Printed circuit board, electrical switch and machine tool

Abstract

Printed circuit board (10) for an electrical switch of a driven machine tool (100) with a drive unit (114), wherein the electrical switch serves to adjust a control variable of the drive unit (114), for example to set a speed of the drive unit (114), comprising: at least one contact area (12), wherein the contact area (12) has at least two contact segments (14, 16, 18) spaced apart from each other on or are arranged on the circuit board (10) and each has a contact surface with a metallic material, wherein the circuit board (10) has at least one conductive surface (20) in at least one area adjacent to the contact area (12) for electrically connecting the at least two contact segments (14, 16, 18) to each other, wherein at least one conductive surface (20) comprises a carbon-containing material; and wherein at least one of the at least two contact segments (14, 16, 18) has a plurality of recesses (24) which create a mesh-like structure, wherein the recesses (24) are bounded by a plurality of meshed connecting webs (26) which form the remaining contact area on the at least one contact segment (14, 16, 18).

Description

[0001] The invention relates to a printed circuit board for an electrical switch, in particular an actuating switch of a driven machine tool, for example a drilling machine. The invention further relates to an electrical switch with a corresponding printed circuit board and to a machine tool with such a switch.

[0002] In switches known from the prior art, a switching operation typically involves pressing a contact spring against a mating contact by actuating a switching element, such as a rotary knob or a button, or by using sliding contacts. To ensure reliable and fail-safe switching over many years, the contacts, usually designed as spring contacts, and the mating contacts must be designed to be wear-resistant. Currently, one known solution is to "refine" both the contacts and the mating contacts, for example, by a gold or silver plating. However, due to high material costs, silver- or gold-plating the contacts / middle contacts is currently comparatively expensive.

[0003] German patent application DE 100 37 155 B4 describes an electrical switch with an actuating lever that acts on two control levers, each of which has an electrical contact area formed, for example, from a carbon pill or an elastic metallic area. When the actuating lever is actuated, the contact areas press against a printed circuit board.

[0004] German patent application DE 100 26 352 A1 discloses an electrical switching device comprising a push button that can be pressed onto a raised switching area of ​​an elastic switching mat. A contact bridge is provided on the inside of this switching area, which, when the push button is pressed, electrically bridges two corresponding contacts on the circuit board. The contact bridge is made of metal and carbon.

[0005] Another solution for providing a switch with a simple design and high wear resistance is disclosed in DE 10 2005 014 934 B3, which is also published in the Fig. 1 and Fig. 2 is shown.

[0006] The electrical switch disclosed therein comprises at least one contact spring with at least one elastic spring tongue, a switching element acting on the at least one contact spring, and at least one mating contact on a printed circuit board associated with the at least one spring tongue. The at least one contact spring and the at least one spring tongue are integrally formed from stainless steel, and the at least one mating contact on the printed circuit board is formed from a carbon layer.

[0007] This switch replaces the previously complex process of silver- or gold-plating a mating contact with a carbon layer applied to a printed circuit board. A key advantage of using this carbon layer is the current cost savings compared to silver- or gold-plating a conventional mating contact. Furthermore, the carbon layer exhibits very high mechanical resistance, ensuring the switch's functionality over extended periods.

[0008] Although the carbon layer has a lower conductivity than, for example, a gold or silver layer, during switching operations of the solution according to DE 10 2005 014 934 B3, only a "quasi-digital" shaft signal (on / off) needs to be evaluated, and not a signal intensity. Therefore, the lower conductivity is of minor importance for a switch that toggles between different switching states (on / off).

[0009] The contact spring with its tongue is made of stainless steel; it is therefore uncoated and not further refined. Furthermore, the mating contact is printed onto the circuit board with a carbon layer thickness of approximately 5 to 12 µm. The printing process, common in circuit board technology, provides a simple method for applying the mating contact.

[0010] According to one possible solution, the circuit board with the integrated mating contact is simultaneously a control board with an integrated circuit pattern. Therefore, no separate circuit board is provided for the mating contacts. Instead, the mating contacts are directly integrated into a complex switching or control board, which, in addition to conductor tracks, may also contain individual control modules or components. The circuit pattern consists primarily of untreated conductor tracks, for example, made of copper.

[0011] Previous solutions used a separate circuit board for each mating contact, where the corresponding short conductor tracks were gold-plated or silver-plated. This circuit board with the mating contacts was then electrically connected to a control board, for example, by soldering a connecting wire or using a plug connector. This somewhat more complex method with the separate circuit board was chosen because otherwise, due to process limitations, all conductor tracks on the control board would have had to be coated.

[0012] In contrast, the application of the carbon layer, particularly using printing technology, is now a circuit board-specific manufacturing process, so that the carbon layer is easily integrated into the manufacturing process of the circuit board and can be applied only in the desired areas.

[0013] In the solution described in German patent application DE 10 2005 014 934 B4, a switching element is designed, for example, as a rotary knob, which is provided on its underside with nubs that, depending on the position of the switching element, press a respective contact spring against the corresponding mating contact. The contact springs are bent towards their free end and spring tongues. Due to the bend and the spring-elastic design, the free end with the spring tongues slides slightly over the contact surface of the mating contacts, thus forming an electrical contact. When actuated by means of the switching element, the spring tongues not only touch the mating contact but are also displaced under pressure on the mating contact, thereby achieving good surface contact between the contact spring and the mating contact.

[0014] Furthermore, it is also known in practice to implement an electrical switch, in particular an actuating switch for a machine tool, using a printed circuit board on which carbon pads for electrical contact with a contact spring or a sliding contact are printed. The carbon pads are connected to different resistors on the back of the circuit board.

[0015] The machine tool has a housing with an externally accessible operating switch. The operating switch, which the user presses with their index finger (to increase the speed by applying increasing pressure, a so-called "trigger switch"), passes over the circuit board of the switch inside the machine tool housing, for example with a spring-loaded wiper, thereby switching different resistances into the closed circuit depending on the position.

[0016] A disadvantage of this well-known solution is that, although the carbon surfaces exhibit higher abrasion resistance than gold or silver, they are still sensitive and wear down quickly due to repeated grinding and the vibrations of a machine tool. This results in undesirable speed fluctuations for the user, which are problematic, for example, during drilling or similar applications.

[0017] From publication DE 43 35 715 A1, a potentiometer is also known which has a plurality of spaced-apart metallic conductor tracks bridged by a film resistor, dividing the film resistor into a series of partial resistances. An associated wiper does not tap the partial resistances directly on the film resistor, but rather via the conductor tracks, so that even mechanically less resistant film resistors can be used while still achieving a long service life for the potentiometer.

[0018] From the publication DE 30 41 944 A1, a switch for a hand drill with potentiometer tap is also known.

[0019] Finally, reference is made to the publication DE 80 11 367 U1, which discloses a variable resistor with a resistance characteristic printed on a substrate.

[0020] In contrast, one object of the present invention is to at least reduce the existing disadvantages of known solutions.

[0021] Accordingly, the invention proposes a printed circuit board for an electrical switch of a driven machine tool with the features of claim 1. Furthermore, the invention also relates to an electrical switch with the features of claim 9 and a machine tool with the features of claim 14.

[0022] The machine tool according to the invention comprises a drive unit and an electrical switch for adjusting a control variable of the drive unit, for example, for setting the speed of the drive unit. Such an exemplary speed controller (also called a "trigger switch") can output a control variable, such as the speed, to the control electronics of the drive unit via an analog signal.

[0023] The electrical switch is connected to the drive unit in a known manner and comprises a printed circuit board with at least one contact area (resistive conductor). Furthermore, the switch includes at least one actuating element for manual operation by a user, and a sliding contact comprising at least one mating contact area.

[0024] The sliding contact is designed to move in sliding contact across the contact area of ​​the printed circuit board (PCB) as a result of manual actuation of the actuator by the user. Furthermore, the sliding contact, at least in the mating contact area, is made of a metallic material, at least partially, to connect to the at least one contact area of ​​the PCB. The sliding contact can be made entirely of one or more metallic materials, for example, copper or copper alloys, or of several metallic material layers, such as a nickel underlayer and a copper plating, or only partially of a metallic material, for example, a non-metallic material with a metal-containing coating in the mating contact area (area of ​​a mating contact surface), such as a gold-hard alloy coating.

[0025] The sliding contact can be designed as a contact spring with one or more spring tongues or as a rigid sliding element and has at least one mating contact surface with which it can make contact with the at least one contact area of ​​the printed circuit board on the (sliding) surface. In the case of multiple spring tongues, for example, two sets of three spring tongues, which can extend in opposite directions in pairs, multiple mating contact surfaces of the springs (two sets of three mating contact surfaces) are provided. The spring contacts do not have to be arranged one behind the other in the direction of movement of the sliding contact, i.e., aligned with each other, but can also be arranged side by side or offset from each other, which makes the sliding contact more compact and saves space. The material of the slider, in particular the spring base material, can include any resilient and conductive materials.The geometry of the sliding contact, in particular the spring base geometry, can also include, for example, individual rounded and springy wires (so-called "multi-wire wiper"). The mating contact surface(s) can have a rounded outline with one or more radii.

[0026] In a known manner, depending on the position of the sliding contact relative to the contact area, a change in electrical resistance, and thus an adjustment of the control variable, such as the speed of the drive unit, can be achieved. The output signal of a corresponding switch is linearly proportional to the length of the contact area of ​​the circuit board up to the contacting surface with the mating contact of the sliding contact.

[0027] By incorporating at least one contact area that is at least partially made of a metallic material, such as copper, a robust and highly conductive mating contact surface is provided. However, this metallic mating surface also results in increased wear. Accordingly, practical experience has shown that in a commonly used carbon contact track (consisting of a soft, conductive carbon paste with a hard, non-conductive epoxy resin), the soft carbon particles are worn away over time, at least in some areas, due to fretting corrosion. The resin remains in these areas, leading to partial interruptions in the current flow and thus affecting, for example, the control variable settings.In known solutions where the speed of the drive unit is controlled, an undesirable drop in speed can occur due to the control electronics.

[0028] According to the invention, to overcome this known problem, it is further provided that the at least one contact area of ​​the circuit board has at least two contact segments (segments of the resistance conductor) which are spaced apart from each other on or at the circuit board and each have a contact surface with a metallic material.

[0029] The contact surface can therefore be made entirely of one or more metallic materials, for example copper or copper alloys, or of several metallic material layers, such as a nickel underlayer and a copper coating, or only partially of a metallic material, for example a non-metallic material with a metal-containing coating in the area of ​​the contact surface, for example with a gold hard alloy coating.

[0030] Furthermore, according to the invention, it can be provided that the printed circuit board has at least one conductive surface for electrically connecting the at least two contact segments to each other in at least one area adjacent to the contact area, wherein the conductive surface comprises a carbon-containing material.

[0031] A key feature here is that the contact segments are initially designed separately and spaced apart from one another. This corresponds to a cascade solution, as is known from the prior art. In known solutions of this type, the individual contact segments are electrically connected to each other via tap points provided in an adjacent area and connecting conductor tracks. The output signal in these solutions is cascaded proportionally and thus stepped.

[0032] Unlike known solutions from the prior art, in the invention the electrical connection of the individual contact segments is not achieved via additional tap points that have to be connected to each other (usually such a connection is provided by resistors on the back of the circuit board), but via at least one conductive surface comprising a carbon-containing material.

[0033] In this way, a solution can be provided that enables a durable and cost-saving design of an electrical switch without external fixed resistors.

[0034] In this design, at least one conductive surface can comprise a carbon layer applied to the printed circuit board, particularly one printed on it. The conductive surface can, for example, consist of a soft, conductive carbon paste with a hard, non-conductive epoxy resin. Unlike in the prior art, the conductive surface is not traversed by the sliding contact, thus eliminating the known problem of fretting corrosion, which can cause the soft carbon particles to be worn away in certain areas. At the same time, the conductive surface represents a particularly simple and cost-effective solution for electrically connecting two or more contact segments of the contact area.

[0035] The sensitive carbon-containing conductive surface thus serves as an electrical resistance material, but is no longer subject to wear, which increases the service life compared to known solutions.

[0036] At least one guiding surface can be formed in a single adjacent area, for example, on one side next to the contact area (one-sided design of one or more guiding surfaces). Alternatively or additionally, several guiding surfaces can be provided in one or more adjacent areas. For example, one or more guiding surfaces can be formed on both sides next to the contact area (two-sided design of one or more guiding surfaces).

[0037] According to a further development of the invention, at least one of the at least two contact segments can comprise an insert made of a copper-containing material and be inserted into the printed circuit board. Alternatively, individual or all contact segments can also comprise a printed metal layer.

[0038] Alternatively or additionally, it can be provided that the at least two contact segments and the conductive surface are arranged on the same side (i.e., front or back) of the (flat) printed circuit board. This again simplifies manufacturing.

[0039] Alternatively or additionally, the contact segments can be designed to be crescent-shaped or annular-arc-shaped. A known design features several parallel, strip-shaped contact segments (viewed from above on the circuit board) arranged, for example, at an angle of 45 to 60 degrees relative to the direction of movement.

[0040] In further development of the invention, instead of such a known solution, the contact segments can also be designed as crescent-shaped or annular arc-shaped – viewed from a top view of the circuit board. The radii of such crescent-shaped or annular arc-shaped contact segments can be designed and arranged such that the at least one mating contact surface of the wiper, particularly if it also has rounded edges, is applied to the contact segments with one or more opposing radii in order to further reduce wear.

[0041] Alternatively or additionally, it can be provided that the at least two contact segments are arranged parallel to each other in a repeating pattern on or around the circuit board.

[0042] According to the invention, at least one of the at least two contact segments has at least one recess, in particular a plurality of recesses, which create a mesh-like structure, wherein the recesses are bounded by a plurality of meshed connecting webs which form the remaining contact area on the at least one contact segment.

[0043] In this case, at least one recess can be polygonal, in particular hexagonal.

[0044] The recesses serve two main functions: firstly, to reduce the contact area and thus the risk of fretting corrosion. This reduces wear and increases service life. A further advantage arises when combined with a lubricant. At least one recess can be designed to hold a lubricant, particularly a lubricating paste. Such a lubricant can further reduce friction and thus fretting corrosion. Additionally, it binds wear-related debris, which can then be pushed to the end positions and to the side by the moving sliding contact. The at least one recess thus has a cleaning effect; with each actuation, the sliding contact is cleaned as it passes over the at least one recess (self-cleaning).

[0045] At least one of the recesses acts as a grease reservoir, allowing the lubricant to collect there as well. Metallic oxides on the sliding contact can be broken up by the structure formed by the numerous recesses, thus preventing a significant increase in the contact resistance between the sliding contact and the contact section.

[0046] In test series, recesses shaped like squares, pentagons, hexagons, heptagons or octagons with a minimum interior angle of at least 90 degrees have proven to be particularly advantageous.

[0047] The recesses create a mesh-like structure, with the recesses bordered by connecting ribs that form the remaining contact surfaces on the contact segments. These have a minimal extent (rib width).

[0048] The recess(s) can be arranged in an orientation relative to the direction of movement of the sliding contact such that none of the connecting webs are arranged parallel to the direction of movement of the sliding contact.

[0049] Furthermore, it has been shown that dimensioning the at least one recess is advantageous such that the remaining contact surfaces (connecting webs) on the contact segments have a minimum extent (web width) of at least 125 µm for sufficiently good conductivity. It has also proven advantageous if the recesses have a maximum extent (maximum diameter) of approximately twice the minimum extent of the remaining contact surfaces on the contact segments, for example, a maximum extent of less than 250 µm.

[0050] In an advantageous embodiment, a multitude of polygonal recesses are provided, which form a multitude of interconnected connecting webs.

[0051] With a bridge width of at least 125 µm and a maximum recess length of 250 µm, the polygonal recesses comprise approximately 1 / 3 of the respective contact segment, while the bridges themselves comprise approximately 2 / 3. This results in a lubricant clearance of approximately 33% of the contact area. Due to the orientation of the recesses relative to the direction of movement of the sliding contact, the sliding contact, guided over the polygonal contact segments, maintains multiple contact points with a multitude of interconnected bridges. This ensures a stable electrical signal despite the large clearance area of ​​33%.

[0052] Alternatively or additionally, the distance between the at least two contact segments in the direction of movement of the sliding contact can be provided with a maximum extent that is smaller than the extent of the at least one mating contact area of ​​the sliding contact in the direction of movement of the sliding contact, in particular less than or equal to half the extent of the mating contact area in the direction of movement of the sliding contact. Thus, despite the mechanical gaps between two contact segments, no drop in the output voltage occurs because the sliding contact is never in an electrically free field.

[0053] Alternatively or additionally, the actuating element may include a slide switch designed to move the sliding contact translationally relative to the circuit board.

[0054] Alternatively or additionally, the printed circuit board may also include a capacitor designed to act as an RC filter together with the conductive surface.

[0055] As explained above, in solutions with multiple contact segments, the output signal is cascaded proportionally and therefore stepped. A simple RC (resistor-capacitor or low-pass) filter can further smooth the output voltage, reducing dips and spikes.

[0056] An innovation according to this further development of the invention is that the resistance component of the RC filter does not have to consist of a conventionally mounted resistance component, but can also consist of a small carbon-containing resistance point, which is significantly cheaper to manufacture.

[0057] Furthermore, the invention also relates to an electrical switch for adjusting a control variable of a drive unit associated therewith, for example for setting a speed of the drive unit of a driven machine tool with the features according to one of claims 9 to 13.

[0058] Finally, the invention also relates to a driven machine tool according to claim 14.

[0059] It should also be noted that terms such as "comprehensive," "exhibit," or "with" do not exclude other characteristics or steps. Furthermore, terms like "a" or "that," which indicate a singular set of steps or characteristics, do not exclude a plurality of characteristics or steps, and vice versa.

[0060] Further features and advantages of the invention will become apparent from the following description of an exemplary embodiment of the invention and from the dependent claims.

[0061] The invention is described in more detail below with reference to the accompanying figures. The figures show several features of the invention in combination with one another. Of course, a person skilled in the art can also consider these features separately and, if necessary, combine them into further meaningful sub-combinations without having to make an inventive step.

[0062] They show schematically: Fig. 1 an exemplary driven machine tool which may be equipped with an electrical switch according to the present invention; Fig. 2 a top view of a printed circuit board of an electrical switch according to the present invention in a first embodiment; and Fig. 3 a top view of the structure of a contact area of ​​a printed circuit board of an electrical switch according to the present invention in a second embodiment.

[0063] The Fig. Figure 1 shows an example of a driven machine tool which may be equipped with an electrical switch according to the present invention.

[0064] The driven machine tool 100 shown is a handheld machine tool comprising a main body 102 and a gripping unit 104. For descriptive purposes, the handheld machine tool 100 has a top side 106, a bottom side 108, a body side 110, and a gripping side 112, wherein the main body 102 is arranged on the body side 110 and the gripping unit 104 is arranged on the gripping side 112.

[0065] A working tool, not shown here, such as a chisel or drill bit, can be attached to the main body 102, which is only partially shown. The main body 102 comprises a drive unit 114 for driving the working tool and an operating element 116 for setting different operating modes (drilling or drilling with a chisel) of the drive unit 114. The drive unit 114 is in Fig. 1 is shown set to a first operating mode.

[0066] Furthermore, the main body 102 includes a locking element 118 which can be moved by means of the control element 116, as well as a guide element 120, on the upper side 106 of which the locking element 118 rests and slides or is guided during its movement in order to block the control element 116 - which here is designed as a rotary switch.

[0067] The gripping unit 104 comprises an electrical switch with an actuating element 130, a damping element 132, and a locking device 134. The switching element 130 serves to switch the drive unit 114 on and off and has a pivot axis 136 and a recess 138. The damping element 132 surrounds the guide element 120 arranged on the main body 102, with respect to which the damping element 132 has a damping-effective range of motion. Together with the pivot joint 128, the damping element 132 and the guide element 120 form a vibration-damped mounting of the gripping unit 104 on the main body 102.

[0068] The locking device 134 comprises a locking button 140, a locking hook 142, and a locking spring 144, designed here as a helical compression spring, to lock the actuating element 130 in its specific position for greater ease of use. The locking button 140 has a recess 146, the upper surface 106 of which is bounded by an inclined wall of the locking button 140. A second contact surface 148 is formed on this wall, which faces the body side 110 and the lower surface 108.

[0069] As already mentioned, the actuating element 130 serves to switch the drive unit 114 on and off. For this purpose, in the exemplary embodiment, the actuating element 130 is pivotably connected to the gripping unit 104 by means of the pivot axis 136. To switch on, the actuating element 130 can be moved from an off position 150 to an on position 152 by pushing it towards the gripping side 112 against a restoring force from a compression spring (not shown). To switch off, the actuating element 130 can be returned from the on position 152 to the off position 150 either by releasing it or by means of the restoring force.

[0070] Furthermore, the rotational speed of the drive unit 114 can be varied depending on the position of the actuating element 130. For example, in a fully depressed position, the maximum rotational speed of the drive unit can be set, while in a slightly depressed position of the actuating element 130, the rotational speed of the drive unit 114 is low, and in the released or unpressed position (off position 150) it is minimal, i.e., equal to zero.

[0071] The rotational speed is adjusted via an inventive design of the actuating element 130 of the electrical switch, as follows.

[0072] The electrical switch comprises the actuating element 130, a sliding contact connected to the actuating element 130 (not shown) and a circuit board 10 (see figure). Fig. 2) According to the invention, at least one contact area 12 is formed on the circuit board 10, which can be electrically contacted by the sliding contact with at least one mating contact area. During the switching movement of the actuating element 130, the sliding contact passes over the contact area 12 of the circuit board 10 with its mating contact area in a direction of movement B, so that, depending on the position of the sliding contact relative to the contact area 12, a percentage of the operating voltage is tapped. The speed of the drive unit 114 is then adjusted according to the tapped operating voltage.

[0073] According to the invention, the contact area 12 has at least two contact segments 14, 16, 18 (in the illustrated embodiment of the Fig. 2. It has a multitude of contact segments 14, as well as a contact segment 16 and a contact segment 18 at the end. These are made of a metallic material or are at least partially made of a metallic material, such as copper. The contact segments can, for example, be designed as inserts that are embedded in the circuit board 10.

[0074] Furthermore, one can recognize from the Fig. 2, that the contact segments 14, 16, 18 are arranged at a distance from one another. The distance between two adjacent contact segments 14, 16, 18 can be chosen such that it has a maximum extent in the direction of the movement B of the sliding contact that is less than the extent of the at least one mating contact area of ​​the sliding contact in the direction of the movement B of the sliding contact, in particular less than or equal to half the extent of the mating contact area in the direction of the movement B of the sliding contact. Thus, despite the mechanical gaps between two adjacent contact segments 14, 16, 18, no drop in the output voltage occurs because the sliding contact is never in an electrically free field.

[0075] The contact segments shown 14 of the Fig. 2 are furthermore designed as crescent-shaped or annular arc-shaped – viewed from a top view of the circuit board 10. The two end-side contact segments 16 and 18 also have a convex (contact segment 16) or concave (contact segment 18) contour adjacent to the contact segments 14. The radii of curvature of the curved contour sections of the crescent-shaped or annular arc-shaped contact segments 14 and of the contact segments 16 and 18 can be designed and arranged such that the at least one mating contact surface of the wiper, particularly if it also has rounded edges, can be applied to the contact segments 14, 16, 18 with one or more opposing radii in order to further reduce wear.

[0076] The contact segments 14 are arranged parallel to each other in a repeating pattern on or around the circuit board.

[0077] Furthermore, contact segments 14, 16, 18 have at least one recess 24 (cf. Fig. 2 and Fig. 3), in the embodiment shown, a plurality of recesses, which are arranged in a repeating pattern on the contact segment and thus form a structure of the contact segment.

[0078] In the embodiment shown, the Fig. 2. These are rectangular (square), while the recesses in the alternative embodiment of the Fig. 3 hexagonal, is formed.

[0079] The recesses 24 serve, firstly, to reduce the contact area of ​​the contact segments against the sliding contact and thus reduce fretting corrosion. This reduces wear and increases service life.

[0080] Another function is to hold a lubricant, in particular a lubricating paste. Such a lubricant can further reduce friction and thus fretting corrosion. Additionally, it binds wear-related abrasion particles, which can be pushed to the end positions and to the side by the moving sliding contact. The at least one recess 24 thus has a cleaning effect; with each actuation, the sliding contact is cleaned as it passes over the at least one recess (self-cleaning).

[0081] At least one of the recesses 24 acts as a grease reservoir, allowing the lubricant to collect there. Metallic oxides on the sliding contact can be broken up by a structure formed by the numerous recesses, thus preventing a significant increase in the contact resistance between the sliding contact and the contact section.

[0082] In test series, recesses shaped in a square, pentagonal, hexagonal, heptagonal or octagonal shape 24 with a minimum interior angle of at least 90 degrees have proven to be particularly advantageous.

[0083] The recesses 24 create a mesh-like structure, with the recesses 24 being bounded by connecting webs 26. The connecting webs 26 are formed by the remaining contact surfaces on the contact segments 14, 16, 18 and have a minimum extent 26a (web width).

[0084] The recess(s) 24 are in the illustrated embodiments of the Fig. 2 and Fig. 3 be arranged in such a way relative to the direction of movement B of the sliding contact that none of the connecting webs 26 are arranged parallel to the direction of movement B of the sliding contact.

[0085] Regarding the exceptions 24 of the Fig. 3. The maximum diameter 24a of the recess 24 can be less than or equal to 500 µm, in particular less than or equal to 300 µm, for example less than or equal to 250 µm. Furthermore, the web width 26a of the connecting webs 26 can be greater than or equal to 50 µm, in particular greater than or equal to 100 µm, for example greater than or equal to 125 µm for sufficiently good conductivity.

[0086] In an advantageous embodiment, a multitude of polygonal recesses are provided, which form a multitude of interconnected connecting webs.

[0087] With a web width 26a of 125 µm of the connecting webs 26, the polygonal recesses 24 of the Fig. The recesses 24 comprise approximately 1 / 3 of the respective contact segments 14, 16, 18, while the connecting webs comprise approximately 2 / 3 of the respective contact segments 14, 16, 18. This results in a clearance for the lubricant of 33% of the contact area. Due to the orientation of the recesses 24 relative to the direction of movement B of the sliding contact, the sliding contact, guided over the polygonal contact segments 14, 16, 18, has multiple contact points with a multitude of the interconnected connecting webs 26. This creates a stable electrical signal, despite the large clearance area of ​​33%.

[0088] In the Fig.Figure 2 shows that, in addition to the contact area 12 of the printed circuit board, at least one, and in the illustrated embodiment two, conductive surfaces 20 are formed, which run along the contact area 12. The conductive surfaces 20 comprise a carbon-containing material and serve for the electrical connection of the individual (cascaded) contact segments 14, 16, 18.

[0089] In this way, a solution can be provided that enables a durable and cost-saving design of an electrical switch without external fixed resistors.

[0090] In this arrangement, at least one conductive surface 20 can comprise a carbon layer applied to the printed circuit board 10, in particular a printed layer. The conductive surfaces 20 can, for example, consist of a soft, conductive carbon paste with a hard, non-conductive epoxy resin. Unlike in the prior art, the conductive surfaces 20 are not traversed by the sliding contact, thus eliminating the known problem of fretting corrosion, which can cause the soft carbon particles to be worn away in places. At the same time, the conductive surfaces 20 represent a particularly simple and cost-effective solution for electrically connecting two or more contact segments 14, 16, 18 of the contact area.

[0091] The sensitive carbon-containing conductive surface 20 thus serves as an electrical resistance material, but is no longer subject to wear, which increases the service life compared to known solutions.

Claims

[1] Printed circuit board (10) for an electrical switch of a driven machine tool (100) with a drive unit (114), wherein the electrical switch serves to adjust a control variable of the drive unit (114), for example to set a speed of the drive unit (114), comprising: at least one contact area (12), wherein the contact area (12) has at least two contact segments (14, 16, 18) spaced apart from each other on or are arranged on the circuit board (10) and each has a contact surface with a metallic material, wherein the circuit board (10) has at least one conductive surface (20) in at least one area adjacent to the contact area (12) for electrically connecting the at least two contact segments (14, 16, 18) to each other, wherein at least one conductive surface (20) comprises a carbon-containing material; and wherein at least one of the at least two contact segments (14, 16, 18) has a plurality of recesses (24) which create a mesh-like structure, wherein the recesses (24) are bounded by a plurality of meshed connecting webs (26) which form the remaining contact area on the at least one contact segment (14, 16, 18). [2] Printed circuit board according to claim 1, wherein at least one of the recesses (24) is polygonal, in particular square, pentagonal, hexagonal, heptagonal or octagonal. [3] Printed circuit board (10) according to claim 1 or 2, wherein at least one of the recesses (24) is designed to receive a lubricant, in particular a lubricating paste. [4] Printed circuit board (10) according to one of claims 1 to 3, wherein at least one of the at least two contact segments (14, 16, 18) comprises an insert with a copper-containing material and is inserted into the printed circuit board (10). [5] Printed circuit board (10) according to one of the preceding claims, wherein the at least one conductive surface (20) comprises a carbon layer applied to the printed circuit board (10), in particular printed on it. [6] Printed circuit board (10) according to one of the preceding claims, wherein the at least two contact segments (14, 16, 18) and the at least one conductive surface (20) are arranged on the same side (10a) of the printed circuit board (10). [7] Printed circuit board (10) according to one of the preceding claims, wherein the contact segments (14, 16, 18) are crescent-shaped or annular-arc-shaped or, viewed from a top view of the printed circuit board (10), have at least one curvature contour. [8] Printed circuit board (10) according to one of the preceding claims, wherein the at least two contact segments (14, 16, 18) are arranged in a repeating pattern parallel to each other on or at the printed circuit board (10). [9] Electrical switch for adjusting a control variable, for example a rotational speed, of an associated drive unit (114) of a driven machine tool (100): the printed circuit board (10) according to one of claims 1 to 8, at least one actuating element (130) for manual actuation by a user, and a sliding contact connected to the actuating element (130), which has at least one counter-contact area designed to as a result of manual actuation of the actuating element (130) by the user, in sliding contact over the contact area (12) of the circuit board (10), wherein the sliding contact is formed at least in the counter-contact area for contacting the contact area (12) at least sectionally from a metallic material. [10] Electrical switch according to claim 9, wherein the distance between the at least two contact segments (14, 16, 18) has a maximum extent in the direction of the movement (B) of the sliding contact which is less than the extent of the at least one counter-contact area of ​​the sliding contact in the direction of the movement (B) of the sliding contact, in particular less than or equal to half the extent of the counter-contact area in the direction of the movement (B) of the sliding contact. [11] Electrical switch according to one of the preceding claims 9 or 10, wherein the actuating element (130) comprises a slide switch which is configured to move the sliding contact translationally relative to the circuit board (10). [12] Electrical switch according to any one of the preceding claims 9 to 11, wherein the recesses (24) of the circuit board (10) are arranged in an orientation relative to the direction of movement (B) of the sliding contact such that none of the connecting webs (26) are arranged parallel to the direction of movement (B) of the sliding contact. [13] Electrical switch according to any one of the preceding claims 9 to 11, wherein the circuit board (10) further comprises a capacitor configured to act as an RC filter together with the conductive surface (20). [14] Driven machine tool (100) with a drive unit (114) and an electrical switch for adjusting a control variable of the drive unit (114), for example for setting a speed of the drive unit (114), according to one of claims 9 to 13.

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