched couplings stages - uses closed loop operational amplifier earthing speech current fluctuations

摘要

In solid state switching of communication paths, thyristors are often used. Once switched on by a selector pulse the thyristor passes a current to act as a low resistance coupling link. Not infrequently spurious disturbances occur on the line which draw a current which opposes the maintaining current of the thyristor. The thyristor loses its conductivity and the connection already established breaks down at this point. To forestall this, diodes (D1 to D8) are provided in serial pairs and placed across the transformer windings (W2,W3) limiting peak to peak voltages to 1.4 volts. Current limitation is achieved by a variable ground resistance circuit consisting of an operational amplifier (OV) having a resistor (R) in the return loop from output to inverting input. For speech current fluctuations, this circuit constitutes a plain ground connection. The effective resistance for larger currents depends on the loop resistor (R).