GB2370423A

GB2370423A - Variable resistors and mounting bush for dimmer

Info

Publication number: GB2370423A
Application number: GB0105505A
Authority: GB
Inventors: Clive Silver; Andrew Deeming
Original assignee: HOME AUTOMATION Ltd
Current assignee: HOME AUTOMATION Ltd
Priority date: 2000-03-06
Filing date: 2001-03-06
Publication date: 2002-06-26
Anticipated expiration: 2021-03-06
Also published as: GB2370423B; GB0105505D0

Abstract

A dimmer comprises a rotary variable resistor and other components mounted on a circuit board located in a casing. The variable resistor comprises a self contained capsule PP1 containing a resistive track (63 Fig. 2) and a wiper (65b,Fig. 3) mounted on and for rotation with a hub which is accessible from outside the capsule. A separate spindle 72 engages the hub and projects from a front of the casing for rotating the hub to control the voltage drop between two mains terminals. The spindle is mounted for rotation in a bush 70 which extends from the front of the casing and has a male thread by which the dimmer can be secured to a face plate by an appropriate nut. The spindle and bush may have co-operating faces preventing excessive axial or rotary motion of the spindle.

Description

VARIABLE RESISTORS AND POTENTIOMETERS This invention relates to variable resistors and potentiometers.

The background of the invention will be explained in relation to dimmers. Some aspects of the invention extend beyond the field of dimmers, more generally to variable resistors and potentiometers.

Dimmers are used to control lighting. In the construction of conventional domestic dimmers, one of the more expensive components is a rotary variable resistor which is used to control the voltage drop between two mains terminals, thus to control the brightness (or dimness) of a lamp. The variable resistor used in conventional dimmers is a unitary structure containing a resistive track and a wiper. The wiper is mounted for rotation by spindle which is part of the unit. The unit may also contain a rotary or push/push switch operable by the spindle.

In accordance with one aspect of the invention, there is provided a dimmer, contained by a casing, a rotary variable resistor and other components of the dimmer being mounted on a circuit board located in the casing, the variable resistor comprising a self contained capsule containing a resistive track and a wiper mounted on and for rotation with a hub which is accessible from outside the capsule, a separate spindle engaging the hub and projecting from the front of the casing for rotating the hub to control the voltage drop between two mains terminals, the spindle being mounted for rotation in a bush which extends from the front of the casing and has a male thread by which the dimmer can be secured to a face plate by an appropriate nut.

This form of variable resistor is significantly cheaper. The form is similar to a preset variable resistor or trimmer.

In one form, the circuit board is located towards the rear of the casing, the bush having a first portion which extends through the front of the casing and bears said male thread, and a second portion defining a receptacle to receive the variable resistor, so mounting it therein, and a shoulder engaging the casing behind the nut, when installed.

In another form, the circuit board is located towards the front of the casing, the spindle passing through a hole in the circuit board, the bush being secured to or formed with the casing.

In order to avoid the cost of a mechanical push/push or rotary mains switch, in one position of the spindle, the whole mains voltage is preferably dropped between the terminals, there being no mechanical mains switch.

One of the problems found in dimmers is that, in general, if the spindle of the variable resistor is given an axial blow, e. g. accidentally, the spindle transmits the force of the blow to the internal components of the resistor causing damage, and may even cause the assembly to fall apart.

Against this background, another aspect of the invention extends to a rotary variable resistor or potentiometer, comprising a resistive track and a wiper mounted on and for rotation with a hub, about an axis, a spindle engaging the hub and projecting through a bush and being rotatable therein about the axis to control the position of the wiper on the track, wherein the spindle and the bush have complimentary surfaces transverse to the axis, such as to limit movement of the spindle in the bush, in an axial a direction towards the hub.

In general, rotary variable resistors and potentiometers have stops intended to limit the rotational movement of the wiper to, for example, less than one revolution.

Conventionally the stops are provided internally in the hub/wiper assembly and are generally not very robust. Another problem which is experienced is that over enthusiastic operators sometimes turn the spindle past the stops, again wrecking the mechanism.

Against this background, yet another aspect of the invention provides a rotary variable resistor or potentiometer, comprising a resistive track and a wiper mounted on and for rotation with a hub, a spindle engaging the hub and projecting through a bush and being rotatable therein to control the position of the wiper on the track, wherein the spindle and the bush have complimentary features which limit rotational movement of the spindle in the bush to less than one revolution.

In order to avoid problems of alignment of conventional stops with those of the invention, the hub itself is preferably free to rotate through unlimited revolutions.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is an exploded pictorial view of a variable resistor bush and spindle assembly; Figure 2 is a perspective view of one part of the variable resistor, showing the resistive track; Figure 3 is a perspective view of another part of the variable resistor, showing the wiper; Figure 4 is a cross section on arrows IV-IV of Figure 3; Figure 5 is a cross section of a first dimmer switch embodying one aspect of the invention; Figure 6 is a cross section of a second dimmer switch embodying one aspect of the invention; Figure 7 is a schematic circuit diagram of the dimmer switch of Figure 1; Figure 8 is. an exploded view, similar to Figure 1, showing a rotary variable resistor or potentiometer embodying other aspects of the invention; Figure 9 is a side view of a further variable resistor or potentiometer embodying the said other aspects of the invention.

Figure 10 is an underneath plan of the variable resistor or potentiometer; Figure 11 is a pictorial view of the bush of the variable resistor or potentiometer of Figure 9; and Figure 12 is a section on arrows XII-XII of Figure 10.

Referring to Figure 5 of the drawings, in which similar reference numbers denote similar parts, a dimmer switch 30 has a casing 32 from which a spindle 34 and threaded part 36 of a bush 37 of a variable resistor PP1 project. The spindle and bush also project through a face plate 40 to which the dimmer switch 30 is secured by means of a nut 42 fitted to the threaded part 36. The face plate 40 is designed to fit a standard electrical wall box 44 which has a standard depth of 16mm. The wall box is shown installed in a plaster wall 45.

Components, which may be conventional, of the dimmer switch are mounted on a printed circuit board 46. The circuit diagram is shown in Figure 4. The dimmer switch replaces a conventional on/off switch between the lamp 48 and the live side L of an a. c. mains supply L N. As will be understood by a reader skilled in the art, the firing point in the a. c. mains cycle of a triac TR1 is controlled by the setting of the variable resistor PP1 The lamp 48 is thus brighter when the triac conducts for more of the cycle and dimmer when the triac conducts for less of the cycle. A mains switch for switching the lamp entirely off is not provided in this example. The range of the variable resistor PP1 is sufficient to prevent the triac TR1 from firing at all when the spindle is turned to an extreme position. There is a break in the variable resistor's resistive track (as explained below) In order to achieve this.

As will be seen from Figure 3, the casing 32 fills most of the depth of the wall box 44. The components in the casing fill most of the depth of the casing. That necessitates the printed circuit board 46 being placed close to the back of the casing and, thus, the back of the wall box.

The variable resistor PP1 is in the form of a self contained capsule 62 containing a resistive track 63. A wiper 65 is mounted for rotation with a hub 64 so as to vary the resistance between a pin 66b and a pin 68 formed integrally with a conducting back plate 69. The wiper 65 has contacts 65a and 65b which make electrical contact between the resistive track 63 and the back plate 69 so that the resistance between the pin 66b and the pin 68 varies according to the position of the wiper. The resistive track ends short of the allowed movement of the wiper so that the wiper can move off the track onto an insulating area 63a. This prevents the triac TR1 from firing. The back plate 69 has a dimple 69a in the path of the contact 65a to act as a detent when the contact 65b is off the end of the track 63.

The pins are soldered to appropriate tracks of the printed circuit on the board 46.

The variable resistor is a standard component being available, for example from Pihere, from their PT15 series. As will be seen from Figure 1 the variable resistor is similar in form to some preset resistors or potentiometers.

The bush 37 locates on the capsule 62 and provides a bearing in which the spindle can rotate. The thread allows the assembly to be mounted on, say, a face plate 40 projecting therethrough.

It is readily apparent that the standard arrangement cannot be used because the printed circuit board 46 would be too close to the front of the casing 32 and to the face plate 40, to allow room for the other components.

In the drawings a bush 70 and spindle 72 are illustrated which overcome this problem. The bush 70 has a body part 74 having a depending skirt 76 forming a receptacle 78 shaped to receive the capsule 62. A boss 80 extends from the body part and has a cylindrical hole 82 therethrough aligned with the hub 64. The spindle 72 is formed at one end 84 to snap fit into an aperture 86 in the hub 64. The spindle 72 is a sliding fit in the hole 82 so that rotation of the spindle rotates the hub 64 varying the resistance between the pins 66a or 66b and 68.

In contrast to the standard bush, the threaded part 36 is sufficiently spaced from the variable resistor to allow the threaded portion to project through the front of the casing 32 and the face plate 40 when the printed circuit board is located at the rear of the casing to accommodate the other components. So that the nut can be tightened on the face plate a shoulder 88, wider than the threaded part 36 is provided by ribs 90 which extend parallel to the rotational axis 92 of the spindle. The shoulder engages the casing behind the nut. Other arrangements to provide the shoulder may be used. For example the boss 80 may be stepped so that a portion between the threaded part 36 and the body part 74 is wider than the threaded part (not illustrated). The shoulder is spaced 10mm to 25mm from the circuit board 46, e. g. 17 mm.

An electrical cable 52 enters the wall box through a side wall. The casing 32 has an aperture 54 giving access to terminals 4. During installation, the insulation is stripped from the ends of the conductors 56 of the cable (only one conductor is shown in the drawings). The stripped ends of the conductors are secured in respective terminals.

The face plate is then secured to the wall box.

Another arrangement is illustrated in Figure 6. Here the circuit board 46 is located close to the front of the casing. Components, including the variable resistor PP1, are mounted on the rear of the circuit board. A hole 100 through the circuit board 46 gives the spindle 34 access to the hub 64 of the variable resistor PP1. In this case, the bush 36 is formed integrally with the casing 32.

Referring now to Figure 8, modifications are shown to the bush 70 and the spindle 72.

The bush 70 has a surface extending transversely to the axis 92. The transverse surface is provided by a more or less annular surface 102 at the upper end of the bush as seen in Figure 8. The spindle is stepped at a shoulder 104 also to provide a surface transverse to the axis 92. The surfaces 102 and 104 act complimentarily to limit the movement of the spindle 72 in the bush 70, in an axial direction towards the hub 64.

The arrangement can be sufficiently robust to absorb axial blows to the spindle and prevent damage to the variable resistor PP1.

Projecting upwardly, as seen in Figure 8, from the surface 102 is a stop member.

As can be seen in Figure 8, the stop member 106 extends in a radial direction, only part way across the surface 102, so as to leave room for the shoulder 104 of the spindle 72.

Another stop member projects from the spindle 72 adjacent the shoulder 104. The stop members 106 and 108 act complimentarily, mutually engaging to limit the rotational movement of the spindle 72 in the bush 70 to less than one revolution. The arrangement can be sufficiently robust to prevent the spindle turning so far as to wreck the variable resistor PP1. Indeed, it is preferable if the variable resistor PP1 has no internal stops, so that in the absence of the stop members 106,108, the hub would be free to rotate through unlimited revolutions.

Referring now to Figures 9 to 12, the spindle 72 is stepped at a shoulder 120 which provides a surface transverse to the axis 92. The cylindrical hole 82 is stepped at a shoulder 122 which provides a surface transverse to the axis 92 and complimentary to the shoulder 120. The shoulders 120 and 122 cooperate to prevent the spindle from being forced inwardly in the hole 82.

Behind the shoulder 122, is a short section 124 of reduced diameter which opens out into a section 126 of increased diameter. This has an aperture 128 in its side. A stop 130 projects from the aperture 128 into the section 126 of the hole 82. This is in the way of a cam 132 projecting from the spindle 72 in the enlarged section 126 of the hole 82. A longitudinal groove 133 in the wall of the hole 82 accommodates the cam 132 to allow the spindle to be inserted from the position shown in Figure 12. The cam 132 and stop 130 provide complimentary surfaces which act to limit the rotational movement of the spindle in the hole 82 as can be seen from Figure 10. The limit in the clockwise direction as seen in Figure 10 corresponds to wiper 65 being positioned at the break provided by the insulating area 63a.

In addition to the stop 130, a follower 134, at the end of a resilient spring member 136 projects through the aperture 128 into the enlarged section 126 of the hole 82. The follower is also in the way of the cam 132 and is urged outwardly as the cam passes producing an audible click. As can be seen from Figure 10, the follower is placed so as to interact with the cam as the cam approaches and leaves the stop 130 corresponding to the wiper being positioned at the break provided by the insulating area 63a, i. e. to the off position. The entire bush including the stop and the follower/spring member is formed in insulating plastics as one integral piece by injection moulding.

In an alternative, not illustrated, the cam is replaced by an indent in the spindle.

Suitable shaping of the indent and the follower, both stop rotation of the spindle and produce an audible click.

Claims

1. A dimmer, contained by a casing, a rotary variable resistor and other components of the dimmer being mounted on a circuit board located in the casing, the variable resistor comprising a self contained capsule containing a resistive track and a wiper mounted on and for rotation with a hub which is accessible from outside the capsule, a separate spindle engaging the hub and projecting from the front of the casing for rotating the hub to control the voltage drop between two mains terminals, the spindle being mounted for rotation in an aperture through a bush which extends from the front of the casing and has a male thread by which the dimmer can be secured to a face plate by an appropriate nut.

2. A dimmer as claimed in claim 1, wherein the circuit board is located towards the rear of the casing, the bush having a first portion which extends through the front of the casing and bears said male thread, and a second portion defining a receptacle to receive the variable resistor, so mounting it therein, and a shoulder engaging the casing behind the nut, when installed.

3. A dimmer as claimed in claim 1, wherein the circuit board is located towards the front of the casing, the spindle passing through a hole in the circuit board, the bush being secured to or formed with the casing.

4. A dimmer as claimed in any preceding claim, in which in one position of the spindle, the whole mains voltage is dropped between the terminals, there being no mechanical mains switch.

5. A dimmer as claimed in any preceding claim, wherein the spindle and the bush have complimentary surfaces transverse to the axis, such as to limit movement of the spindle in the bush, in an axial a direction towards the hub.

6. A dimmer as claimed in claim 5, wherein the complimentary transverse surfaces are provided by a step in the aperture and a shoulder on the spindle.

7. A dimmer as claimed in any preceding claim, wherein the spindle and the bush have complimentary features which limit rotational movement of the spindle in the bush to less than one revolution.

8. A dimmer as claimed in claim 7, wherein the hub itself is free to rotate through unlimited revolutions.

9. A dimmer as claimed in claim 7 Or 8, wherein the complimentary features which limit rotational movement, comprise a projection into the aperture in the way of a projection from the spindle.

10. A dimmer as claimed in claim 4 or any preceding claim dependent thereon, wherein the spindle and the bush have complimentary features which engage to produce an audible click as the spindle rotates into the position of the spindle in which the whole mains voltage is dropped between the terminals.

11. A dimmer as claimed in claim 10 in which the complimentary features which engage to produce an audible click are a follower resiliently biased into the aperture and a projection from or an indent in the spindle.

12. A rotary variable resistor or potentiometer, comprising a resistive track and a wiper mounted on and for rotation with a hub, about an axis, a spindle engaging the hub and projecting through a bush and being rotatable therein about the axis to control the position of the wiper on the track, wherein the spindle and the bush have complimentary surfaces transverse to the axis, such as to limit movement of the spindle in the bush, in an axial a direction towards the hub.

13. A variable resistor or potentiometer as claimed in claim 12, wherein the complimentary transverse surfaces are provided by a step in the aperture and a shoulder on the spindle.

14. A rotary variable resistor or potentiometer, comprising a resistive track and a wiper mounted on and for rotation with a hub a spindle engaging the hub and projecting through a bush and being rotatable therein to control the position of the wiper on the track, wherein the spindle and the bush have complimentary features which limit rotational movement of the spindle in the bush to less than one revolution.

15. A variable resistor or potentiometer as claimed in claim 14, wherein the spindle and the bush have complimentary features which limit rotational movement of the spindle in the bush to less than one revolution.

16. A variable resistor or potentiometer as claimed in claim 14 or 15, wherein the hub itself is free to rotate through unlimited revolutions, when not restrained by the spindle.

17. A variable resistor or potentiometer as claimed in claim 14 to 16, wherein the complimentary features which limit rotational movement, comprise a projection into the aperture in the way of a projection from the spindle.

18. A variable resistor or potentiometer as claimed in any of claims 14 to 17, wherein the spindle and the bush have complimentary features which engage to produce an audible click as the spindle rotates into an extreme position at one end of its movement.

19. A variable resistor or potentiometer as claimed in claim 18 in which the complimentary features which engage to produce an audible click are a follower resiliently biased into the aperture and a projection from or an indent in the spindle.

20. A variable resistor or potentiometer as claimed in claim 14, wherein the hub itself is free to rotate through unlimited revolutions, when not restrained by the spindle.