GB2122414A

GB2122414A - Cathode ray tubes incorporating a protective resistor

Info

Publication number: GB2122414A
Application number: GB08217361A
Authority: GB
Inventors: Maurice Ronald Bennett
Original assignee: THORN EMI BRIMAR Ltd
Current assignee: THORN EMI BRIMAR Ltd
Priority date: 1982-06-15
Filing date: 1982-06-15
Publication date: 1984-01-11

Abstract

A cathode ray tube has a flared glass envelope 2 with an internal coating 5 which acts as a wall anode for the electron gun assembly 6 by linking the coating to the last, collimating anode 11 of assembly 6. This link is made via a current limiting resistor 12 to protect external circuitry from damage by surge currents in the equipment. The resistor 12 is formed of a disc shaped substrate of alumina ceramic on which is printed a track of resistive material (14, Fig. 3, not shown). A central portion (15) of the disc is removed to provide for unhindered passage of the electron beam. <IMAGE>

Description

SPECIFICATION Improvements relating to cathode ray tubes This invention relates to cathode ray tubes and more especially to current limiting resistors therefor.

In a cathode ray tube, an internal cathode gun assembly is connected to external circuitry. On occasions, large currents can surge between the assembly and circuitry resulting in damage to the latter. To avoid this a current limiting resistor is included in the tube. Such a resistor is typically coupled between the last anode of the gun assembly and an internal anode wall coating of the cathode ray tube, which is linked to the external circuitry. It is known to use a hollow cylindrical resistor as a current limiting resitor, the cathode rays passing through and along an axis of the cylindrical resistor. This naturally increases the overall length of the gun assembly when the limiting resistor is placed in circuit between the last gun assembly anode and the internal coating of the tube.

It is an object of this invention to provide a current limiting resistor permitting a reduction in the length of the overall cathode gun assembly.

According to one aspect of the invention there is provided a cathode ray tube including a cathode gun assembly and input connection means and output connection means, the tube also including a current limiting resistor, located in a circuit between said input and said output means, and arranged to protect external circuitry from inadvertant current surges through said a circuit; the limiting resistor comprising a pattern of resistive material applied to a substantially planar element of electrically insulating material.

In a preferred embodiment the limiting resistor comprises a disc having a central aperture and a pattern of resistive material provided thereon. Preferably the limiting resistor is located on the cathode gun assembly.

According to another aspect of the invention there is provided a current limiting resistor suitable for use in a cathode ray tube, the limiting resistor comprising a pattern of resistive material applied to a substantially planar element of electrically insulating material.

For a better understanding of the present invention and to illustrate how the same may be carried into effect, reference will now be made, by way of example only, to the accompanying drawings, in which: Figure 1 illustrates a section through a cathode ray tube including a surge limiting resistor in accordance with one example of the invention.

Figure 2 illustrates in enlarged scale the area defined in the circle A according to Fig.

1.

Figure 3 illustrates one example of a surge limiting resistor.

Referring to the drawings, Fig. 1 illustrates a cathode ray tube, which comprises a neck portion 1 opening onto the apex of a flared glass envelope 2. A screen 4 is located on an internal face of the base 3 of the envelope.

An internal coating 5 covers the sides of the envelope and extends partway into the neck portion. The coating, which may comprise graphite or aluminium is arranged to act as a wall anode for an electron gun assembly 6 located at an end of the neck portion remote from the envelope. The electron gun assembly is connected by means of an adjacently disposed connector 7 to external circuitry (not shown). The assembly is of conventional form and includes an axially located thermionic cathode, accelerating anodes and collimating anodes. An electron beam from the assembly is indicated by a line 8, the beam being coincident with the symmetry axis of the neck portion as illustrated.

External surface areas 9, of the envelope, which correspond to the internal surfaces 5 are also coated in order to establish a suitable capacitance between the internal and external coating. An external area, in the vicinity of a contact 10 which is hermetically sealed into the envelope wall, is not coated externally since the contact 10 forms the electrical connection with the internal coating 5. As disclosed herein above, the internal coating 5 is arranged to act as a wall anode for the electron gun assembly. This can be achieved by linking this coating, through an a current limiting resistor 12, to a collimating anode 11 being the last anode of the electron gun assembly. The function of the limiting resistor 1 2 is to protect external circuitry (not shown) from damage should large induced voltages result from circulating currents within the equipment.

Referring to Figs. 2 and 3, the limiting resistor 1 2 comprises a disc shaped substrate of an electrically insulating material, for example an alumina ceramic, and is typically 1 mm thick. A central portion 1 5 of the disc is removed to allow the disc to be capable of permitting unhindered passage of electrons therethrough. A track of resistive material 14 can be provided on the disc 1 3 to produce the required anti-surge resistive capability.

The track can be printed on utilising for example thick film techniques, and different patterns may be produced, for example a spiral. The resistive material is preferably comprised of a cermet, and the track has a breadth of 0.5mm with a 0.5mm gap between adjacent turns thereof. A suitable size of resistance between one end of the track and the other end, is 0.5M ohms.

The diameter of the disc 1 3 is selected having regard to the size of the neck portion.

The resistor 1 2 is maintained in circuit with the internal coating 5 and last anode 11 by means of contact springs 1 6 and 17, which are arranged to hold the disc firmly and establish an electrical link from one end of the track 14 to the collimating anode 11. Further spring clips 1 8 and 1 9 are arranged to also hold the disc and establish an electrical link from the other end of the track 1 4 to the internal coating 5.

As a consequence of utilising the limiting resistor in disc format, the overall length of the electron gun assembly 6, that is to say, including the resistor, is reduced in length, allowing thereby a reduction in length of the neck portion and a consequent reduction in depth of the cathode ray tube as a whole.

It will be understood that the embodiment illustrated shows an application of the invention in one form only for the purposes of illustration. In practice the invention may be applied to many different configurations, for example any substantially planar element may form the substrate for the limiting resistor and the resistive track may be applied to the element by it being embedded in, printed on, or laid on the substrate or within grooves therein; the detailed embodiments being straightforward for those skilled in the art to implement.

Claims

1. A cathode ray tube including a cathode gun assembly and input connection means and output connection means, the tube also including a current limiting resistor, located in a circuit between said input and said output means, and arranged to protect external circuitry from inadvertant current surges through said circuit; the limiting resistor comprising a pattern of resistive material applied to a substantially planar element of electrically insulating material.

2. A cathode ray tube according to Claim 1, wherein the limiting resistor comprises a disc having a central aperture and a pattern of resistive material provided thereon.

3. A cathode ray tube according to Claim to Claim 1 or Claim 2, wherein the limiting resistor is located on the cathode gun assembly.

4. A current limiting resistor suitable for use in a cathode ray tube, the limiting resistor comprising a pattern of resistive material applied to a substantially planar element of electrically insulating material.

5. A cathode ray tube substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

6. A current limiting resistor substantially as hereinbefore described with reference to Fig. 3 of the accompanying drawings.