GB1164786A - Degaussing Circuit - Google Patents

Abstract

1,164,786. Cathode-ray tube circuits. STANDARD TELEPHONES & CABLES Ltd. 8 Feb., 1968, No. 6306/68. Heading H4T. [Also in Division H1] A degaussing circuit includes a degaussing coil connected in series with a bidirectionally conducting switch comprising at least one unidirectionally conducting device, the switch being set into its conducting condition to initiate a degaussing cycle. The invention is described in connection with the degaussing of a shadowmask colour television picture tube. In one embodiment the degaussing coil 6 (Fig. 1), which is suitably located on the tube, is connected in series with a switch 5 consisting of a thyristor 7 in parallel with an oppositely poled diode 8, the series arrangement being connected across a capacitor 4 which is charged via resistor 3 (30K) to a potential of about 30 volts derived from a suitable point in the television receiver. To initiate a degaussing cycle switch 5 is closed, by applying a suitable voltage to the gate of thyristor 7 so that coil 6 together with the associated resistance and capacitor 4 form an L.C. circuit producing a damped oscillatory wave the amplitude of which decreases to about 1% of the initial value after about 100 to 500 oscillations. In a second embodiment (Fig. 2) the circuit utilizes an A.C. supply, e.g. the mains supply to the receiver, connected at 11, 12 and the switch comprises a pair of parallel connected oppositely poled thyristors 14, 15. To initiate a degaussing cycle (by operation of switch 17) the gate 15 is supplied from a circuit 16 with a trigger pulse which is phase-time controlled such that the switch is turned-on at later-and-later instants in successive half-cycles of the A.C. current. Thus the current flowing in the degaussing coil 13 is progressively reduced to a minimum.