Electric lamp and discharge devices: systems – Cathode ray tube circuits – Cathode-ray deflections circuits
Patent
1986-02-06
1988-02-09
Blum, Theodore M.
Electric lamp and discharge devices: systems
Cathode ray tube circuits
Cathode-ray deflections circuits
358169, H01J 2952
Patent
active
047243643
DESCRIPTION:
BRIEF SUMMARY
This invention relates to the control of cathode ray tubes (CRT's) and particularly, though not exclusively, to the control of CRT's in colour television receivers.
In use of CRT's and particularly colour CRT's the voltages and currents required to be applied to a CRT change both in the course of a single session of use (e.g. as the CRT "warms up") and during the life of the CRT as it ages. Thus it is desirable to monitor these signals while the CRT is in operation and adjust signals accordingly.
It is known to use such automatically adjusting loops to set the level of the tube cathode current at which the tube is nominally considered to be cut-off. However, such known loops may result in impractical cathode voltages.
It is an object of this invention to provide a circuit for controlling a cathode ray tube wherein the above-mentioned disadvantage may be overcome or at least alleviated.
In accordance with the invention, a circuit for controlling a cathode ray tube comprises: nominal cut-off level of the tube cathode current accordingly; and adjustment and to a grid of the tube so as to bring the nominal cut-off level control loop within its range of adjustment.
Preferably the changing means is arranged to change the voltage applied to the G.sub.2 grid of the tube.
Preferably the changing means comprises a shunt feedback regulator connected in parallel with the voltage applied to the G.sub.2 grid.
The shunt feedback regulator may be a high voltage transistor with resistive shunt feedback or preferably a low voltage transistor with a cascode high high voltage thyristor or triac.
One control cirucit in accordance with the invention for controlling a cathode ray tube will now be described, by way of example only, with reference to the accompanying drawing, which is a schematic block diagram of the circuit.
Referring now to the drawing, the control circuit includes a differential amplifier comparator arrangement 2 for operatively monitoring the cathode voltage V.sub.K of the cathode ray tube and for determining during the flyback period whether the cathode voltage is too low or too high.
The arrangement 2 provides two outputs respectively indicating that the cathode voltage is too high or too low with respect to predetermined reference voltages V.sub.REF1 and V.sub.REF2. The two outputs of the monitoring arrangement 2 are connected to an up/down counter 4 to increment the counter up or down respectively. The count of the counter 4 is applied in parallel as a multi-bit word to a digital-to-analog converter 6. The D/A converter 6 provides at its output a voltage which is representative of the required nominal cut-off level of the tube cathode current. The output of the D/A converter 6 is applied to tube cathode current generating circuitry (not shown).
The output of the D/A converter 6 is also applied to a differential amplifier camparator arrangement 7. The comparator arrangement 7 detects when the output of the D/A converter 6 is outside of a predetermined range with respect to predetermined reference voltages V.sub.REF3 and V.sub.REF4. This predetermined range is chosen so as to include substantially all of the range of adjustment provided by the up/down counter 4.
The comparator arrangement 7 provides two outputs respectively indicating that the present level of the cathode voltage V.sub.K is above or below the range of adjustment provided by the up/down counter 4. The two outputs of the comparator arrangement 7 are connected to an up/down counter 8 to increment the counter up and down respectively. The count of the counter 8 is applied as a multi-bit word to a digital-to-analogue converter 9. The output of the digital-to-analog converter 9 is connected to the control input of a shunt feedback regulator 24. The shunt feedback regulator 24 includes a low voltage transistor 10. The base of transistor 10 is also connected via a current source S to ground. The emitter of transistor 10 is connected to ground and the collector of transistor 10 is connected via a thyristor 12 and resistors 14 and 16 to a source of hi
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patent: 3598913 (1971-10-01), Janssen
patent: 3898381 (1975-08-01), Amsen et al.
patent: 4126884 (1978-11-01), Shanley, II
patent: 4277798 (1981-07-01), Hinn
patent: 4318034 (1982-03-01), Nakamura
patent: 4344021 (1982-10-01), Johnston
patent: 4461983 (1984-07-01), Lees
Bingham Michael D.
Blum Theodore M.
Cain David
Motorola Inc.
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