Patent GB-1184926-A (GB)
Title: Improvements in or relating to Protection Circuits for High-Tension Stages in Television Receivers
IPC: H04N3/18
Filed: 1967-04-13
Abstract: 1,184,926. Cathode-ray tube protection circuits. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 13 April, 1967 [16 April, 1966], No. 17019/67. Heading H4T. [Also in Division H2] In a television receiver in which the EHT is derived by rectification of the voltage across the secondary winding of a transformer the primary winding of which is supplied with a control voltage derived from the line deflection transformer, the production of EHT is inhibited by interrupting the control voltage when the EHT voltage exceeds a predetermined level. Thus in Fig. 1, pulses derived from a winding 5 on the line deflection transformer 2 (e.g. of a projection or a colour television receiver) are supplied to triode 7 the output of which, comprising the control voltage of approximately sawtooth shape, is fed to a pentode 12 having the primary winding of the EHT transformer 15 in its anode circuit. Variations in the EHT output due to any cause, e.g. due to variations in the supply voltage + V or due to beam current variations, are eliminated by effecting a compensating variation in the, negative, bias potential of pentode 12 by the operation of known circuit means including a triode 18 and capacitor-resistor combinations 21, 22 as described in Specification 1,152,458. Additionally, by utilizing the booster voltage across capacitor 3 in the line deflection circuit 1 as an additional control for triode 18 a fixed relationship between the EHT voltage and the focusing voltage provided by the line deflection circuit 1 is established. In accordance with the present invention if the EHT voltage exceeds a predetermined level the control supplied to the primary winding of transformer 15 is interrupted by increasing the negative bias on the control grid of pentode 12 such that the latter is cut-off. In the embodiment of Fig. 1 this is effected by a photo-sensitive resistor 34, connected between the control grid of pentode 12 and the control grid of triode 7 the potential of which, due to grid rectification, is highly negative, the resistance of resistor 34 falling from a high (dark) value to a very low value when illuminated by a gas filled tube 31 triggered by a potential proportional to a predetermined level of EHT and derived from potentiometer 30. When the tube 31 ignites on the occurrence of a voltage equal to or exceeding the predetermined level and the EHT is interrupted, resistors 32, 33 maintain a voltage across tube 31 greater than the extinction voltage (but less than the striking voltage) so that the operation of the EHT circuit continues to be inhibited until the apparatus is switched off and then switched-on again, the illuminated tube 31 serving as an indication that this action is necessary. In a second embodiment a thyristor 45 (Fig. 2) operates when triggered by a voltage proportional to a predetermined level of EHT (in this case derived via an additional winding 40 on transformer 15 and rectifying and smoothing elements 40 . . . 44) to connect the grid of pentode 12 to the grid of triode 7 and so bias 12 to cut-off and thus interrupt the EHT. When the latter occurs thyristor 45 continues to conduct by virtue of the connection of the gas (neon) tube 47 between the negative anode of 45 and earth, this tube also serving as an indication that the apparatus must be switched off and then on again to restore the EHT.