Variable Resistance Assembly

Abstract

An assembly having a variable resistance for providing a variable voltage dividing function, and; hence, a controllable voltage such as to the anode of a television picture tube.

Description

BACKGROUND OF THE INVENTION

In present cathode ray tubes, and more specifically, in most television picture tubes, it is desirable to provide an assembly to control and adjust the voltage provided to the focus anode. Various types of prior art resistance assemblies or variable voltage dividers have been provided for this purpose. Variable voltage dividers usually comprise resistance elements, a contactor adjustably engaging the resistance elements, a knob for changing the position of the contactor relative to the resistance elements, and several terminals for connecting the voltage divider to the associated electronic circuitry. Prior art devices known to the inventors are relatively expensive, and are also susceptible to unstable operation caused by changes in temperature and humidity conditions.

For use with present day electronic equipment, it is important that voltage dividers be relatively small, compact, readily mountable on the associated chassis, and that such dividers exhibit stable operating characteristics that are independent of the humidity or temperature of the surrounding environment.

The inventive variable resistance assembly is further directed to a so-called thick-film type of device. A thick film may be conveniently of a conductive material of metal deposited on a ceramic substrate. It has been found that variable resistance assemblies or voltage dividers formed of thick film provide a stable reliable resistance.

A metallic conductive pad or terminal is normally formed on the substrate to facilitate the making of electrical connections from the film to the associated electronic circuitry. More specifically, electronic wire terminals can be soldered to the conductive pads.

For one specific application, the inventive resistance assembly is used to supply a constant voltage to the focus electrode of a television picture tube. In such application, the input ends of the resistance assembly is connected to a relatively high potential supply in the 20KV to 30KV range and the other end of the divider is connected to a ground or reference. An intermediate output terminal of the assembly connects an adjustable voltage in the 0KV to 20KV range to the focus electrode of the picture tube.

Accordingly, it is an object of the present invention to provide an improved variable resistance assembly that is reliable even while operating in various different environmental conditions.

It is still another object of the present invention to provide an improved variable resistance assembly that is readily assembled, and is relatively inexpensive.

It is a further object of the present invention to provide a new and improved variable voltage divider having a plurality of thick film resistive paths.

Still another object of the present invention is to provide an improved voltage divider assembly having a conductive film formed on a ceramic substrate and having an adjustable contactor mounted on the substrate for varying the effective resistance provided by the film.

The foregoing objects and other features of the invention will be apparent from the following more particular description of a preferred embodiment of the invention as depicted in the accompanying drawings wherein:

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the substrate portion of a variable resistance assembly in accordance with the invention;

FIG. 2 is a plan view of the rotatable electrical contactor for the variable resistance assembly;

FIG. 3 is a cross sectional view taken along the lines 3--3 of FIG. 2 and reversed in orientation to conform with the showing of FIG. 4;

FIG. 4 is an isometric view of the assembled variable resistance assembly of the invention; and,

FIG. 5 is an electrical representation of the structure of FIG. 1.

DESCRIPTION OF THE INVENTION

The present invention is directed to a variable resistance assembly on which are formed various thick film resistive paths of a suitable electrical resistance, known in the art.

Refer now to the drawings, FIGS. 1-4 illustrate the inventive variable resistance assembly or voltage divider, generally indicated by the numeral 11.

The variable resistance assembly 11 comprises a ceramic substrate 12 which may be formed of a suitable material such as alumina. The substrate 12 may have any suitable shape and in the embodiment shown, comprises a modified rectangle with a reduced portion as at 38 to provide a stop surface for the associated contactor assembly 17, shown in FIG. 2.

Electrically resistive paths 25, 27, 29 and 36 are deposited on the substrate 12 in a desired pattern. The resistive paths each comprise a thick film; that is, a compound of electrically resistive material with a suitable dispersion of metal. Several commercially available materials may be used to form the thick film.

As can be seen in FIG. 1, the resistive path 25 has one end electrically in contact with a terminal pad 19 formed of a metallic base material which is deposited by any suitable known means onto the ceramic substrate 12. The terminal pad 19 is preferably formed of a material to which the conductive connector 20 (see FIG. 4) may be conveniently soldered for making electrical connections thereto.

In the present embodiment, two other terminal pads 21 and 23 similar to pad 19 are mounted in spaced relation on one edge of the substrate 12. Terminal pad 19 is the high voltage terminal of the assembly; terminal pad 21 provides an adjustable voltage to the focus anode of the associated television picture tube as will be described; and, terminal pad 23 is connected to reference or ground potential.

The resistive path 25 which is in electrical contact with terminal pad 19, extends from pad 19 along the ceramic substrate 12 and terminates in a free, unconnected or open end 34. The free end of resistive path 25 is conveniently formed in a circular shape.

A second resistive path 27 which is in electrical contact with terminal pad 21 extends from pad 21 and joins or connects to resistive path 36 which is electrically connected to terminal pad 23. The junction of resistive paths 29 and 36 extend into and join resistive path 27 which terminates in a free, unconnected or open end 40. The free end portion of resistive path 27 is conveniently formed in a circular shape.

An electrical contactor 31 having two outwardly extending leaves 33 and 35, (see FIG. 2) is mounted for rotation on a suitable pin 32 to make electrical contact with the resistive path portions 25 and 27 adjacent the open ends 34 and 40, for purposes to be explained hereinbelow. Since, in this embodiment, contactor 31 is arranged to be rotatable, the resistive path portions, which it contacts are conveniently circular in shape, as mentioned above. It will, of course, be understood that the contactor 31 could be arranged to be movable along an essentially straight line and in such arrangement the resistive paths could be straight.

Refer now to FIG. 4. In the completed assembly, a cover or housing 13 is placed over substrate 12. Suitable apertures 14 in housing 13 accomodate outwardly extending electrical connectors 20, 22 and 24 which connect to terminal pads 19, 21 and 23 respectively. Housing 13 includes a circular portion 16 which houses the contactor assembly 17. The portion 16 includes a central opening through which the knob 18 of the contactor assembly 17 can extend.

The details of the contactor assembly 17 are more clearly shown in FIGS. 2 and 3. Contactor assembly 17 includes the conductive contactor 31 having the two outwardly extending flexible leaves 33 and 35, as mentioned above. Leaves 33 and 35 are mounted on the lower surface, as oriented in FIG. 3, of contactor 17 and when the contactor is mounted in position, in assembly 11, the leaves 33 and 35 will be resiliently compressed against the planar surface of the substrate 12 to thus make contact with the circular portions of the respective resistive paths 25 and 27. The contactor 17 is manually rotatable to be set at a selected position by knob 18 and contactor 17 is constrained in its rotation by a stop 26 formed on its periphery which engages the side of substrate 12 as its opposed limits as at 28 and 30.

The operation and function of the structure of FIGS. 1-4 can be better understood from reference to FIG. 5 which shows an electrical circuit representative of the resistive paths 25, 27, 29 and 36 as electrical resistors. The reference characters in FIG. 5 are intended to correspond to the reference characters in FIGS. 1-4. Note also that terminal pads 19, 21 and 23 are indicated as electrical connections.

Referring to FIG. 5, it will be appreciated that if a high voltage is impressed across terminals 19 and 23, a given voltage will be developed at the intermediate or focus anode terminal 21. If the voltage across terminals 19 and 23 remains essentially constant, while the resistance between terminals 19 and 21 is decreased, and the resistance between terminals 21 and 23 remains essentially constant, it will be appreciated that the voltage on terminal 21 will be changed. When the contactor 31 is in the position shown in FIG. 5, a maximum resistance is effected between terminal 19 and terminal 21; accordingly, the voltage output at terminal 21 will be at a given potential. If contactor 31 is rotated in a clockwise direction, the effective resistance between terminal 19 and terminal 21 will be reduced and the voltage output on terminal 21 will be changed. Thus, by properly setting contactor 31 (and leaves 33 and 35), the voltage output on terminal 21 can be adjustably controlled. The contactor 17 is manually rotatable to be set at a selected position by a stop 26 on its periphery which engages the side of substrate 12 as at limit points 28 and 30.

As mentioned, variable resistance assembly 11 is particularly suitable for use in the focus circuit of a cathode ray tube. In a typical color television picture tube application, the input to the variable resistive assembly 11 is connected to terminal pad 19 as a DC voltage of, say, approximately 25 KV volts and the low potential termination is connected to ground reference terminal pad 23. The output is connected from terminal pad 21 to the focus anode of the picture tube. The tube is then focused by adjusting the contactor 17 until a voltage in the range of 0-20KV is applied to the focus anode of the tube.

Of additional interest with respect to this invention, is that arcing intermittently occurs between the high voltage anode of a picture tube and the focus grid in the tube. Should such arcing occur, a damaging high frequency pulse of very high KV potential may be impressed upon the power supply connected to the input of the voltage divider. Accordingly, a series resistance such as resistive path 29 can be provided to function as a surge limiting resistor which is effective to create a voltage drop when arcing occurs, and to thus limit the magnitude of the damaging pulse at the input of the voltage divider, and at the power supply.

While the invention has been particularly shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

What is claimed is:

1. A variable resistance assembly comprising an electrically non-conductive substrate having at least three terminal pads, first and second electrically resistive paths on said substrate, said paths being electrically separated one from the other, the first resistive path having one end connected to one terminal and the second resistive path dividing into a first portion which connects to a second terminal pad and another portion which connects to a third terminal pad, and the resistive paths each having an electrically open or unconnected end, and an electrically conductive contactor engaging said first and second paths, said contactor being selectively adjustable to increase and decrease the effective length of the resistive paths and hence the effective electrical resistance thereof.

2. A variable resistance assembly as in claim 1 wherein the open end portions of the resistive path are in a circular shape, and the conductive contactor is rotatable relative to engage the resistive paths to change the effective resistance of said resistive paths.

3. A variable resistance assembly as in claim 1 wherein a terminal pad and the first resistive path is connectable to a source of high voltage, the one portion of said second resistive path and the associated terminal pad is connectable to provide a focus anode voltage, and the other portion of said second resistive path is connectable to a reference potential whereby by adjusting the contactor the voltage provided to the focus anode can be controlled.

4. A variable resistance assembly as in claim 1, said resistive paths have one end connected to a conductive pad and the other end is open.

5. A variable resistance assembly as in claim 1 wherein said resistive paths are formed of a thick film material.