FOIL KEYBOARD FOR A PROTECTIVE DEVICE IN THE AREA OF ELECTRIC ENERGY TRANSMISSION AND DISTRIBUTION

Description

[0001] The invention relates to a membrane keypad for a protective device in the field of electrical power transmission and distribution, comprising a front decorative film, a shielding layer arranged behind the decorative film, and a printed circuit board arranged behind the shielding layer, which has a central electrode and at least one outer electrode that are at different electrical potentials, wherein a snap switch can be moved from its initial position to its contact position by means of finger pressure, and the snap switch in its contact position rests against the central electrode, from which it automatically returns to its initial position when finger pressure is omitted, in which it does not contact the central electrode.

[0002] The invention further relates to a protective device with such a membrane keyboard.

[0003] Such a membrane keyboard is known to experts and is used in Figure 1The diagram shows a schematic representation. The membrane keypad 1 depicted comprises an upper, or in other words, front decorative film 2 with a front surface that, during operation, faces a user or operator of a protective device in which the membrane keypad 1 is installed. On this front, user-facing surface of the decorative film 2, areas are marked with letters, numbers, or symbols. Below these surface areas of the decorative film 2 are switches (not shown) that can be pressed with a finger. The switches are designed as snap switches and automatically return to their initial position when the finger is removed.

[0004] Below the decorative film 2, an adhesive film 3 is visible, which is coated on both sides with an adhesive layer, so that the decorative film 2 is firmly bonded to a shielding layer 4. In the illustrated prior art, the shielding layer is designed as a shielding film 4 and consists of an insulating film material with a silver conductive coating. The shielding film 4 is firmly bonded to a circuit board 5 via two further adhesive films 3. The circuit board 5 is in turn arranged in a plastic front cap 6 of a protective device (not shown). A shielding spring plate 8, formed with elastically flexible contact tongues 7, is clamped to the rear of the front cap 6 and is conductively connected to a metallic housing of the protective device.

[0005] To protect the keyboard wiring located on the circuit board 5 from capacitive discharge displacement, the electrically conductive silver conductive lacquer layer is galvanically connected to the housing of the protective device. According to the prior art, this is achieved by folding over an edge tab of the shielding foil so that it extends across the layers at the edge to the circuit board 5, thus electrically connecting the conductive layer of the shielding foil to the shielding plate. For protection against electrostatic discharge (ESD), the circuit board 5 is provided with a circumferential conductor ring 9 that completely encircles the circuit board 5 at its edge. The conductor ring 9, which is also connected to the contact surface of the shielding layer, is conductively connected to the shielding spring plate 8 and, in the installed state, is at ground potential, or in other words, at the housing potential.

[0006] Further prior art is disclosed in US 4 461 934 A, EP 0 834 993 A2 and US 4 638 133 A.

[0007] The disadvantage of this state of the art is that the surrounding conductor ring forms an induction loop, which could lead to unintended interference with the electronics of the protective device. Furthermore, the assembly of the shielding layers with the folded edge flaps is complex.

[0008] The object of the invention is to provide a membrane keyboard of the type mentioned above that offers reliable protection against electrostatic discharges and capacitive discharge displacement, while being simple in design and therefore cost-effective.

[0009] The invention solves this problem by designing the shielding layer as a shielding sheet which is electrically and mechanically connected to metallic conductor connecting means, wherein the conductor connecting means can be connected to the earth potential, so that a low-resistance, mechanically stable current path to the earth potential is provided by means of the conductor connecting means.

[0010] According to the invention, a solid shielding plate is arranged directly behind the decorative foil, serving as protection against electrostatic discharges. Due to its position directly behind the decorative foil and in front of the circuit board, the keyboard wiring on the circuit board is protected from capacitive discharge shifts. Induction loops and the resulting unwanted electromagnetic interference are thus avoided. The circuit board can be connected to ground potential via conductor connections. During operation of the membrane keyboard, i.e., when it is installed in a protective device, these conductor connections extend between the shielding plate and the housing of the protective device, which is at ground potential. In this way, a low-resistance and mechanically robust current path is provided between the shielding plate and the housing, ensuring that electrostatic discharges can be safely dissipated to ground.

[0011] Advantageously, the conductor connecting elements have an electrical resistance of less than 0.9 ohms or preferably less than 0.5 ohms. According to a preferred further development, the conductor connecting elements are designed as dimensionally stable metallic conductors.

[0012] Advantageously, the conductor connectors are designed as metallic contact studs that protrude from the side of the shielding plate facing away from the decorative foil and can be electrically connected to a housing of the protective device. The contact studs are advantageously cylindrical or frustoconical in shape and extend longitudinally. Their ends are mechanically and electrically firmly connected to the shielding plate. The end of the stud facing away from the shielding plate is, for example, connected to an electrically conductive housing connector, which allows the membrane keypad to be connected to the housing of the protective device.

[0013] According to a particularly advantageous further development, a front cap made of an insulating material is arranged behind the circuit board, and behind the front cap is an electrically conductive shielding spring plate. During operation or intended use of the membrane keypad according to the invention, the shielding spring plate contacts a housing of the protective device, being electrically connected to the contact pins. As described above, the shielding spring plate has elastically flexible clamping tongues with which it can be clamped to the housing of a protective device. This provides a low-resistance, mechanically stable current path from the shielding plate, via the pins and the shielding spring plate, so that electrostatic discharges can be reliably dissipated despite potentially occurring high magnetic bending forces.

[0014] Advantageously, the contact bolts are designed as threaded bolts and have a thread with which they are attached to the shielding plate. Alternatively, the contact bolts are inserted into the shielding plate or riveted in place.

[0015] Advantageously, the shielding plate has a thickness between 0.05 mm and 0.3 mm or between 0.1 mm and 0.2 mm. The thickness of the shielding plate is critical within the scope of the invention. On the one hand, the shielding plate must be thick enough to reliably dissipate or prevent electrostatic discharges and capacitive discharge displacements. However, if the thickness of the shielding plate is too great, problems may arise regarding the contact of the electrodes by the snap switches, which must ensure a reliable electrical contact in their contact position while simultaneously maintaining their reset potential so that they reliably spring back from their contact position to their initial position.

[0016] According to a further development in this area, the shielding plate is made of stainless steel. Stainless steel is cost-effective, sufficiently conductive, and weather-resistant.

[0017] It is advantageous to have at least one double-sided adhesive film placed between the decorative film and the shielding plate and / or between the shielding plate and the circuit board. These adhesive films prevent unwanted horizontal displacement of the films, the plate, and the shielding plate within the film stack and fix the position of the membrane switches horizontally and vertically.

[0018] Advantageously, the shielding plate has through-holes which, when glued to the circuit board, allow the snap switches to pass through the shielding plate to contact at least one of the center electrodes. According to this embodiment of the invention, through-holes are provided in the shielding plate. In a preferred embodiment of the invention, these through-holes are projected by the snap switches, which are advantageously electrically connected to the shielding plate. The snap switches thus provide additional protection for the circuit board.

[0019] According to a preferred embodiment of the invention, each center electrode forms two sub-electrodes which are electrically insulated from each other and arranged on the circuit board, each forming a potential point of a matrix circuit. Each snap switch has a marking with which, in its contact position, the snap switch electrically connects the two center electrodes. According to the invention, pressing a snap switch electrically connects two sub-electrodes of a center electrode that are at different logic potentials. Preferably, one sub-electrode is at the potential of a row of a matrix circuit, and the other sub-electrode is at the potential of a column of said matrix circuit. Alternatively, each column of the matrix circuit is connected to the base of a transistor, the collector of which is connected to a sub-electrode. The emitter of the transistor is at ground potential.Preferably, a coupling resistor and a coupling capacitor are arranged between the collector and the partial electrode. The other partial electrode is again connected to a row of the matrix circuit via a coupling capacitor. The columns of the matrix circuit continuously switch the transistor on at a defined clock rate, so that one partial electrode is grounded at a predetermined rate. By pressing the snap-action switch, these potentials of the partial electrodes are connected together. Thus, one of the rows of the matrix circuit is also continuously and periodically short-circuited, so that the function associated with the pressed button can be assigned via a decoding circuit.

[0020] According to an advantageous further development, at least one outer electrode is at ground potential. In this further development of the invention, each base point of the snap switch is connected to an outer electrode. The snap switch is thus at ground potential in its initial position. Since its dome-shaped central section extends above the electrodes, they are additionally protected.

[0021] According to a suitable further development in this regard, four outer electrodes are provided, which surround a central electrode in a circular fashion, with each outer electrode being at earth potential.

[0022] According to the invention, two central partial electrodes are provided at logic potential. A logic potential at one of the terminals of the snap switch, as is common in the prior art, can be omitted according to the invention. Therefore, according to the invention, it is possible to connect the terminal or all terminals of the snap switch to ground potential. The snap switch itself is therefore also at ground potential in its initial position and thus contributes to the protection of the circuit board.

[0023] Advantageously, at least one partial electrode is connected to a row of the matrix circuit via a coupling capacitor.

[0024] According to a practical further development in this regard, one of the partial electrodes is connected to a transistor via a coupling capacitor and a coupling resistor, the base of the transistor being connected to a column of the matrix circuit. Each column of the matrix circuit is continuously clocked and therefore connects the collector of the transistor, which is connected to one of the partial electrodes, to the emitter of the transistor, which is at ground potential.

[0025] Advantageously, the shielding layer, designed as a shielding plate, and the snap switches are firmly connected to each other, with the snap switches spanning a recess in the shielding plate like a shield.

[0026] Advantageously, each snap switch is made of a metallic material. Preferably, each snap switch is made of stainless steel.

[0027] According to a practical further development in this regard, the snap switches are integrated into the shielding plate. In other words, the snap switches are part of the shielding plate. In this embodiment of the invention, the snap switches have foot sections that are integrally formed with a flat or planar area of ​​the shielding plate. Rising from the feet is a dome-shaped central section of the snap switch, which can be moved into its contact position by finger pressure.

[0028] Advantageously, each snap switch has several contact feet, each of which electrically contacts an outer electrode, with the snap switch extending over a dome-shaped, outwardly bulging section between the contact feet.

[0029] Further advantageous embodiments and benefits of the invention are the subject of the following description of exemplary embodiments of the invention with reference to the figures of the drawing, wherein components acting in the same way are provided with the same reference numerals and wherein Figure 1 shows a membrane keyboard according to the prior art, Figure 2 shows an embodiment of the membrane keyboard according to the invention, Figure 3 shows a shielding plate of the membrane keyboard according to the invention. Figure 2 from above Figure 4 the shielding plate according to Figure 3 from below figure 5 with the shield plate according to Figure 4 connected bolt, Figure 6 a central electrode and surrounding outer electrodes of a printed circuit board according to the prior art, Figure 7 central electrode and surrounding outer electrodes of a printed circuit board according to the invention, Figure 8 the printed circuit board according to Figure 7 with attached snap switches, Figure 9 a matrix circuit for a center electrode according to Figure 7Figure 10 shows the voltage curve of the matrix circuit after pressing a button, Figures 11 and 12 show exemplary embodiments of stamped snap switches according to the present invention.

[0030] Figure 1 shows a previously known membrane keyboard 1, which was already explained above in connection with the evaluation of the state of the art.

[0031] Figure 2 An example of the membrane keyboard according to the invention is shown. As with the prior art - compare Figure 1 The membrane keyboard comprises a stack of sheets. The outermost sheet, which faces the user or viewer during normal use of the membrane keyboard 1, is a decorative sheet 2. The decorative sheet 2 has print areas 10, each printed with a symbol, number, letter, or the like. A symbol, for example, is an upright triangle whose central point faces a specific direction.

[0032] Below each print area 10 is a Figure 2 A snap-action switch (not shown) is arranged such that applying a pressure force with a finger of the user's hand, hereinafter referred to as finger pressure, moves said snap-action switch from its initial position to a contact position. In the contact position, the snap-action switch is connected to two logic potentials.

[0033] The decorative film 2 is firmly and securely bonded to a shielding plate 11 via an adhesive film 3. According to the invention, the shielding plate 11 forms the shielding layer instead of a separate shielding film. The shielding plate 11 is made of stainless steel and has a thickness of 0.1 mm. The shielding plate 11 will be described in more detail later.

[0034] The shielding plate 11 is connected via two further adhesive films 3 to a printed circuit board 5, which has a keyboard wiring in the form of a matrix circuit and electrodes. The printed circuit board 5 is arranged in a front cap 6 made of insulating material, which is connected at the rear to a shielding spring plate 8, which has mechanically flexible and elastically spring-loaded contact tongues 7.

[0035] In the installed state of the membrane keyboard 1, the contact tongues 7 of the shield spring plate 8 are electrically connected to a metallic housing of a protective device not shown in the figure.

[0036] Figure 3Figure 1 shows the shielding plate 11 in a perspective view. In this view, the surface of the shielding plate 11 facing the decorative foil 2 is visible. The shielding plate 11 has circular through-openings 12, which allow a snap switch, in its contact position, to pass through these through-openings 12 in order to electrically connect electrodes formed on the circuit board 5.

[0037] Furthermore, contact bolts 14 are visible, which are connected to their Figure 3 The visible end 15 of the shielding plate is mechanically fixed to the shielding plate 11. One end 15 of the shielding plate of the contact bolt 14 is aligned with the side visible to the observer. Figure 3 aligned with the facing surface of the shielding plate 11, so that an overall flat or planar top of the shielding plate 11 is provided.

[0038] Figure 4Figure 1 shows the shielding plate 11 from its underside, which faces away from the decorative foil 2 in the installed state. It can be seen that the contact pins 14 protrude from the back of the shielding plate 11 and extend towards their free end 16. These longitudinally extending contact pins 14, in the installed state, pass through openings provided in the circuit board 5 and rest against the metallic shielding spring plate 8. Since, in the installed state, the shielding spring plate 8 also contacts the housing, a low-resistance current path is provided from the shielding plate 11 to the housing, so that electrostatic discharges from the shielding plate 11 are dissipated via the contact pins 14 and the shielding spring plate 8 into the housing and thus to ground potential.

[0039] Figure 5 shows the embodiment according to Figure 4in a detailed view. Here it can again be seen that the contact bolt 14, made of a metallic material such as a precious metal, is firmly connected to the shielding plate 11, which is made of stainless steel. The contact bolt 14 extends longitudinally from the back of the shielding plate to its free bolt end 16. The decorative foil 2, the adhesive foils 3, the shielding plate 11, the circuit board 5, the front cap 6, and the shielding spring plate 8 are also stacked on top of each other in this longitudinal direction. The outer surface of the contact bolt 14 has an external thread 17 for screwing on a nut to fasten the shielding spring plate 8 or directly to a housing. All contact bolts 14 of the shielding plate 11 are aligned with the one shown in the diagram. Figure 5 The contact bolt 14 shown is designed accordingly.

[0040] Figure 6Figure 1 shows a section of a printed circuit board 5 according to the prior art. A central electrode 18 is visible, surrounded by four outer electrodes 19, the outer electrodes 19 being interconnected via a circular conductor path 20. One of the outer electrodes and the central electrode are at a logic potential.

[0041] According to the prior art, the four outer electrodes 19 are each connected to the snap-action switch (not shown) via a base point. When the snap-action switch is moved from its initial position to its contact position, it is connected to the center electrode 18, thus creating a short circuit between the outer electrodes 19 and the center electrode 18. Since the center electrode 18 and the outer electrodes 19 are at the potential of a matrix circuit (not shown), a row of the matrix circuit is electrically connected to a column of said matrix circuit. A specific key can be uniquely assigned to this intersection point via a decoding circuit.

[0042] Figure 7Figure 1 shows a corresponding section of the printed circuit board 5 according to the present invention. It can be seen that the central electrode 18 consists of two sub-electrodes 21 and 22, which are separated from each other by a narrow insulating bridge 23. All outer electrodes 19 are connected to each other and are at a ground potential 20. The sub-electrodes 21 and 22, on the other hand, are at a logic potential. More precisely, one of the sub-electrodes, for example sub-electrode 21, is electrically connected to a row of a matrix circuit, and the sub-electrode 22 is connected to a column of said matrix circuit. When a snap switch arranged above the central electrode 18 is moved into its contact position, it connects the logic potentials of the sub-electrodes 21 and 22, so that a specific key, or in other words, a specific snap switch, can be identified using a decoding circuit.In this context, it should be noted that a column or a row of the matrix circuit can also be connected to a transistor or other electronic component, with one of the sub-electrodes also being connected to this electronic component.

[0043] Figure 8 The circuit board shows according to Figure 7 with a snap switch 24 arranged above the central electrode 18 and its two sub-electrodes 21 and 22. The snap switch 24 is provided with an internal embossing 25 which has two intersecting longitudinal sections. If the snap switch 24 were moved into its contact position by means of finger pressure, the sub-electrodes 21 and 22 arranged below it would be connected, which in Figure 7 are shown.

[0044] Figure 9Figure 1 shows an example of a matrix circuit. Four switches 26, 27, 28, and 29 are visible, designed as snap-action switches. Each snap-action switch 26, 27, 28, and 29 is in its initial position, in which it is at ground potential, while the partial electrode 21 below snap-action switch 26 is connected via a coupling capacitor 30 to a row 33a of a matrix circuit. The same applies to the partial electrode 21 of snap-action switch 27. The second partial electrode 22 of snap-action switch 26 is connected via a coupling capacitor 30 and a coupling resistor 31a to the collector of a transistor 32a, which is driven by column 34a of the matrix circuit. The emitter of the transistor is connected to ground potential. The same applies to the partial electrode 22 of snap-action switch 28.The partial electrode 22 of the snap switch 27 is connected via a coupling capacitor 30 and a coupling resistor 31b to the collector of a transistor 32b, which is controlled by column 32b of the matrix circuit. The same applies to the partial electrode 22 of the snap switch 29. The partial electrodes 21 of the snap switches 28 and 29 are connected via a coupling capacitor 30 to one of the rows 33b of the matrix circuit.

[0045] Figure 10The diagram shows a ordinate whose ordinate corresponds to the voltage V across series 33a. The abscissa represents time t. Furthermore, columns 34a and 34b are regularly energized, i.e., in a common clock cycle, so that transistors 32a and 32b continuously switch, causing the collector, and thus the partial electrodes 22 of switches 26 and 28, to be briefly brought to ground potential at regular intervals. If no switch is pressed, the voltage Vcc across resistor 35a remains constant. However, if switch 26 is pressed, the line is briefly brought to ground potential during the continuous switching of transistor 32a. A zero voltage therefore drops briefly across series 33a. The coupling capacitors ensure a gradual increase in the voltage until Vcc is reached again. It is advantageous not to detect the zero voltage itself, but rather a voltage between Vcc and zero, which is measured in Figure 10is denoted by V ILmax. The following applies: 0 < V ILmax < Vcc

[0046] Advantageously, V ILmax = VCC / 3. If V ILmax is detected, it is possible to deduce the location of the respective button or, in other words, the corresponding snap switch.

[0047] In Figures 11 and 12 The metallic snap switches 24 with their markings 25 are shown. While the marking of the snap switch 24 according to Figure 11 which again consists of two crossed elongated grooves, the embossing 25 of the snap switch 24 according to Figure 12a circular embossing. Within the scope of the invention, it must be ensured that the embossing is designed such that the partial contacts arranged below the embossing are effectively connected to each other by the embossing. Point contact of the center electrode, as in the prior art, is not sufficient within the scope of the invention to connect the two partial electrodes. Therefore, within the scope of the invention, the embossing forms an elongated or even planar connection with both partial electrodes. The embossing thus enables reliable contact between both partial electrodes and therefore reliable switching by the snap switches 24.

Claims

1. Membrane keyboard (1) for a protective device in the field of electrical energy transmission and distribution, having - a front decorative film (2), - a shielding layer arranged behind the decorative film (2), and - a circuit board (5) arranged behind the shielding layer and which comprises a centre electrode (21, 22) and at least one outer electrode (19) which are at different electrical potentials, wherein a snap-action switch (24) is able to be transferred from its initial position to its contact position by means of finger pressure, and the snap-action switch (24) in its contact position lies against the centre electrode (21, 22), from which it automatically returns to the initial position in which it is not in contact with the centre electrode (21, 22) in the absence of finger pressure, characterized in that the shielding layer is embodied as a shielding plate (11) which is electrically and mechanically connected to metallic conductor connecting means (14), wherein the conductor connecting means (14) are able to be connected to the earth potential such that a low-resistance mechanically stable current path to the earth potential is provided by means of the conductor connecting means (14).

2. Membrane keyboard (1) according to one of the preceding claims, characterized in that the conductor connecting means are embodied as metallic contact bolts (14) which protrude on that side of the shielding plate (11) that faces away from the decorative film and which are able to be electrically conductively connected to a housing of the protective device.

3. Membrane keyboard (1) according to Claim 2, characterized in that a front cap (6) made of an insulating material is arranged behind the circuit board (5) and an electrically conductive shielding spring plate (8) is arranged behind the front cap (6), which shielding spring plate makes contact with a housing of the protective device during operation, wherein the shielding spring plate (8) is electrically conductively connected to the contact bolts (14).

4. Membrane keyboard (1) according to Claim 2 or 3, characterized in that the contact bolts (14) are embodied as threaded bolts and have a thread with which they are fastened to the shielding plate (11).

5. Membrane keyboard (1) according to one of the preceding claims, characterized in that the shielding plate (11) has a thickness of between 0.05 mm and 0.3 mm or of between 0.1 mm and 0.2 mm.

6. Membrane keyboard (1) according to one of the preceding claims, characterized in that at least one adhesive film (3) that is adhesive on both sides is arranged between the decorative film (2) and the shielding plate (11) and / or between the shielding plate (11) and the circuit board (5).

7. Membrane keyboard (1) according to one of the preceding claims, characterized in that the shielding plate (11) has through holes (12, 13) which, in a state adhesively bonded to the circuit board (5), allow the snap-action switch (24) to reach through the shielding plate (11) in order to make contact with at least one of the centre electrodes (21, 22).

8. Membrane keyboard (1) according to one of the preceding claims, characterized in that the centre electrode (21, 22) forms two electrode elements (21, 22) which are arranged on the circuit board (5) electrically insulated from each other and which each form a potential point of a matrix circuit, wherein each snap-action switch (24) has embossing with which the snap-action switch (24), in its contact position, electrically connects the two centre electrodes (21, 22) to each other.

9. Membrane keyboard (1) according to Claim 8, characterized in that at least one outer electrode (19) is at an earth potential.

10. Membrane keyboard (1) according to Claim 9, characterized in that provision is made for four outer electrodes (19) which surround a centre electrode (21, 22) in a circular manner, wherein each outer electrode (19) is at an earth potential.

11. Membrane keyboard (1) according to one of Claims 8, 9 and 10, characterized in that one of the electrode elements (21, 22) is connected to a row (33a, 33b) of the matrix circuit via a coupling capacitor (30).

12. Membrane keyboard (1) according to Claim 11, characterized in that one of the electrode elements (21, 22) is connected to a transistor via a coupling capacitor (30) and coupling resistor (31a, 31b), wherein the base of the transistor is connected to a column (34a, 34b) of the matrix circuit.

13. Membrane keyboard (1) according to one of the preceding claims, characterized in that the shielding layer embodied as a shielding plate (11) and the snap-action switches (24) are permanently connected to each other, wherein the snap-action switches (24) are arranged over a recess (12) in the shielding plate (11).

14. Membrane keyboard (1) according to the preceding claim, characterized in that the snap-action switches (24) are embossed into the shielding plate (11).

15. Membrane keyboard (1) according to the preceding claim, characterized in that each snap-action switch (24) has a plurality of contact feet, each of which make electrical contact with an outer electrode (19), wherein the snap-action switch (24) extends between the contact feet over a coupling section that protrudes from the contact feet in a dome-shaped manner.

16. Protective and / or automation device for connecting to an electrical energy distribution or transmission network, wherein the protective device protects components arranged in the network against overcurrents and / or overvoltages, characterized by a membrane keyboard (1) according to one of Claims 1-15.