GB945145A - Improvements relating to circuits for protecting the luminescent coatings of cathoderay tubes - Google Patents

Abstract

945,145. Cathode-ray tube circuits. TELEFUNKEN G.m.b.H. Feb. 19, 1960 [Feb. 19, 1959 ; Oct. 30, 1959 (2)], No. 5883/60. Heading H4T. In a cathode-ray tube (C.R.T.) circuit comprising means for operating the C.R.T. to produce a trace on its luminescent screen and including a source of positive voltage for the wall anode of the C.R.T. and means for biasing the other C.R.T. electrodes, the biasing means including a network for biasing one control electrode relative to the cathode, the network is such that, when the circuit is switched-off so as to disable the operating means, two different transient voltages arise at two points of the network and so influence the decay of the bias voltage at the control electrode that such voltage increases momentarily in the sense to increase C.R.T. beam current, or at least decays at a slower rate than that corresponding to the decay of the voltage by which the biasing means is energized; the discharge of the wall anode voltage source is thereby facilitated. All the embodiments relate to television receivers. In one embodiment, Fig. 1, a negative voltage is produced by a diode 18 or by an existing source; when the receiver is switched off the voltage at control grid 10 falls relatively slowly, Fig. 2 (not shown), or if the negative voltage discharge current is made sufficiently high, a brief increase in control grid bias ensues, Fig. 3. In another embodiment, Fig. 5, a resistor 19 is provided to avoid sudden brightness variation resulting from mains voltage fluctuation; after switching-off the receiver the control grid voltage falls slowly as depicted in Fig. 4 (not shown). In a further embodiment, Fig. 6a, the control grid voltage after switching-off the receiver varies as the algebraic sum of curves A, B, Fig. 6b. In a still further embodiment, based on the circuit of Fig. 6a, point 20 is additionally connected to a diode held normally conductive to stabilize the picture brightness against supply voltage fluctuations, Fig. 7 (not shown). In yet another embodiment, Fig. 8, a diode 25 is normally non-conducting ; when the receiver is switchedoff voltage - U B collapses more rapidly than voltage +U B and the diode then conducts. The inclusion of a non-linear resistor 23 leads to a reversed sawtooth-like voltage at the control grid, Fig. 9 (not shown). In more embodiments a triode is utilized as a voltage-dependent resistance, Figs. 10, 11 (neither shown).