Patent GB-693457-A (GB)
Title: Improvements in or relating to electric circuits for use with cathode-ray tubes and for like purposes
IPC: H04N3/185
Filed: 1950-12-28
Abstract: 693,457. Automatic voltage control systems for rectifiers. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Dec. 28,1950 [Dec. 31,1949], No. 31485/50. Classes 38 (ii) and 38 (iv). [Also in Group XL (c)] A circuit arrangement for producing a sawtooth current in a deflection coil system of a cathode-ray tube in which the voltage pulse appearing across the deflection coil system during the fly-back of the saw-tooth current is rectified to produce a direct voltage, is characterized in that during the fly-back period, but before the rectifiers are cut off, the voltage pulse is damped by a load which is variable in a sense opposite to the damping produced by the cathode-ray tube load. The high direct voltage across the load 38-representing the variable load presented by the beam current of a cathode-ray tube of a television receiver-and the amplitude and/or the linearity of the sawtooth deflection current are arranged to be substantially constant and unaffected by the variations in the load 38. This is effected by causing the damping exhibited by the circuit associated by the valve 33 to be variable in a sense opposite to the damping exhibited by the load 38. A control voltage, dependent upon the load 38, is taken from a condenser 39 and, together with a positive bias voltage through resistor 43, is fed to the anode 44 of the diode portion of a valve 31. A capacitor 45 is also placed in this anode circuit. If no control voltage from the capacitor 39 is present and if the diode is not conductive, the capacitor 45 becomes charged over a circuit comprising resistor 43 and inductance 46. When the control voltage 30 is operative on the control grid of the valve 31 the potential of the cathode thereof increases during the forward stroke of the control voltage 30 and the diode remains unconductive. During the fly-back of the control voltage 30 the potential of the cathode drops until it reaches the potential of the anode 44 when the diode becomes conductive and the capacitor 45 discharges through cathode resistor 32 and the inductance 46. The pulse thus produced in the resistor 32 is taken to the control grid of the valve 33 to cause that valve to conduct temporarily during the fly-back period but before the rectifier 36 has cut off, thereby producing the desired damping of the oscillation in the deflection coil system. The instant at which the diode conducts is controlled by the voltage fed to the anode 44 from the condenser 39. Voltage variation across the load 38 may also be caused by losses in the rectifier system and this is obviated by taking a proportion of the pulse produced in the resistor 32 and feeding it to the control grid of a valve 47 which, together with valve 49, causes a phase shift of the pulse which is then fed to the control grid of valve 33 so that the valve 33 becomes conductive a second time during the fly-back period, this time, however, after the rectifier has been cut off. In other embodiments is shown the use of an " efficiency diode " in conjunction with these circuits.