GB2264587A

GB2264587A - Cathode ray display tube.

Info

Publication number: GB2264587A
Application number: GB9203771A
Authority: GB
Inventors: Terence Keith Gibbs; Graham Luck; David John Eagle; Anthony John Bateman; Tony Eric Lanham
Original assignee: International Business Machines Corp
Current assignee: International Business Machines Corp
Priority date: 1992-02-21
Filing date: 1992-02-21
Publication date: 1993-09-01
Also published as: JPH0644918A; KR930018640A; GB9203771D0

Abstract

In a cathode ray display tube the front of the display screen (20) is shielded from the electric fields generated by the electromagnetic deflection coils (40) by an electrically conductive extension (60') of the usually provided exterior conductive coating 60 on the tube funnel, the extension coating (60') being disposed between the deflection coils (40) and the cathode ray tube envelope (10). Preferably a frustoconical insulating insert 90 electrically insulates the deflection coils from the coating extension 60'. Alternatively, Fig. 3 (not shown), the coating 60 is effectively extended by a conductive coating (62), on the inner face of a flexible plastics frustoconical insert (61), in electrical contact with and secured to the coating 60. <IMAGE>

Description

CATHODE RAY DISPLAY TUBE The present invention relates to a cathode ray display tube (CRT) adapted to reduce electric field emissions.

A conventional CRT, for a computer visual display unit or a television receiver, comprises a bell shaped glass envelope closed at its widest end by a display screen, and carrying at its narrowest end, remote from the screen, an electron gun assembly. Deflection coils are disposed around the envelope towards its narrowest end. The screen is internally coated with a electrically conductive layer, or "final anode." The outer surface of the envelope, between the screen and the coils is clad with a electrically conductive coating known as

or simply "dag' The dag terminates short of the coils leaving an annular gap between the dag and the coils.

In operation, electrons are accelerated in a beam extending from the electron gun assembly to the screen by an Extra High Tension (EHT) voltage introduced between the electron gun assembly and the final anode. The dag is connected to ground to form a capacitor with the final anode on the screen. The capacitor smoothes the EILT voltage. The dag also forms a protective ground plane offering an immediate path to ground to arcing or "flashover currents" from the final anode.

Magnetic fields produced by deflection currents flowing in the coils move the beam across the screen. In a raster scanned display, such as a computer visual display unit or television receiver, the currents flowing in the coils have sawtooth waveforms. Because the voltages across the coils are dependent upon the rate of change of the currents in the coils, the sawtooth CUrre.lltS produce high amplitude voltage pulses across the coils. The width of the annular gap is selected to prevent arcing between the coils and the dag. The currents in the coils also generate electric fields that can extend from the plane of the coils towards and beyond the screen.

In accordance with the present invention, there is provided a cathode ray display tube in which the front of the display screen is shielded from the electric fields generated by the electromagnetic deflection by an electrically conductive extension of the aquadag, the extension being disposed between the deflection coils and the cathode ray tube envelope.

The present invention stems from a realis at ion that the electric fields extending in front of the screen of a CRT from the deflections coils can be significantly reduced, at advantageously negligible expense, by extending the dag coating until it interposes between the coils and the envelope. The extension of the dag acts as a barrier positioned between the coils and screen that attenuates the electric fields to an acceptable level.

Viewing the present invention from another aspect, there is provided a cathode ray display tube comprising: a display screen; a bell shaped envelope closed by the display screen; a conductive coating on an outer surface of the envelope; beam generating means within the envelope for generating a charged particle beam directed at the display screen; a plurality of electromagnetic deflection coils disposed around the envelope for deflecting the beam across the screen and for generating an electric field extending in front of the screen; characterised in that the conductive coating on the outer surface of the envelope extends beneath, and is electrically insulated from, the coils to reduce the electric field extending in front of the screen.

In the absence of a gap between the coils and dag, arcing between the coils and the dag can be simply, and therefore advantageously, prevented by interposing a layer of electrically non conductive material such as plastic material between the extension of the dag and the coils.

The insulating layer may be an frustoconical insert of flexible plastics material. The layer may thus be advantageously inexpensive to manufacture. Furthermore, the layer can be disposed around the envelope to conceal the dag extension from the coils before the coils are positioned on the CRT in an additional, but advantageously trivial, process step in the manufacture of the CRT.

The dag extension may be provided simply, and thus advantageously cheaply, by extending the area of the envelope to be covered by the dag material during the dag application step in the manufacture of the CRT.

The dag extension may alternatively be provided by a frustoconical insert having: an inner surface coated with an electrically conductive material connecting to, at its flared end, and thus effectively extending, the dag; and an outer surface coated with an electrically non-conductive for electrically insulating the dag from the coils. This advantageously permits existing CRTs to be modified in accordance with the present invention through retrofitting of such an insert.

It will therefore be appreciated that the present invention extends to a frustoconical insert, for interposing between deflection coils and a bell shaped envelope of a CRT, the insert comprising an inner surface coated with an electrically conductive material; an outer surface coated with an electrically non-conductive material; and means for electrically connecting the inner surface to a conductive coating on the outer surface of the envelope.

It will further be appreciated that the present invention extends to a display device, such as a computer visual display unit, television receiver or the like, comprising a cathode ray display tube having: a display screen; a bell shaped envelope closed by the display screen; a conductive coating on an outer surface of the envelope; beam generating means within the envelope for generating a charged particle beam directed at the display screen; a plurality of electromagnetic deflection coils disposed around the envelope for deflecting the beam across the screen and for generating an electric field extending in front of the screen; characterised in that the conductive coating on the outer surface of the envelope extends beneath, and is electrically insulated from, the coils to reduce the electric field extending in front of the screen.

Preferred embodiments of the present invention will now be described with reference to the accompanying drawings in which: Figure 1 is a cross sectional view of a CRT of the prior art; Figure 2 is a cross sectional view of a CRT of the present invention; Figure 3 is cross sectional view of another CRT of the present invention.

Referring first to Figure 1, a conventional CRT comprises a bell shaped glass envelope 10 closed at its widest end by a display screen 20, and carrying at its narrowest end, remote from the screen, an electron gun assembly 30. Horizontal and vertical electron beam deflection coils 40 are disposed around the envelope towards its narrowest end. The deflection coils 40 are wound onto a toroidal bobbin or "yoke" 70. The screen 10 is internally coated with a phosphor layer 50 and a conductive layer (not shown) in superimposition. The conductive conventionally formed of indium tin oxide and is generally known as the "final anode." The outer surface of the envelope, between the screen 20 and the deflection coils 50 is clad with a conductive coating 60 known as "aquadag" or simply "dag". Conventionally, the dag 60 terminates short of the deflection coils 40 to leave an annular gap 80.

In operation, electrons are accelerated in a beam extending from the electron gun assembly to the phosphor layer 50 by an Extra High Tension (EHT) voltage introduced between the electron gun assembly and the final anode on the screen 20. The screen 20 is illuminated by electrons impinging on the phosphor layer 50. The dag 60 is connected to ground to form a capacitor with the final anode on the screen 20. The capacitor smoothes the extra high tension voltage. The dag 60 also forms a protective ground plane that offers an immediate path to ground for potentially hazardous arcing or "flashover" currents from the final anode. Magnetic fields produced by deflection currents flowing in the deflection coils move the beam across the phosphor layer 50. In a raster scanned display device, the deflection currents flowing in the deflection coils have sawtooth waveforms.Because the voltages across the deflection coils is dependent upon the rate of change of the currents in the deflection coils, the sawtooth deflection currents produce high amplitude voltage pulses across the deflection coils. For example, the voltage pulses across the horizontal deflect6ion coils are typically lkV peak amplitude. The width of the annular gap 80 is therefore selected to prevent arcing between the deflection coils 40 and the dag 60. The deflection currents in the deflection coils also generate an electric field E that extend from the deflection coils towards and beyond the screen.

Referring now to Figure 2, in an example of cathode ray display tube of the present invention, the annular gap 80 is removed and the dag is extended at 60' to terminate beneath the coils 40. The extended portion of the dag 60' may be formed by applying an electrically conductive compound such as nickel based paint to the outer surface of the envelope. A frustoconical insert 90 of electrically non conductive material is interposed between the coils 40 and the extended portion of the dag 60'.

In operation, the extended portion of the dag 60' acts as a grounded shield that substantially attenuates the electric field E radiating from the coils 40 towards the screen 20. The insert 90 electrically insulates the deflection coils from the extended portion of the dag 60' to prevent arcing between the coils and ground via the dag 60.

Referring now to Figure 3, in another example of the present invention, the dag 60 effectively extended beneath the coils 40 by bridging the annular gap 80 with an electrically conductive frustoconical insert 61. The insert 61 has an electrically conductive inner face 62 and an electrically non conductive outer face 63. The inner face 62 is secured and electrically connected to the dag 60 along annulus 64. The insert 61 may be formed from a sheet of flexible plastics material having one side coated with an electrically conductive compound such a nickel based paint. The insert 61 may be secured and electrically connected to the dag along annulus 64 by an electrically conductive adhesive, press stud, or the like.

In operation, the inner face 62 acts as a grounded shield that substantially attenuates the electric field E radiating from the coils 40 towards the screen 20. The outer face 63 electrically insulates the coils 40 from the inner face 62 connected to the dag 60 to prevent arcing between the coils 40 and the dag 60.

Claims

1. A cathode ray display tube in which the front of the display screen is shielded from the electric fields generated by the electromagnetic deflection coils by an electrically conductive extension of the aquadag, the aquadag extension being disposed between the deflection coils and the cathode ray tube envelope.

2. A cathode ray display tube as claimed in claim 1 comprising a layer of electrically non-conductive material interposed between the aquadag extension and the coils.

3. A cathode ray display tube as claimed in claim 2 wherein the non-conductive layer comprises a frustoconical insert of flexible sheet material.

4. A cathode ray display tube as claimed in claim 2 wherein the aquadag extension comprises a frustoconical insert having: an inner surface coated with an electrically conductive material connecting to, at its flared end, and thus effectively extending, the aquadag; and an outer surface coated with an electrically non-conductive material for electrically insulating the aquadag extension from the coils.

5. A cathode ray display tube substantially as hereinbefore described with reference to Figures 2 or 3 of the accompanying drawings.

6. A visual display device comprising a cathode ray tube as claimed in any preceding claim.

7. A frustoconical insert for a cathode ray display tube, the insert being formed from sheet material and having having: an inner surface coated with an electrically conductive material connecting to, at its flared end, and thus effectively extending, the aquadag of the cathode ray display tube; and an outer surface coated with an electrically non-conductive material for electrically insulating the aquadag extension from the electromagnetic deflection coils of the cathode ray tube.