Abstract: An improved, D.C. shut-off control circuit for a thyristor is provided by means of a transformer having a tapped primary which is connected between a D.C. supply and the thyristor and which has two secondary windings: one is used during shut off to generate an opposing magnetic field in the transformer core which cancels the magnetic field generated by the flow of current through the primary winding to the thyristor. When current to the first secondary winding is shut off, this secondary magnetic field collapses and causes a reverse current in the primary which effectively blocks current to the thyristor and simultaneously generates a voltage in the second secondary winding of the transformer which biases a transistor switch to connect the primary tap to the circuit ground, thereby using the magnetically induced voltage in the primary to reverse bias the thyristor and positively shut it off.