Patent GB-932616-A (GB)
Title: Improvements in or relating to circuit arrangements for generating sawtooth currents
IPC: H04N3/18
Filed: 1959-12-24
Abstract: 932,616. Automatic voltage control systems. PHILIPS ELECTRICAL INDUSTRIES Ltd. Dec. 24, 1959 [Dec. 30, 1958], No. 43884/59. Class 38 (4). [Also in Group XL (c)] In a combined sawtooth current and high voltage generator comprising an amplifier and an efficiency diode, the high voltage is stabilized by variation of the slope and/or flattening of the peak of the sawtooth waveform fed to the amplifier in conjunction with variation of the bias applied to the amplifier input so that the peak amplifier current varies only sufficiently to cut off the efficiency diode at the end of the sweep, whilst the mean amplifier current remains substantially the same as the mean diode current. Fig. 3 shows a line time-base circuit for a television receiver. The deflection coils 7 are coupled via an autotransformer 4 to the amplifier 2 fed with waveform 6. The H.T. supply to valve 2 is augmented by the efficiency diode 5. The flyback pulses are rectified by valve 8 to provide an E.H.T. supply to feed a load 10 simulating a cathode-ray tube. Whenever the E.H.T. voltage tends to rise because of a reduction in the cathode-ray tube beam current, the voltage across the voltage dependent resistor 19 becomes more negative as explained in Specification 890,805, reducing the mean anode current of valve 2. The cathode voltage of valve 2 (smoothed by capacitor 27) falls, increasing the impedance of valve 15 so that capacitor 13 charges less rapidly, reducing the slope of waveform 6 and correcting the E.H.T. voltage. Valve 15 and resistor 14 may be replaced by a voltage-dependent resistor fed with a voltage derived by rectifying the anode voltage of diode 8 via a capacitive divider. A graphical explanation is given. Fig. 6 shows an alternative circuit in which the brightness control 37 is mechanically coupled to a further potentiometer 36. The voltage at the slider of potentiometer 36 together with the mean video signal voltage at the anode of valve 34 is fed to diode 32 to control the level at which waveform 6 is clipped. It is arranged that when the anode voltage of valve 34 corresponds to black level and potentiometers 36, 37 are at the extreme left-hand settings, the voltage fed to diode 32 corresponds to the peak value of waveform 6 so that no clipping occurs. If the E.H.T. voltage tends to fall because of a reduction in the mean video voltage at the anode of valve 34 or due to a variation in the brightness control setting, the voltage at the cathode of diode 32 will fall hence the upper part of waveform 6 will be clipped. The bias on valve 2 will also become less negative because of the action of the voltagedependent resistor 19 as in Fig. 3, both these effects tending to stabilize the E.H.T. voltage. Modifications.-The peak to peak value of sweep current in coils 7 may be allowed to decrease when the E.H.T. voltage falls, so that the raster size is maintained. Capacitor 18 may be omitted in specified circumstances. Combinations of the circuits shown in Figs. 3, 6 may be employed. Valve 2 may be replaced by a transistor fed with a current waveform similar to waveform 6.