Electrical switch assembly

The present invention relates to an electrical switch for controlling the operation of an electrical appliance.

BACKGROUND OF THE INVENTION

The operating condition of certain electrical appliances, such as speed or intensity, can be controlled after the appliance has been switched on. In some cases, it may be necessary or prudent to adjust the operating condition continuously or as required depending on the working situation.

Electrical switch assemblies have been known in general, which include a manual operating member that is arranged for initial movement to switch on an electrical appliance, such as an electric hand drill, and for subsequent movement to adjust the operating condition. Switch assemblies of this type usually incorporate a variable resistor in the control circuit, which is coupled to the operating member for direct control during operation. The variable resistor is typically of the sliding kind, which requires the operating member to have a relatively long operative distance, which may not be suitable for some appliances.

The invention seeks to mitigate or at least alleviate such a shortcoming by providing an improved electrical switch assembly.

SUMMARY OF THE INVENTION

According to the invention, there is provided an electrical switch assembly for controlling the operation of an electrical appliance, which assembly comprises at least first and second electrical elements. The first electrical element comprises an on/off switch for initially switching on said appliance. The second electrical element comprises a pressure-sensitive variable resistor for subsequently adjusting the operating condition of said appliance. The variable resistor comprises a first part having a resilient deformable and electrically conducting resistive surface, and a second part having a surface including at least two contacts and a resistive element connecting from one of said contacts to the other contact. One of the parts is supported for movement to press against the other part such that their surfaces bear against one another, thereby causing the resistive surface to deform against the surface of the second part over an area of contact and causing electrical connection between the resistive surface and the resistive element. The resistive surface and element together then provide a resultant resistance between the two contacts of a value that reduces as said area of contact increases corresponding to the pressure acting upon the two parts.

The assembly includes an operating mechanism for operating the first and second electrical elements, which incorporates manual operating-means arranged for initial movement to operate the on/off switch and subsequent movement, while the on/off switch is on, to operate the variable resistor.

It is preferred that the resistive surface includes fine carbon powder.

It is preferred that the resistive surface has a convex shape facing the surface of the second part of the variable resistor.

Preferably, the first part of the variable resistor comprises a portion made of a resilient deformable and electrically conducting resistive material to provide the resistive surface.

More preferably, the resistive material includes fine carbon powder.

In a specific construction, the first part of the variable resistor comprises a resilient deformable cup-shaped body including a base having an inner side on which the resistive surface is provided.

More specifically, the cup-shaped body includes a substantially frusto-conical peripheral wall that is foldable.

More specifically, the resistive surface includes fine carbon powder.

It is preferred that the resistive element includes fine carbon powder.

It is further preferred that the resistive element comprises a carbon film.

Preferably, said one part is supported for movement to press against the other part in a direction substantially perpendicular to their surfaces.

Preferably, the first part of the variable resistor is supported for movement to press against the second part, and the second part is fixed.

More preferably, the second part of the variable resistor is provided by a portion of a printed circuit board.

The first and second electrical elements may have relatively larger and smaller current ratings respectively.

As an example, the on/off switch comprises a micro-switch.

In a first embodiment, the resistive surface and the resistive element are arranged to come into electrical contact with each other when the surfaces of the first and second parts of the variable resistor bear against one another.

In a second embodiment, the resistive surface and said at least two contacts are arranged to come into electrical contact with each other when the said surfaces of the first and second parts of the variable resistor bear against one another.

In the second embodiment, the surface of the second part of the variable resistor includes more than two said contacts arranged close together for electrical contact with the resistive surface, and a corresponding said resistive element connecting across the adjacent contacts of each pair.

Conveniently, the operating mechanism includes a spring resiliently biasing the manual operating means against operating the first and second electrical elements.

In a preferred embodiment, the manual operating means comprises first and second parts for operating the on/off switch and the variable resistor respectively, the first part having a relatively shorter operative length than the second part.

More preferably, the first and second operating parts are separate parts.

It is further preferred that the first and second operating parts are covered by a resiliently deformable sheet element for operation through a single pressing action at the sheet element.

It is further preferred that one of the first and second operating parts has a portion engaging the other operating part for moving the other operating part during operation.

In a specific embodiment, the electrical switch assembly comprises one said on/off switch and two said variable resistors, wherein the manual operating means comprises three separate press members for operating the on/off switch and the two variable resistors respectively.

In this embodiment, the press member for the on/off switch is positioned between the press members for the two variable resistors.

In this embodiment, the press member for one of the variable resistors has a first portion engaging the press member for the on/off switch for simultaneous operation, and the press member for the other variable resistor has a second portion engaging the first portion and in turn the press member for the on/off switch for simultaneous operation.

Also in this embodiment, the two press members for the variable resistors are covered by resiliently deformable sheet means, said means having two regions covering the two press members respectively for individual depression to operate one or both variable resistors.

Further in this embodiment, the sheet comprises a single sheet including a portion that is between the two regions and supported by a fixed member against depression.

The invention also provides an electrical appliance incorporating the aforesaid electrical switch assembly. The appliance comprises a casing in which the switch assembly is located such that the operating mechanism is accessible, an electrical device located in the casing for performing a function of the appliance, and an internal electronic control circuit for controlling the operation of the electrical device. The on/off switch is connected to the electrical device for switching on the electrical device, and the variable resistor is connected to the control circuit for adjusting the operating condition of the electrical device.

Preferably, the casing includes a resiliently deformable wall portion, immediately behind which the operating mechanism is located for operation through depression at the wall portion.

As an example, the electrical device comprises an electric motor.

Conveniently, the casing is elongate and acts a handle.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

FIG. 1

is a partially cross-sectioned side view of an electrical appliance incorporating a first embodiment of an electrical switch assembly in accordance with the invention;

FIG. 2

is a cross-sectional side view of the switch assembly of

FIG. 1

;

FIG. 3

is a top plan view of a printed circuit board of the switch assembly of

FIG. 2

;

FIG. 3A

is a top plan view of an alternative printed circuit board for use in place of the circuit board of

FIG. 3

;

FIGS. 4A and 4B

are cross-sectional side view and top plan view corresponding to

FIGS. 2 and 3

, showing the switch assembly in a switched-off condition;

FIGS. 5A and 5B

are cross-sectional side view and top plan view corresponding to

FIGS. 4A and 4B

, showing the switch assembly in an initial switched-on condition;

FIGS. 6A and 6B

are cross-sectional side view and top plan view corresponding to

FIGS. 5A and 5D

, showing the switch assembly in an intermediate switched-on condition;

FIGS. 7A and 7B

are cross-sectional side view and top plan view corresponding to

FIGS. 6A and 6B

, showing the switch assembly in a fully switched-on condition;

FIG. 8

is an electrical operating circuit of the electrical appliance of

FIG. 1

;

FIG. 9

is a cross-sectional side view of a second embodiment of an electrical switch assembly in accordance with the invention;

FIG. 10

is a partially cross-sectioned side view of an electrical appliance incorporating a third embodiment of an electrical switch assembly in accordance with the invention;

FIG. 11

is a cross-sectional side view of another embodiment of an electrical switch assembly in accordance with the invention; and

FIG. 12

is a cross-sectional side view of yet another embodiment of an electrical switch assembly in accordance with the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring initially to

FIGS. 1

to

8

of the drawings, there is shown a first electrical switch assembly

100

embodying the invention for use in an electrical appliance such as, for example, a hand-held food mixer

10

. The food mixer

10

has an upright elongate casing

11

that houses an electrical motor with an associated gearbox and also acts as a handle for gripping by a user. The casing

11

has an upper end including a resiliently deformable wall portion

12

on one side, immediately behind which the switch assembly

100

is located. A mixer implement is to be connected to the lower end of the casing

11

for rotation by the motor via the gearbox.

The switch assembly

100

has a plastic casing

101

which has upper and lower sides/walls

102

and

103

. The switch assembly

100

includes a first electrical element

110

in the form of an on/off switch

110

housed at a central position in the casing

101

and a pair of two (at least one) second electrical elements

120

in the form of pressure-sensitive variable resistors

120

housed on opposite left and right sides therein. The three elements

110

and

120

are mounted on a horizontal printed circuit board

130

that extends internally across and closes the lower side

103

of the casing

101

. The upper wall

102

of the casing

101

is formed with a central hole

104

and a pair of left and right vertical bush portions

105

upstanding therefrom.

The on/off switch

110

is a micro-switch

110

that has a pair of terminals

111

extending downwardly through respective holes

131

of the circuit board

130

. The micro-switch

110

has an internal moving contact electrically connected to one of the terminals

111

for electrical connection to the other terminal

111

. The moving contact is supported on a leaf spring which is normally bowed in one direction (against its resilience) and is arranged to momentarily bow in the opposite direction, when it is being pressed upon by means of an external press-knob

112

, to connect the two terminals

111

. The micro-switch

110

includes an external operating lever

113

to operate the press-knob

112

. The construction and operation of the micro-switch

110

are generally known in the art.

Each of the variable resistors

120

comprises a first part in the form of a resiliently deformable rubber cup

121

which has a relatively thin frusto-conical peripheral wall

122

and faces upside down and rests on the circuit board

130

. The cup

121

includes a resiliently deformable soft knob

123

located centrally on the inner surface of the upturned base of the cup

121

. The knob

123

is made of an electrically conducting but resistive material including carbon powder bonded by a suitable bonding agent, and has a part-spherical convex surface

124

. The surface

124

is electrically resistive by nature of the material and faces the circuit board

130

, acting as a moving contact.

Each variable resistor

120

includes a second part co-operable with the aforesaid first part, which is provided by a flat portion

125

at each end of the circuit board

130

directly opposite the resistive surface

124

. The circuit board portion

125

is provided with a pair of copper contact pads

132

and a elongate (I-shaped) carbon film track

133

that extends across and electrically interconnects the two contact pads

132

. The track

133

comprises fine carbon powder bonded by a suitable bonding agent, and has a specific resistance acting as a flat resistive element across the contact pads

132

.

The cup

121

of the variable resistor

120

normally expands by virtue of resilience such that its internal resistive surface

124

is spaced apart at a small distance from, or at close proximity to, the resistive track

133

on the circuit board

130

. When the cup

121

is compressed downwardly at its upturned base, the peripheral wall

122

will be folded and the resistive surface

124

immediately pressed down into overlapping contact with the resistive track

133

below. As the knob

123

is resiliently deformable, the cup

121

can further be compressed to have the knob surface

124

pressed flat on and bearing against the circuit board

130

.

The resistive surface

124

flattens through resilient deformation against the fixed flat resistive track

133

, and the area over which they overlap with each other will increase (or decrease) as the pressure acting on them increases (or decreases). While the two resistive surface

124

and track

133

are in contact and overlap with each other, their resultant resistance across the two contact pads

132

will be reduced dependent upon, in a reverse relationship, their area of overlapping contact.

The two pairs of contact pads

132

on the circuit board

130

are connected in series by means of three copper tracks

134

as shown, which results in series connection of the two variable resistors

120

.

The switch assembly

100

includes an operating mechanism

140

supported by the casing

101

above and for operating all three electrical elements

110

and

120

. The operating mechanism

140

incorporates a central manual operating member

141

for primarily closing the micro-switch

110

and a pair of left and right manual operating members

146

for operating the respective variable resistors

120

. The operating members

141

and

146

are horizontally aligned with the corresponding elements

110

and

120

below.

The central operating member

141

has a vertical cylindrical plastic shaft

142

including a reduced lowermost end

143

which is inserted downwardly through the hole

104

into the casing

101

and engages the operating lever

113

of the micro-switch

110

. The operating member

141

includes a horizontal plastic top bar

144

that extends integrally across the uppermost end of the shaft

142

to form a T-shape. A compression coil spring

145

is disposed around the shaft

142

and co-acts between the top bar

144

and the casing wall

102

to resiliently bias the overall operating member

141

upwards, such that the micro-switch

110

is normally open.

Each of the left and right operating members

146

consists of a vertical cylindrical plastic rod

147

which passes through, and is thus supported by, the corresponding bush portion

105

for relative sliding movement. The rod

147

has an integral annular flange

148

at mid-length within the casing

101

, which retain the rod

147

in the bush portion

105

. The flange

148

is positioned such that a lowermost end

149

of the rod

147

is adjacent, or just touching, the cup

121

of the respective variable resistor

120

, such that the cup

121

is normally uncompressed. The self-expanding force of the cup

121

is sufficient to overcome the weight of the rod

147

as required, such that the value of the variable resistor

120

is normally the specific resistance of the carbon film track

133

on the circuit board

130

.

The top bar

144

of the central operating member

141

extends lengthwise in opposite directions to reach over and engage from above the uppermost ends of the rods

147

of the left and right operating members

146

. Upon depression at the top bar

144

, the central operating member

141

will slide downwards, which in turn will, simultaneously or instantly afterwards, move both of the side operating members

146

downwards, all against the action of the spring

145

. Upon release, the central operating member

141

will slide back upwards under the action of the spring

145

, and both side operating members

146

will follow suit under the self-expanding force of the corresponding cups

121

below.

As shown in

FIG. 8

, the micro-switch

110

is connected in series with a load

13

i.e. the electric motor of the food mixer

10

, and an AC/DC power source

14

, for switching on and off the motor. The variable resistors

120

are connected in series together and then to, or form part of, a control circuit

15

that in turn operates a solid-state switch

16

, such as a triac or silicon-controlled rectifier, for controlling the speed of the motor. The solid-state switch

16

adjusts the conduction angle of an alternating current in the case of an AC power source, or the duty cycle of a pulsating direct current for a DC power source.

It is necessary for the micro-switch

110

to have a relatively larger current rating in order to handle the load current, whereas the variable resistors

120

are only required to have a relatively small current rating to handle the control current. Although two variable resistors

120

have been included, it is clear that only one can be used instead, depending on the circuit and/or mechanical design, for example the resistance value required in the control circuit

15

and/or physical balance in the switch assembly

100

.

The switch assembly

100

is located within the upper end of the casing

11

of the food mixer

10

, with its operating bar

144

lying right against the inner surface of the deformable wall portion

12

. A user is to depress the wall portion

12

in order to operate the switch assembly

100

.

Upon initial depression of the operating bar

144

(FIGS.

4

A and

4

B), the central operating member

141

will first be pushed inwards to close the micro-switch

110

to switch on the motor (FIGS.

5

A and

5

B). Upon further depression, the two side operating members

146

will follow and operate the variable resistors

120

, while the micro-switch

110

is on, by compressing the corresponding cups

121

in a direction perpendicular to the resistive surfaces

124

and tracks

133

. During this action, the resistive surfaces

124

come into initial contact with the corresponding resistive tracks

133

centrally over a relatively small area of contact X (FIGS.

6

A and

6

B). This results in a reduced resultant resistance in the control circuit

15

, and the motor runs at a relatively low speed. Upon further or complete depression, the resistive surfaces

124

will be pressed harder against the corresponding resistive tracks

133

over a gradually larger or the largest possible area X (FIGS.

7

A and

7

B), thereby resulting in the motor running at a progressively higher or the full speed.

If all three operating members

141

and

146

are arranged to move initially at the same time, their operative lengths, i.e. the distance to travel before actual operation, may be made slightly different such that the central operating member

141

will close the micro-switch

110

first before the two side operating members

146

operate the variable resistors

120

.

By nature of the construction, the resistive surfaces

124

of the variable resistors

120

can be spaced apart at a small distance from, or at close proximity to, the corresponding resistive tracks

133

on the circuit board

130

. Also, the knobs

123

only need to be compressed slightly to have their resistive surfaces

124

flatten against the circuit board

130

. As a result, the operating mechanism

140

can be arranged, as is the case in the described embodiment, to move or travel over a relatively short length, i.e. a rather short operative distance, before it operates all three electrical elements

110

and

120

, thereby providing a soft touch feel.

FIG. 3A

shows an alternative printed circuit board

135

for use in place of the circuit board

130

described above. Each end of the circuit board

135

has a flat portion

125

′ that constitutes the second part of the respective variable resistor

120

. The circuit board portion

125

′ bears twelve mostly slant copper contact pads

136

arranged generally in a row and eleven carbon film resistive elements

137

each of which bridges across and electrically interconnects the adjacent contact pads

136

of a corresponding pair. The copper pads

136

include respective copper tracks

138

which extend to a region directly opposite the resistive surface

124

of the corresponding variable resistor

120

, where free ends of the tracks

138

are packed close together but spaced apart in a co-parallel arrangement for contact by the resistive surface

124

.

All eleven resistive elements

137

are in effect connected in series, with their ten junctions

136

and the two outermost contact pads

136

at opposite ends extended by means of the corresponding copper tracks

138

to the aforesaid region for contact by the resistive surface

124

. The two outermost contact pads

136

constitute a pair of terminals for each variable resistor

120

, and the two pairs of terminals are connected by means of three copper tracks

139

as shown such that the two variable resistors

120

are connected in series.

In operation of each variable resistor

120

, when the resistive surface

124

comes into initial contact with the copper tracks

138

centrally over a relatively small area of contact Y, the resistive surface

124

overlaps with some (six as shown) of the tracks

138

at the middle. This results in parallel connection of successive portions of the resistive surface

124

, by means of the tracks

138

in contact with the resistive surface

124

, to the corresponding associated resistive elements

137

, such that the resultant resistance across the terminals of the variable resistor

120

is reduced. This causes a reduction in the relevant resistance in the control circuit

15

, and the motor runs at a relatively low speed. Upon further depression, the resistive surface

124

will be pressed harder against the tracks

138

over a gradually increasing area such that more and eventually all of the tracks

138

will be connected, thereby resulting in the motor running at a progressively higher and finally the full speed.

It should be note that, with the use of the design of the second circuit board

135

, the variable resistors

120

operate like an electrical switch, i.e. closing the open circuit between the copper tracks

138

.

Referring next to

FIG. 9

of the drawings, there is shown a second electrical switch assembly

100

A embodying the invention, whose construction is in part similar to that of the first switch assembly

100

, with equivalent parts designated as shown by the same reference numerals suffixed by a letter “A”. In the second switch assembly

100

A, the central operating member

141

A for the micro-switch

110

A does not engage with the left and right operating members

146

A for the variable resistors

120

A.

More specifically, the top part

144

A of the central operating member

141

A is much shorter across and falls just completely within an upper gap between the two bodies

147

A of the side operating members

146

A, together forming a combined uppermost surface that is slightly convex. All three operating members

141

A and

146

A are individually and independently slidable with respect to the casing

101

A.

The switch assembly

100

A is located such that its said combined uppermost surface lies right against the inner surface of the deformable wall portion

12

of the food mixer

10

. Upon depression of the wall portion

12

by a user, although the operating members

141

A and

146

A are independently slidable, they will be pressed inwards practically at the same time, through a single depressing action, by reason of the wall portion

12

covering and engaging all their uppermost parts.

The operative lengths of the operating members

141

A and

146

A are made slightly different such that the central operating member

141

A will close the micro-switch

110

A before the two side operating members

146

A operate the variable resistors

120

A. The construction and operation of the variable resistors

120

A remain the same as that of the previous resistors

120

.

Compared with the first switch assembly

100

, the second switch assembly

100

A includes certain other differences. The bodies of the operating members

141

A and

146

A are relatively shorter and are supported for vertical sliding movement in individual upright cavities defined by a cradle

105

A snap-fitted from above into the casing

101

A. The circuit board

130

A is located at a relatively higher position in the casing

101

A, with the micro-switch

110

A located under the circuit board

130

A on the casing bottom wall

103

A.

Referring now to

FIG. 10

of the drawings, there is shown a third electrical switch assembly

100

B embodying the invention, whose construction is in part similar to the second switch assembly

100

A, with equivalent parts designated as shown by the same reference numerals having a different suffix letter “B”. In the third switch assembly

100

B, the central operating member

141

B for the micro-switch

110

B is engaged by the left and right operating members

146

B for the variable resistors

120

B. Similar engaging arrangement is absent from the second switch assembly

100

A but can be found, though different, in the first switch assembly

100

.

More specifically, the central operating member

141

B is much shorter than the two side operating members

146

B. The outer end of the left or upper (as shown) operating member

146

B has a first arm

146

B-

1

that extends laterally to reach over and engage from outside the central operating member

141

B. The outer end of the right or lower (as shown) operating member

146

B has a second arm

146

B-

2

that extends laterally to reach over and engage from outside the first arm

146

B-

1

and in turn the central operating member

141

B. The central operating member

141

B is thus enclosed between the two side operating members

146

B by their arms

146

B-

1

and

146

B-

2

.

The two outer ends of the side operating members

146

B together form a combined outermost surface that lies adjacent the inner surface of a rubber cover

12

B attached on the side wall at the upper end of the food mixer

10

. The cover

12

B has upper and lower regions

12

B-

1

and

12

B-

2

which are defined by three grooves

12

B-

3

in the inner surface of the cover

12

B and cover the outer ends of the upper and lower operating members

146

B respectively. Due to the presence of the grooves

12

B-

3

, together with a fixed bar

12

B-

4

engaged by the middle groove

12

B-

3

(between the two regions

12

B-

1

and

12

B-

2

) and acting as a support against depression, the two cover regions

12

B-

1

and

12

B-

2

can readily and individually be pressed inwards.

Upon depression of the lower region

12

B-

2

of the cover

12

B, the central operating member

141

B and both the two side operating members

146

B will be simultaneously pressed inwards, by reason of the aforesaid engagement of the second arm

146

B-

2

upon the first arm

146

B-

1

and in turn upon the central operating member

141

B. This will result in closing of the micro-switch

110

B and then operation of both variable resistors

120

B while the micro-switch

110

B is on, as described above. As both variable resistors

120

B come into operation to reduce the relevant resistance in the control circuit

15

, the motor can run at a speed in the full range, depending on how hard the cover region

12

B-

2

is depressed.

On the other hand, depression of the upper cover region

12

B-

1

will cause the central operating member

141

B and only the upper operating member

146

B to be simultaneously pressed inwards, by reason of the aforesaid engagement of the first arm

146

B-

1

upon the central operating member

141

B. This will result in closing of the micro-switch

110

B and then operation of the upper variable resistor

120

B while the micro-switch

110

B is on. As only one of the two variable resistors

120

B comes into operation reducing the relevant resistance in the control circuit

15

to a lesser extent, the motor can only run at a speed in the lower range, depending on how hard the cover region

12

B-

1

is depressed.

Reference is finally made to

FIGS. 11 and 12

, which show two further electrical switch assemblies

100

C and

100

D embodying the invention, both sharing the same basic concept as the three earlier embodiments

100

/

100

A/

100

B, comprising an on/off switch

110

C/D and at least one pressure-sensitive variable resistor

120

C/D. Each assembly

100

C/D includes an operating mechanism

140

C/D supported for initial movement to close the on/off switch

110

C/D and for subsequent movement, while the switch

110

C/D is on, to adjust the resistance of the variable resistor

120

C/D. The operating mechanism

140

C/D includes first and second parts

142

C/D and

147

C/D for operating the switch

110

C/D and variable resistor

120

C/D respectively, in which the first part

142

C/D has a relatively shorter operative length compared with the second part

147

C/D.

The variable resistor

120

C/D has the same construction as that of the three earlier variable resistors

120

/

120

A/

120

B and operates in the same manner, but the on/off switch

110

C/D is not a micro-switch. One switch

110

C is a press-button switch that includes a pair of fixed contacts and a moving contact arranged to short-circuit the fixed contacts. The other switch

110

D is a rocker switch including a fixed contact and a moving contact that is pivotable about a fulcrum and acted upon by a spring-loaded slider on the rear side. The slider rocks, while riding across opposite sides of the fulcrum, the moving contact into or out of contact with the fixed contact.

It should be understood that the subject switch assembly is not limited to the use in electrical appliances that incorporate an electric motor, and can be used in all types of electrical appliances as appropriate, including a torch or flashlight for example.

The invention has been given by way of example only, and various other modifications of and/or alterations to the described embodiments may be made by persons skilled in the art without departing from the scope of the invention as specified in the appended claims.