GB2327812A - Bi-stable bridge switch - Google Patents

Abstract

An electrical switch 10 comprises a first fixed terminal 12 having a first contact surface; a second fixed terminal 12 having a second contact surface angled with respect to the first contact surface; a pivotally mounted bridge 20 having spaced apart contact engaging surfaces and which is moveable between a first engaged position in which the contact engaging surfaces respectively engage the first and second contact surfaces simultaneously thereby providing a circuit between the first and second contacts, and a second non-engaged position; and a bridge actuator 26 for effecting movement of the bridge 20 between these positions. The switch 10 is bistable and provides a large air gap when in its non-engaged position.

Description

This invention relates to electrical switch mechanisms.

BACKGROUND Electrical switches are widely used both domestically and in industry. Many designs have been conceived and are produced serving different needs and markets.

Commonly switches employ mechanical means to selectively engage (close) and disengage (open) conducting elements referred to as contacts.

Important features of electrical switches include: the size and number of air gap(s) between contacts and the mechanical action between contacts at the point of their engagement and disengagement.

Larger air gaps are desirable to minimise arcing. Multiple air gaps are desirable since they result in more rapid arc extinguishment which is particularly important for highly inductive loads (such as provided by fluorescent lights). The mechanical action between contacts at the point of their engagement and disengagement influences the degree of selfleaning and the degree of contact bounce.

In practice, it is also desirable to produce switches which are compact, reliable, and simple to manufacture. It is in this area that many switch designs are lacking.

Another problem with existing switches is control over their movement Many switches are mechanically stable in a position (or positions) between fully open and closed. This can result in small air gaps and associated arcing.

It is therefore an object of the invention to provide a compact switch with a configuration of elements that when open, provides at least two air gaps, the cumulative size of which is large.

It is a further object of the invention to provide a compact switch which is substantially self cleariing.

It is a still further abject of the invention to provide a switch which minimizes contact bounce and arcing.

It is a still further object of the invention to provide a switch which is bi-stable, that is, a switch which is only stable in an open or a closed position.

SUMMARY OF THE INVENTION In its broadest form the invention is an electrical switch comprising a first fixed terminal having a first contact surface; a second fixed terminal having a second contact surface angled with respect to said first contact surface; a bridge having spaced apart contact engaging surfaces and which is movable between a first engaged portion in which said contact engaging surfaces respectively engage said first and second contact surfaces simultaneously thereby providing a circuit between said first and second contacts, and a second nonengaged position in which the contact engaging surfaces are respectively spaced from said first and second contact surfaces; and a bridge actuator for effecting movement of said bridge switch between said positions.

Preferably contact surfaces of the first and second fixed termiflais define an included angle which is acute.

Preferably the bridge is pivotally mounted.

Preferably there is interposed between the bridge and the bridge actuator a compression spring arranged such that the bridge is bi-stable.

A specific embodiment of the invention will now be described in some further detail with reference to and as illustrated in the accompanying figures. This embodiment is illustrative, and is not meant to be restrictive of the scope of the invention BRIEF DESCRIPIION OF TE DRAWINGS Fig. 1 is a plan view of a switch with its actuator removed according to the preferred embodiment of the invention in its open (off) position.

Fig. 2 is a sectional view "A-A" of the switch shown in Fig. 1.

Fig. 3 is a plan view of a switch with its actuator removed according to the preferred embodiment of the invention in its closed (on) position.

Fig. 4 is a sectional view "B-B" of the switch shown in Fig. 3.

Fig. 5 is an exploded perspective view of the switch shown in the preceding figures, but with the actuator shown Fig 6 is a perspective view of the bridge component of the switch shown in the preceding figures.

Fig 7 is an isometric view of the actuator of the switch shown in Figs 1-5.

Fig 8 is a rear view of the actuator of Fig 7.

Fig 9 is a sectional view of the actuator of Fig 7 through plane B-B.

DEI DESCRIPTION OF AN EMBODIMENT OF THE INVENTION A preferred embodiment of the invention is a double breaking switch as shown in Figs 1-9 and comprises a moulded plastic base 11 supporting two brass fixed terminals 12, each supporting a contact surface 13; a movable bridge 20 pivotally mounted within a recess 19 in the base 11; a compression coil spring 25; and a pivotally mounted moulded plastic actuator 26. Also shown in Figs 1-5 is a cradle 21, this being an optional component, which in practice has been found to be unnecessary.

The non-parallel contact surfaces 13 are preferably dome-shaped as shown in Figs 1 and 3 and define an included angle of approximately 50 degrees between tangents through their centres. The bridge 20 shown in Fig 6 comprises a central limb having a proximal end terminating in a straightedge 23; a pair of oppositely inclined, laterally projecting lugs which form contact engaging surfaces 22 that are electrically connected to each other and remote from the proximal end; and a distal end 24 adapted to receive and locate the end of a coil spring 25. Straight edge 23 rests loosely within recess 19 allowing bridge 20 to pivot The acute angle between the contact engaging surfaces 22 (Producing a wedge-shape) matches the acute angle between contact surfaces 13 to allow good contact to be made when the switch is closed.

Preferably the bridge 20 is formed from a single copper blank and is silver or silver plated over a continuous region including the region arranged to engage contact surfaces 13.

Each terminal 12 has a cylindrical hole 14 providing through entry for up to two A5 square millimetre cables as shown in Fig. 5. Screws 15 (which engage threads tapped into termirls 12) are provided to anchor the cables securely within holes 14. The orientation of the holes 14 is such that the cable wiring is through the side of the switch. This is advantageous in many applications, particularly where switches are required in narrow wall cavities.

The switch is actuated by a rocker actuator 26. Rocker actuator 26 has two protrusions 27 which engage holes 28 within the side walls of base 11. The side walls 11a of base 11 are constructed such that they have sufficient flexibility and resilience to allow outwards deformation during assembly of the switch Specifically, they are able to deform to allow insertion of protrusions 27. As shown in Figs 7,8 and 9, actuator 26 also has U-shaped spring surround 28 and a protrusion 29 which extends towards movable bridge 20.

Compression coil spring 25 is mounted between the upper end 20a of bridge 20 and actuator 26 as shown in Fig. 5. This arrangement results in a snap-acting switch with a bistable bridge and hence a switch which is only highly stable in either an open position (as shown in Figs 1 & 2) or a closed position (as shown in Figs 3 & 4). This minimises 1;he possibility of an in-between condition arising in which arcing is more prone to occur.

In its open position, an air gap is created between each contact 13 and the contact engaging surfaces 22 of bridge 20 (the cumulative air gap being in excess of 3 mm) creating an extremely high electrical resistance to current flow between the two contacts 13.

In its closed position, the bridge engages with both contacts 13 resulting in an electrical circuit being formed between the two contacts 13.

It is anticipated the switch 10 will most often be installed in a manner such that the axis about which the actuator 26 rotates is horizontal; the actuator face 26a is vertical when the actuator is in its (unstable) middle position; and in its open (off) position the face 26a is facing slightly upwards. The operation of the switch will now be described based on this orientation.

To operate the switch, the operator pivots the actuator 26 upwards or downwards by pressing on face 26a.

To move the switch from its open (off) position to its closed (on) position, the operator pushes down on face 26a causing actuator 26 to rotate about an axis through holes 28 working against compression coil spring 25 until bridge 20 snaps across in an arc about its pivot within recess 19 to its dosed position as shown in flogs 3 & 4. Protrusion 29, which extends from actuator 26 towards the distal end 24 of the movable bridge 20, is orientated such that it applies a lateral load to coil spring 25 as actuator 26 is pivoted downwards to ensure that the above described snap action occurs.

The wedge-shaped bridge, arranged to move in an arc, has a number of advantages.

It allows the switch to be compact while at the same time providing two large air gaps. It also creates a geometry which promotes self cleaning of the contacts 13 and the corresponding contact engaging surfaces 22 to occur each time the switch is open and closed. The wedge shape also helps reduce contact bounce.

The switch has numerous applications for both alternating and direct currents. It has been found to be particularly effective in switching alternating current circuits of domestic voltages (110 Volts to 250 Volts). The large double air gaps make the switch particular effective in circuits with high inductive loads such as those provided by fluorescent lights.

While the present invention has been described in terms of a preferred embodiment in order to facilitate better understanding of the invention, it should be appreciated that various modifications can be made without departing from the principles of the invention Therefore, the invention should be understood to include all such modifications within its scope.

Claims (11)

The claims defining the invention are as follows:

1. An electrical switch comprising: a first fixed terminal having a first contact surface; a second fixed terminal having a second contact surface angled with respect to said first contact surface; a bridge having spaced apart contact engaging surfaces and which is movable between a first engaged portion in which said contact engaging surfaces respectively engage said first and second contact surfaces simultaneously thereby providing a circuit between said first and second contacts, and a second nonengaged position in which the contact engaging surfaces are respectively spaced from said first and second contact surfaces; and a bridge actuator for effecting movement of said bridge switch between said positions.

2. An electrical switch according to claim 1 wherein the contact engaging surfaces of said bridge define an included angle which is acute.

3. An electrical switch according to claim 2 wherein said acute angle is substantially matched by the included angle defined between said first contact surface and said second contact surface of the fixed terminals.

4. An electrical switch according to claim 3 wherein said acute angle is between 30 degrees and 70 degrees.

5. An electrical switch according to daim 3 wherein said acute angle is between 40 degrees and 60 degrees.

6. An electrical switch according to any one of daims 1 to 5 wherein said bridge is pivotally mounted allowing movement of its contact engaging surfaces through an arc between said first position and said second position.

7. An electrical switch according to claim 6 wherein said bridge comprises: a central limb having a proximal end terminating in a straight edge, and a distal end adapted to receive and locate the end of a coil spring, said coil spring being compressed between said bridge actuator and said distal end of said bridge in such a way as to cause a snap action in the movement of said bridge, and a pair of oppositely inclined, laterally projecting lugs which form said contact engaging surfaces remote from said proximal end.

8. An electrical switch according to daim 6 or 7 wherein said actuator is hinged about an axis substantially perpendicular to the direction of said movement

9. An electrical switch according to any one of claims 6 to 8 wherein said bridge is stable only in said first position and in said second position

10. An electrical switch according to any one of the preceding claims wherein said bridge when in its second non-engaged position, forms an air gap between each contact surface and its associated bridge contact engaging surface, the sum of the gaps having a minimum length of 3 millimetres.

11. An electrical switch as hereinbefore described with reference to and as illustrated in the accompanying drawings.