GB441762A - Improvements in or relating to circuits for cathode-ray tubes - Google Patents

Abstract

441,762. Protective arrangements. RADIOAKT.-GES. D. S. LOEWE, 10, Wiesenweg, Steglitz, Berlin. July 19, 1934, No. 21085. Convention date, July 19, 1933. [Class 38 (iv)] [See also Group XL] In a television system employing a cathoderay tube and energized from the mains, a safety device is provided to ensure that if, after being switched off, current is switched on again while the cathode is still hot, the tube shall not be damaged by the occurrence of an excessive transient positive potentialdifference between the cathode and the Wehnelt cylinder. The current supply from the mains is rectified by a diode 1, Fig. 1, smoothed by a filter 2, 3, 4, and controlled by a switch 13. The negative bias of Wehnelt cylinder 7 is tapped off the smoothing-resistance 4 and were the connection 16, 21 permanent, a transient potential-difference calculated to cause the tube to be damaged, would be set up between the cylinder 7 and cathode 6 if the switch wiper 13 were closed while the cathode was hot. The additional switch wiper 16 is therefore provided and connects the cylinder 7 to the rectifier-anode 28 through contact 20 until the potential of the cathode 6 has reached its final negative value. The switches may be coupled as shown in Fig. 2, so that during clockwise movement of the wipers 13, 16, the cylinder 7 is connected to anode 28 through segment 20 as soon as wiper 13 is closed upon segment 19 ; and not till the end of the movement is the cylinder connected through segment 21 to the tapping on resistance 4. In modifications the switch 16 is omitted and (i) a high resistance may be temporarily inserted in the earth connection during the switching-on process ; or (ii) the tapping for the cylinder connection, instead of being taken off resistance 4, may be taken off an additional resistance connected between the anode 6 and terminal 30 of smoothing- condenser 3.