GB1269388A - Beam current limiting arrangement - Google Patents

Abstract

1,269,388. Cathode-ray tube circuits. INTERNATIONAL STANDARD ELECTRIC CORP. 23 July, 1969 [31 July, 1968], No. 36987/69. Heading H4T. The beam current of a colour television cathode-ray tube is limited by deriving a D.C. signal when the beam current exceeds a predetermined value and supplying this D.C. signal to the tube via the video amplifier with the polarity appropriate to reducing the beam current. Thus, as shown in Fig. 1, a D.C. voltage produced across resistor 22 connected in series with the H.V. winding 20 of the line transformer renders a transistor 26 non- conducting when the voltage exceeds a value indicative of a normal beam current. Transistor 31 then becomes conductive and the D.C. voltage of its collector is then applied to the input of stage 5 of the video amplifier where it is added to the D.C. voltage produced by the D.C. restoration circuit consisting of the elements 11 to 17, the polarity being such that the output voltage from stage 8 which is supplied to one of the guns of the tube 18 ensures that the normal beam current is not exceeded. The onset of beam-current limitation may be adjusted by a D.C. voltage applied to terminal 28. In a second embodiment (Fig. 2), D.C. restoration is effected by means of a keyed rectification circuit comprising diode 11a and the elements 15, 16, 17, 45, 46, and 51 line flyback pulses being applied at terminal 52. When the beam current exceeds the normal value the collector current of transistor 37 is reduced, the cathode potential of diode 44 increases and the amplitude of the keying pulses (line fly-back pulses) increases to the same extent so that the D.C. voltage in the video amplifier increases thus limiting the beam current. The elements 48 and 50 are connected to respective video amplifiers associated with the other guns of tube 18.