Cathode ray tube

Details Cathode ray tube Cathode ray tube

2000-05-15

2001-07-10

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

C315S383000, C315S085000

Reexamination Certificate

active

06259216

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a cathode ray tube having at least one electron gun including a cathode, a control grid and a focusing unit, and a strip made of electrically conductive material applied to the cathode ray tube in parallel to the outer coating of the cathode ray tube and connected to ground potential.
BACKGROUND INFORMATION
Undesirable high-voltage sparkovers can occur in cathode ray tubes of monitors; such sparkovers are caused by increased field intensities between the focusing unit and the anode, i.e., the inner coating of the tube or by impurities in the tube. Depending on the type of tube and the level of anodic voltage, very high currents may flow within very short times, permanently damaging the monitor electronics. Furthermore, these currents generate magnetic fields that induce voltages in the electronic components close to the tube. This creates the hazard of permanent damage to these components as well.
Circuit design measures can largely reduce the effect of a high-voltage sparkover. In this case, the amplitude of the current during sparkover is usually reduced using series resistors. These protective measures, however, limit the video band width of the monitor, with the result that high image resolutions, e.g., resolutions of up to 4000 lines for band widths of up to 600 MHz, are difficult to implement.
One cathode ray tube is described in German Utility Model 89 05 156. In order to reduce the damaging effects from high-voltage sparkovers, this German Utility Model describes a strip made of electrically conductive material and connected to ground potential be applied is parallel with the inner coating to the outer coating of the cathode ray tube. In the event of a high-voltage sparkover, which usually occurs between the anode and the focusing unit of the tube, the current flows in the tube via the focusing unit, the screen grid, the control grid, the cathode, and the cathode heater to ground, these elements, particularly the cathode and the outer protective circuit of the monitor electronics, being subject to damage.
SUMMARY
An object of the present invention is to provide a cathode ray tube in which the harmful effects of high-voltage sparkovers can be largely eliminated.
It is advantageous that, with the cathode ray tubes according to the present invention, monitors having a high video band width can be designed.
The current induced by a high-voltage sparkover flows away via the focusing unit, which, in “normal” monitor operation, works with a focusing voltage of approximately 7 kV, the additional grid, via the voltage limiting means or directly to the strip and to the outer coating. The high-voltage sparkover has no effect on the screen grid and control grid, on the cathode, the cathode heater and the monitor electronics.
In one embodiment of the present invention the strip is a copper or aluminum foil. This foil is applied directly to the glass surface of the tube. Thus, a connection is created that is parallel to the tube axis and forms a conductive loop having a very small current linkage surface, so that in the event of a high-voltage sparkover, the induced voltage is also small. In the event where the glass bulb of the tube is already provided with an outer coating, e.g., in the form of a graphite coating, the foil can be applied to this coating.
In another embodiment of the present invention, the voltage limiter is implemented using simple circuit design means and include a capacitor and a Zener diode connected in parallel thereto.
In another embodiment of the present invention the, the diameter of the additional grid is approximately equal to the inner diameter of the tube neck. Thus, the cathode and the control grid are properly shielded from the focusing unit, which is usually at a high potential.


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Yuzuru Kobori et al., “A novel arc-suppression technique for cathode ray tubes”,IEEE Transactions on Consumer Elektronics, vol. CE-26, Aug. 1980.
Patent Abstracts of Japan, vol. 009, No. 049 (E-300) Mar. 2, 1985 & JP 59 191241 A (Sanyo Denki KK, others: 02) Oct. 30, 1984.
Smith, “Protecting a cathode-ray tube during internal arcing”, IBM Technical Disclosure Bulletin, vol. 13, No. 12, May 1971.

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