GB1409580A - Cathode-ray tube driving circuit - Google Patents

Abstract

1409580 D.C.-D.C. converters MATSUSHITA ELECTRIC INDUSTRIAL CO Ltd 19 Dec 1972 [23 Dec 1971 24 Dec 1971 (3)] 58664/72 Heading H2F [Also in Division H4] In a cathode-ray tube system in which the E.H.T. voltage is derived by rectifying fly-back pulses and a focusing voltage is derived via a potential divider from the full E.H.T. or, when a voltage multiplier rectifier is employed, from an intermediate tap on the latter a desired ratio between the focusing voltage and the E.H.T. voltage is maintained, despite variations in the beam current of the tube, by producing a voltage the magnitude of which varies with variation in the beam current and combining this voltage with the voltage across the potential divider to produce a focusing voltage the ratio of which with respect to the E.H.T. voltage decreases as the beam current increases. Fig. 3 curve b shows this characteristic in comparison with the characteristic, curve a, of a prior cut system which is described with reference to Figs. 1, 2a and 2b (none shown). Fig. 4 shows a first arrangement in which the voltage developed across resistor 13 and which varies with variation in beam current Ib is combined with the voltage at terminal F of a three-stage multiplier rectifier 8 (supplied with fly-back pulses from fly-back transformer 7) via a potential divider network comprising resistors 10, 11, and 12 to provide the required focusing voltage VF<SP>1</SP>. In operation, when the beam current increases, the E.H.T. voltage VA and the voltage at terminal F decrease whilst the voltage across resistor 13 increases. Thus when the latter voltage is combined via the resistor chain 10, 11, 12 and by suitable adjustment of resistor 12 the rate of decrease in the voltage VF<SP>1</SP> may be made greater than the rate of decrease in VA so that the ratio VF<SP>1</SP>/VA decreases with increasing beam current. Variations of this circuit in which various ones of the capacitors in the multiplier rectifier 8 are removed are described with reference to Figs. 6 to 9 (none shown), and a further variation in which the resistor chain 10, 11, 12 is replaced by a similar resistor chain connected to terminal A is described with reference to Fig. 10 (not shown). Fig. 11 shows a second arrangement in which the voltage which varies with variation in beam current is derived by means of a rectifier 21 (such voltage being also usable as the screen voltage for the tube) and variations of this embodiment in which a two-stage multiplier and a single rectifier stage are employed are described with reference to Figs. 13 and 14 (neither shown). Fig. 12 shows a third arrangement in which the current dependent voltage is produced by means of a tertiary winding 7a and Fig. 15 shows a fourth arrangement in which such voltage is produced across a resistor 31 in series with the secondary winding of transformer 7. A variation of this latter arrangement in which the multiplier-rectifier 8 is replaced by a single rectifier stage is described with reference to Fig. 16 (not shown).