GB1225365A - - Google Patents

Abstract

1,225,365. Step wave generation; cathoderay tube circuits. TEXAS INSTRUMENTS Inc. 22 March, 1968 [31 March, 1967; 6 April, 1967], No. 14024/68. Headings H3T and H4T. A three-level stepped voltage is generated across a capacitive load by connecting the load to the output of an inductive circuit and feeding to the input a first pulse in a first direction, a second pulse in the opposite direction and then a third pulse in the first direction, the sum of the pulses in the first direction equalling the pulse in the opposite direction and the pulses being terminated by reversal of the input current due to resonance of the capacitive load with the inductive circuit. As shown, the capacitance is that of the screen of a cathode-ray tube connected to winding W2. Assuming the capacitance has been charged to a given level less than the supply voltage, rendering Q1 conductive connects the supply across the primary winding of W3. Due to resonance of the capacitance with the relatively small value leakage inductance, the secondary voltage rapidly swings in value until the primary voltage has risen above the supply voltage after which the current tends to reverse, and Q1 is thus extinguished. This level is held until Q2 is rendered conductive whereupon the voltage is resonantly reversed and Q2 is extinguished when the current attempts to reverse. The voltage is held until Q3 is rendered conductive and the voltage is again resonantly reversed to a value which, because of losses, is intermediate the other two. The sequence is now repeated, the first pulse restoring the losses. A Zener diode may be connected in series with Q3 to displace the value of the voltage reached on conduction of Q3 (Fig. 5, not shown), while in Fig. 1a (not shown) Q3, for the same purpose, is connected to an intermediate voltage developed across two capacitors (C4, C5), a shunt resistor (R3) being variable to control the capacitor current and thus the intermediate voltage value and the level of the voltage reached when Q3 conducts. The generator is described as applied to a colour television system in which the phosphors are in three layers evicted in accordance with the E.H.T. supply voltage applied to a final anode which includes a conductive layer on the phosphor layers. The stepped voltage is applied in series with an E.H.T. bias from 42 to provide the required stepped supply voltage. In order to compensate for the different deflections for the different electron velocities at the three steps the stepped voltage is applied in opposite sense to an intermediate conical anode layer 33.