GB1601988A - Rotary switch - Google Patents

Description

(54) ROTARY SWITCH (71) We, STANDARD TELEPHONES AND CABLES LIMITED, a British Company of 190 Strand, London W.C.2. England, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to rotary electric switches of the type in which the switching action is achieved by the use of a coded movable member having alternate insulating and conducting areas which cause a set of stationary spring loaded contacts to be alternately connected or disconnected in sliding across the switch, depending on member position and the coding used.

In practice this function has normally been achieved by the use of (subtractive) printed circuit board techniques for the members where etched copper track has been used as the conductor with noble metal overplating to protect contacts and conductors from oxidation (and subsequently variable switch performance). Subtractive copper clad PCB manufacture, although widespread in its use, remains comparatively expensive to operate.

A reduction in both the material cost and processing costs is therefore desirable provided acceptable switch performance can be maintained.

According to the present invention there is provided a rotary electric switch having a coded movable member with alternate insulating and conducting areas, in sliding rotary contact with a set of stationary electrical contacts, in which at least the insulating or the conducting areas are fabricated from a printed abrasive material.

The use of abrasive materials results in automatic cleaning of the contacts each time the member is moved, thus enabling the use of contacts which do not need to be plated but which can satisfactorily be made of base metals.

Embodiments of the invention will now be described with reference to the accompanying drawings, in which: Fig. 1 illustrates a simple arrangement of a coded rotating disc and a set of stationary electrical contacts, Fig. 2 is a cross-section through the disc of Fig. 1, Fig. 3 illustrates one form of rotary switch construction utilising the simple arrangement of Fig. 1, and Fig. 4 illustrates an alternative form of rotary switch incorporating additional circuit elements in the switch.

A basic form of rotary switch is shown in Fig. 1, in which a rotating disc 1 has coded areas of electrically conducting material 2 alternating with electrically insulating material 3. Spring loaded electrical contacts 4 are pressed against the coded disc so that when the latter is rotated in either direction different electrical connections are established between the contacts via the electrically conducting material on the disc.

In one embodiment of the invention the disc is fabricated from a rigid sheet of electrically insulating material 10, e.g. a short strand random laid glass fibre material bonded with an epoxy resin. This substrate is typically 1.6 mm thick. One face of the substrate is first provided with an electrically conducting layer 11. A suitable material for the overlayer is a silver loaded epoxy resin material. The silver conductor is in the form of flake particles of particle size distribution nominal 35 micrometers with 50 micrometers a maximum particle size and 10 micrometers a realistic minimum. The flake thickness varies between 1 micrometer and 10 micrometers and should not exceed the maximum stated dimensions. The epoxy resin is polymerized by the addition of either a solid or liquid aromatic amine, or other suitable hardener. 4,4'-diaminodiphenyl methane (DDM) mixed either as a freely dispersed solid (flake) material or as a solution in a suitable solvent such as methyl ethyl ketone would be a preferred embodiment of the latter type of hardener.

Flake silver is mixed intimately with the resin-plus-hardener in an amount calculated to ensure 70 to 80 weight percent of silver in the cured resin. A non-reactive modifying agent will then be added in order to render the whole printable through any fine mesh, patterned in any pre-described geometry so as to facilitate the laying down of an electrically continuous conducting film on to the desired substrate material. The conducting layer so defined will then be caused to rapidly harden off by the application of heat, (i.e. fan assisted oven) at a temperature and time as determined by the type of resin used.

For a Bisphenol A resin and DDM hardener this would be 150"C for 1--3 hours. Over the conducting layer 11 there is then deposited an electrically insulating layer 12, comprising an epoxy resin matrix, with either DDM or an aromatic amine hardener, mixed with a predefined proportion of a suitable thixotropic agent. The thixotropic agent is specifically chosen so as to impart distinct, well defined properties to the insulating layer.

When mixed together with the resin + the hardener (where the hardener would preferably be in solution in a suitable solvent such as methyl ethyl ketone) it allows the whole to be printed through any fine mesh into any predefined pattern where the thixotropic agent will cause the printed film to remain in the predefined geometry during any and all subsequent thermal processing cycles.

The insulating layer as defined above, possesses a second important characteristic necessary for the reliable operation of a switch made in the described manner. It relates to the preferred wear characteristics of the stationary contacts. In order that reliable contact conditions be maintained between contacts and substrate the presence of insulating oxide films at that point must be avoided. Using unplated contacts, the hardness difference between the contact material and silica thixotrope is used to advantage.

The (mildly) abrasive insulating layer will remove the metal oxides that may form on the contact faces. The controlled addition of silica powder to the insulator is thus desirable (and necessary) for reliable switch operation. The use of (costly) noble metalplated contacts may therefore be avoided.

The fixed electrical contacts will be fabricated from a hardened phosphor bronze alloy, this material possessing the desired wear characteristics when used in conjunction with the conducting and insulator resins as previously detailed.

The assembled switch will thus possess the following characteristics: (1) That the conductor is in the form of a screen printable silver loaded epoxy resin (containing a silver flake of the correct size, shape and particle size distribution, dispersed within the desired chemically formulated epoxy resin).

(2) That the insulating medium is screen printable, and possess . both the required degree of abrasiveness and geometrical stability after printing (and during printing).

Moreover, it is necessary that the insulator film be geometrically stable when printed on to a surface such as that produced in the printing of a thick, viscous underlayer, e.g.

stable edge definition on surfaces with roughness peaks 25 micrometers in height.

The special property of the insulating resin (as defined above) will result from the use of 3 to 5.5 wt% of the special finely divided silica in the pre-defined epoxy resin mix.

(3) That the substrate be a cold punchable laminate (e.g. random laid short strand glass fibre-epoxy board). It must possess geometrical and thermal stability at the elevated temperatures used to cure the overprinted layers. It must be chemically compatible with those layers.

(4) The switch will provide acceptable electrical switching with the use of unplated phosphor bronze contacts, in the range typically 50 volts 10 mA (1.10-2 Amps) D.C.

Typical life of such a device is 40000 disc revolutions under constant applied electrical load and 60000 revolutions with zero electrical load. The device may well operate at higher loads, 50V 10 mA may not be the upper limit.

(5) The substrate + conductor + insulator concept differs from that of existing switch rotors in that coding is achieved by a printed conductor overprinted with a compatible insulator. The conductor layer is therefore continuous, so forming a low resistance conduction path. A typical switch resistance would be 130my (0.13Q).

A completed switch is illustrated in Fig. 3 and comprises a switch housing 20, which in this case is a compartment formed of one or more moulded piece parts which hold the coded disc 1 and electrical contacts 4 in the correct positions with respect to each other, and provides the necessay electrical isolation between the individually mounted contacts 4. The latter are initially located by pins 21 when assembling the switch. A hole 22 is provided for the central spindle (not shown) which has a keyed portion engaging a suitably shaped hole in the centre of the disc 1, the spindle being the means by which the disc is rotated. The whole is held together by screws 23.

A further advantage of using the materials and methods of manufacture described above lies in the relative ease with which additional circuit elements, such as resistors and capacitors can be incorporated, particu larly if they are formed as part of a printed circuit board which is coplanar with the rotating disc, as shown in Fig. 4. In practice it is convenient to fabricate the disc 1 as part of a printed circuit board 30, from which it is subsequently punched out prior to assembly of the switch. The board 30 is provided with conductor tracks 31 which terminate at the board edge in a conventional printed circuit connector configuration 32. The other ends of the tracks are provided with printed circuit components as required. When the switch is assembled the disc 1 and board 30 are placed in position on the lower parts of the housing 33 and the stationary electrical contacts 34 are located in position such that one end of each contact bears against the disc and the other end is laid in electrical contact with a conductor track 31 on the board 30. When the final upper part of the housing is secured in place the other ends of the contacs 34 are trapped under pressure against the conductor tracks 31.

Multiple layer circuits may thus be built up from individual printed layers, either on separate boards and/or on double side boards with either separate and/or double sided discs, with all individual elements remaining both thermally and electrically compatible with each other.

WHAT WE CLAIM IS: 1. A rotary electric switch having a coded movable member with alternate insulating and conducting areas, in sliding rotary contact with a set of stationary electrical contacts, in which at least the insulating or the conducting areas are fabricated from a printed abrasive material.

2. A switch according to claim Im wherein the electrically insulating areas are formed from a printed epoxy resin material loaded with an abrasive additive.

3. A switch according to claim 1 or 2 wherein the electrically conducting areas are formed from a printed epoxy resin material loaded with an electrically conducting additive.

4. A switch according to claim 1, 2 or 3 wherein the member is formed from an insulating substrate at least one face of which is first completely covered with a layer of electrically conducting material and is then partially covered with a layer of electrically insulating material.

5. A switch according to any one of claims 24 wherein the abrasive additive is silica powder.

6. A switch according to any one of claims 3-5 wherein the electrically conducting additive is silver particles.

7. A switch according to any of claims 46 wherein the insulating substrate comprises a short strand random laid glass fibre material loaded with an epoxy resin.

8. A switch according to any preceding claim wherein the stationary contacts are made of a phosphor bronze alloy.

9. A switch according to any preceding claim including a switch having comprising a compartment formed of one or more moulded piece parts which hold a coded disc and electrical contacts in the correct positions with respect to each other, the housing having a hole through which a central spindle having a keyed portion may engage a correspondingly keyed hole in the centre of the disc to effect rotation thereof.

10. A switch according to any preceding claim including a printed circuit board incorporating electric circuit elements, said board having an aperture within which the movable member is located coplanar with the board, the stationary electrical contacts being arranged so that one end of each contact bears against a conductor on the printed circuit board and the other end bears against the coded face of the member.

11. A switch according to claim 10 wherein the member is fabricated as part of the board and is subsequently separated therefrom.

12. A rotary electrical switch sustantially as described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (12)

**WARNING** start of CLMS field may overlap end of DESC **. larly if they are formed as part of a printed circuit board which is coplanar with the rotating disc, as shown in Fig. 4. In practice it is convenient to fabricate the disc 1 as part of a printed circuit board 30, from which it is subsequently punched out prior to assembly of the switch. The board 30 is provided with conductor tracks 31 which terminate at the board edge in a conventional printed circuit connector configuration 32. The other ends of the tracks are provided with printed circuit components as required. When the switch is assembled the disc 1 and board 30 are placed in position on the lower parts of the housing 33 and the stationary electrical contacts 34 are located in position such that one end of each contact bears against the disc and the other end is laid in electrical contact with a conductor track 31 on the board 30. When the final upper part of the housing is secured in place the other ends of the contacs 34 are trapped under pressure against the conductor tracks 31. Multiple layer circuits may thus be built up from individual printed layers, either on separate boards and/or on double side boards with either separate and/or double sided discs, with all individual elements remaining both thermally and electrically compatible with each other. WHAT WE CLAIM IS:

1. A rotary electric switch having a coded movable member with alternate insulating and conducting areas, in sliding rotary contact with a set of stationary electrical contacts, in which at least the insulating or the conducting areas are fabricated from a printed abrasive material.

2. A switch according to claim Im wherein the electrically insulating areas are formed from a printed epoxy resin material loaded with an abrasive additive.

3. A switch according to claim 1 or 2 wherein the electrically conducting areas are formed from a printed epoxy resin material loaded with an electrically conducting additive.

4. A switch according to claim 1, 2 or 3 wherein the member is formed from an insulating substrate at least one face of which is first completely covered with a layer of electrically conducting material and is then partially covered with a layer of electrically insulating material.

5. A switch according to any one of claims 24 wherein the abrasive additive is silica powder.

6. A switch according to any one of claims 3-5 wherein the electrically conducting additive is silver particles.

7. A switch according to any of claims 46 wherein the insulating substrate comprises a short strand random laid glass fibre material loaded with an epoxy resin.

8. A switch according to any preceding claim wherein the stationary contacts are made of a phosphor bronze alloy.

9. A switch according to any preceding claim including a switch having comprising a compartment formed of one or more moulded piece parts which hold a coded disc and electrical contacts in the correct positions with respect to each other, the housing having a hole through which a central spindle having a keyed portion may engage a correspondingly keyed hole in the centre of the disc to effect rotation thereof.

10. A switch according to any preceding claim including a printed circuit board incorporating electric circuit elements, said board having an aperture within which the movable member is located coplanar with the board, the stationary electrical contacts being arranged so that one end of each contact bears against a conductor on the printed circuit board and the other end bears against the coded face of the member.

11. A switch according to claim 10 wherein the member is fabricated as part of the board and is subsequently separated therefrom.

12. A rotary electrical switch sustantially as described with reference to the accompanying drawings.