IE51835B1 - A method of manufacturing a miniature variable inductor,and an inductor thus manufactured - Google Patents

Abstract

1. A method for manufacturing a miniature inductance which is adjustable by means of a ferromagnetic dip core and is sealed in a plastic material, method according to which a winding (5) of a wire coated with a thermoadhesive insulating material is spooled under hot air on a temporary mandrel, the temporary mandrel is withdrawn and a ferromagnetic rod (9) is realized having a smaller diameter than the winding (5) and bearing a headless screw (10) made of plastic material and moulded onto one of its ends, characterized in that the method further includes the steps of - manufacturing two contact pieces (1, 2) for the terminals (19, 20) of the winding (5), these pieces being definitely positioned one with respect to the other and constituting two opposite sides of a frame cut out of a metal strip, the two flanks of these sides being bent back onto the upper face of the strip and being attached to the residual frame by constrictions (17, 18) enabling their later separation, - tin-coating and bending the terminals (19, 20) of the winding (5), - locating the winding (5) on the frame, approximately in the definite position, and securing the terminals (19, 20) to the contact pieces (1, 2), - locating the winding in a mould having a parallelepipedic cavity (30) which is open on a face having smaller size than the frame and having two opposed lateral walls (37, 38) of the same distance as the distance between the contact pieces (1, 2) of a frame, this frame supporting the winding (5) which is located in the cavity (30) of the mould and being applied on the open face of this cavity, whereby the contact pieces (1, 2) project into the cavity (30) along said lateral walls (37, 38), - placing a bar (31) inside the mould cavity (30) through a lateral opening (39) aligned to the axis of the final position of the winding (5), the bar having a threaded zone (34) of a diameter and thread type adapted to those of the headless screw (10) which is moulded onto an end of the ferromagnetic rod (9), this zone being followed via a shoulder (33) by a smooth end stem (32), the diameter of which exceeds that of the ferromagnetic rod but is smaller than that of the winding (5) and which engages in the latter, the bar (31) providing an exact positioning of the winding (5) and a clearance space for the ferromagnetic rod (9) and for its headless screw (10), - filling the cavity (30) of the mould with a plastic filling material, - withdrawing the bar (31) by unscrewing after set-up of the plastic material, - unmoulding of the winding and mounting of the ferromagnetic rod (9) with its headless screw (10) in the recess left free by the bar (31).

Description

The present invention reiates to the manufacture of a miniature electrical component having variable self inductance obtained by means of a coil associated with a ferromagnetic core which penetrates therein to an adjustable depth.

Miniature variable inductors potted in plastic material are already available. In general, they include a large number of small components in addition to the coil and the moving core: a base member for fixing the connection terminalsj a maundrel for supporting the coil; a bracket for mounting the winding on the base member; and a set of components for supporting the core and enabling it to move inside the winding. Such small components make assembly a fastidious and expensive job. Further, such inductors generally have enough stray capacitance to make them difficult to use in systems operating at high frequencies, e.g. in 560 Mbit/s transmission systems.

Preferred embodiments of the present invention provide miniature variable inductors that are cheap and reliable, and have a minimal number of components. Such miniature variable inductorscan be made with a value of stray capacitance that is low enough for them to be used at high frequencies.

The present invention provides a method of manufacturing a miniature variable inductor potted in a plastic material and having a ferromagnetic core, said method consisting in: - taking conductor wire that is coated in thermo-adhesive insulation, winding it under hot air into a coil on a temporary maundrel, and then removing the temporary maundrel; - tinning and bending the ends of the coil; - providing a ferromagnetic rod of smaller diameter than the coil and moulding a grub screw of plastic material onto one of the ends of the rod; - providing a pair of connection pads, one for each end of the coil, the pads being formed in opposite sides of a frame cut out in a tape of metal, said sides having their edges folded upwardly from the metal tape, and said pads being held in their final positions relative to each other by the rest of the frame to which they are joined by isthmuses to enable their subsequent separation therefrom; - placing the coil on the frame approximately in its final position, and fixing the ends of the coil to the connection pads; - placing the winding in a mould having a cavity of parallelipiped shape and having an open face of smaller size than the frame, two opposite sides of the cavity being the same distance apart as the connection pads on a frame, and the frame hoiding the coil inside the mould cavity through the open face thereof with the connection pads lying inside the cavity along said opposite sides thereof; - threading a plug through the inside of the mould cavity via a aide opening lying an the axis of the final position of the coil, said plug having a threaded portion of the same diameter and thread as the grub screw that is moulded onto the end of the ferromagnetic rod, a shoulder, and a smooth end portion of greater diameter than the ferromagnetic rod but of smaller diameter than the coil, the smooth end portion being threaded through the coil and the plug serving to hold the coil accurately in position and reserving empty space for the ferromagnetic rod and its grub screw; - filling the mould cavity with a plastic potting material; - removing the plug by unscrewing it after the plastic potting material has hardened; and - unmoulding the block of hardened material and inserting the ferromagnetic rod and its grub screw in the void left by the plug.

According to a preferred implementation the mould used for potting is a flexible mould, gravity filled with pla3tic potting material, and threaded by the plug.

The present invention also provides a miniature variable inductor of parallelipiped shape potted in a plastic material, obtained using the method defined above and comprising; - two electrical connection pads each formed from a thin strip of metal folded into a channel section shape with two flanges of differing profile and height interconnected by a web, the larger flange terminating in a finger folded towards the other flange, said tabs being symmetrically disposed opposite each other on two opposite sides of the parallelepiped, the web of each pad being against the lower edge of the side on which it is found, with the larger flange being flush with the surface of said side to provide a flange surface accessible from outside the paralielipiped, and with the smaller flange penetrating into the base of the paralleliped and anchoring the pad therein; - a coil disposed inside the paralielipiped with its ends connected to the fingers of the connection pads; - the remainder of the paralielipiped being filled with plastic potting material, except for the inside of the coil and for two cylindrical access openings, one of which is tapped and of greater diameter than the coil; and - a ferromagnetic rod of smaller diameter than the coil, with a plastic grub screw moulded on one end, the diameter and thread of the grub screw corresponding to the said one of the access openings.

Preferably the large flange of each pad occupies the entire width of one of the sides of the paralielipiped, and extends half way up it.

Other characteristics and advantages of the invention appear from the accompanying claims and from the following description of one implementation given by way of example. The description is made with reference to the accompanying drawings, in which: - figure 1 is a perspective view of a miniature variable inductor with its inner parts shown dotted and with its magnetic core removed; - Figure 2 shows the fixing pads of the miniature variable inductor of Figure 1, at various stages in manufacture; - Figure 5 shows the connection pads and the coil during assembly of the miniature variable inductor of Figure lj - Figure 4 shows various stages in moulding the plastic material in which the miniature variable inductor of Figure 1 is potted; and - Figure 5 ia a section through a mould for use in moulding the plastic material in which the miniature variable inductor of Figure 1 ia potted.

The various components are shown in the figures on enlarged scales since the leading dimensions of the miniature variable inductor are less than one centimeter. The scales are different from one figure to the next, depending on the features to be illustrated.

The miniature variable inductor shown in Figure 1 is potted in plastic material and has the general shape of a rectangular parallelepiped. It comprises: two connection pads 1,2 facing each other at the bases of two opposite side faces 3 and 4 of the parallelepiped; a cylindrical coil 5 placed at the centre of the parallelepiped perpendicularly to said side faces 3,4 and having its ends connected to the connection pads 1,2; plastic material 6 filling the parallelepiped except for the inside of the coil 5 and for two access openings 7 and 8 provided in the side faces 3 and 4, one of which openings, 7, being of larger diameter than the coil 5 and being tapped; and a ferromagnetic rod 9, of smaller diameter than the coil 5, said rod being provided with a plastic grub screw 10 moulded on one end thereof, the diameter and thread of the grub screw 10 matching the access opening 7 and the grub screw having a diametral actuating slot 16.

The connection pads 1, 2 are identical. Their shape can be seen more clearly in Figure 3. Each is made from a thin strip of metal folded into a channel section shape with two flanges 11, 12 of differing profile and height, interconnected by a web 13. The larger of the flanges 11 is flush with one of the side faces of the parallelepiped, and it occupies the entire width of the lower half thereof. Its profile is substantially rectangular with a wide rounded notch 14 in the middle of its edge to ensure that the access openings 7 and 8 to the inside volume of the coil 5 are kept clear. To one end of the flange's edge there is a finger 15 bent towards the other flange and to which one end of the coil 5 is fixed. The smaller flange 12 also ha3 a rectangular profile, it is centred about the same point as the larger flange and serves to anchor the pad in the plastic potting material. Its smaller width ensures that, in the event of poor adhesion between the smaller flange and the plastic potting material, shear forces are applied to its own edges rather than to the finger 15 of the larger flange. For improved anchoring, the smaller flange may be provided with side notches.

The absence of a coil-supporting maundrel makes it possible to use a coil of smaller diameter giving rise to less stray capacitance for a given value of inductance. The fact that the connection pads are disposed perpendicularly to the axis of the coil also contributes to reducing the value of stray capacitance.

The miniature variable inductor which has just been described is connected to the tracks of a circuit by means of its connection pads, e.g. by means of a fillet of solder or by means of brackets as is common practice for other components in the form of a block with connection pads flush with their side surfaces. Manufacture of the inductor is illustrated in Figures 2 to 5 and does not require any additional assembly components.

The coil 5 is made by winding conductor wire that is coated in thermo-adhesive insulation on a temporary maundrel under hot air. After winding the temporary maundrel is removed and the ends of the coil are tinned and one of them is also bent.

The core is made from a ferromagnetic rod of smaller diameter than the coil, and from a plastic grub screw moulded onto one end of the rod, said grub screw having a diametral actuating slot.

The connection pads are made by cutting and folding a metal tape. The tape is, for example, a copper tape with a thickness of 0.1 mm, that is cut out by a photogravure technique, is tinned by an electrolytic technique, and is then folded. Figure 2 shows sections of the tape at differing stages of manufacture.

The left hand section in the Figure shows the tape immediately after cutting out. It is then ladder shaped. The connection pads are made from the strips that constitutes the rungs of the ladder. The strips are attached to the risers of the ladder via isthmuses 17, 18 that facilitate their subsequent removal therefrom. The sides of the strips are each cut out in the form of a generally rectangular tongue, one of which extends over the entire length of the rung and is provided with a finger 15 and a wide rounded notch 14 in the middle of its end, while the other is both shorter and narrower and is centred on the middle of the 'rung'. The connection tabs are grouped in symmetrically disposed pairs with the smaller tongues of each tab facing the shorter tongue of the other pad of the pair. The pads of each pair are for use in the same miniature variable inductor. Together with the 'riser' portions of the copper tape, the pads constitute a rigid frame in which the pads are held apart at the final spacing that they retain inside the miniature variable inductor.

The middle section of Figure 2 shows the copper tape after cutting out and after a first folding operation. Each ’rung' of the ladder now has its side tongues folded upwardly from the copper tape to constitute a channel section member with two flanges of differing profile and height, the larger flange 11 corresponding to the larger tongue and being on the ouside of each pair of connection pads.

The right hand section of Figure 2 shows the tape after a second folding operation in which the fingers 15 have been folded towards the inside of the connection pads.

A winding 5 is the soldered to the fingers 15 of each pair of connection pads while they remain connected to the tape and are held in their final positions relative to each other by the frame constituted by the tape. As can be seen in Figure 3, one of the ends 19 of the coil 5 is not bent, and it leaves the S1835 coil tangentially to arrive perpendicularly at the finger 15 of one of the connection pads, while the other end 20 of the coil is bent parallel to the axis of the coil 5 after leaving it tangentially, and arrives axially at its finger 15. This arrangement ensures that the coil is nearer to one of the connection pads than to the other thereby leaving enough space between the coil 5 and said other connection pad for the movement of the ferromagnetic rod and its grub screw which together constitute the moving core.

Each of the frames as constituted by a pair of connection pads and the portions of tape 21 that interconnect them, together with the coil 5 soldered thereto, is then separated from the tape for individual moulding operations in which they are potted in plastic material.

Figure 4 shows a base 41 supporting three identical moulds together with three coil-supporting frames at different stages in the moulding operation.

Each mould comprises a cavity 50 that determines the outside shape of the miniature adjustable inductor. The cavity Z0 has a plug 51 passing through it to reserve the space in which the ferromagnetic rod and its grub screw head move.

The plug 51 has a smooth end portion 52 projecting from a shoulder 55 at the end of a threaded portion 54 of larger diameter, the threaded portion is at the end of a smooth shank 35 whose other end terminates in a screw head 36. The smooth end portion 32 is of smaller diameter than the coil 5, but is of larger diameter than the ferromagnetic rod. The diameter and thread of the threaded portion 34 correspond to those of the grub screw 10 (see Figure 1), and the smooth shank has the same diameter as the threaded portion 34.

The cavity 30 is shaped like a rectangular parallelepiped. its upper face is open and two opposite side faces 37 and 38 are the same distance apart as are the pads of a pair of pads on a frame. The side faces 37, 33 are the same width as the connection pads, i.e. they are as wide as the larger flanges 11 thereof. The two side faces 37 and 38 are pierced in their centres by respective access openings 39 and for insertion of the plug 31. The entire plug 31 passes through the access opening 39 in the side face 37 which therefore has a diameter equal to that of the shank 35, while only the smooth end portion 32 passes through the access opening 40 in the side face 38, so its diameter ia smaller than that of the other opening 37 and is equal to that of the smooth portion 32.

The left hand mould in Figure 4 is shown before the frame is inserted therein. The plug 31 is removed frcm the cavity 30 and the frame is located upeidedown above the cavity 30 with the connection pad that ie connected to the bent end 20 of the coil 5 being located above the side face 37 and the other connection pad above the side face 38.

The mould in the middle of Figure 4 is shown with the coil inside it. The coil 5 is inserted into the cavity and hangs in place supported by the frame which rests on the upper edges of the cavity 30. The connection pads occupy the upper part of the cavity 30, lying against the side faces 37 and 38. The plug 31 is then inserted into the cavity via the access opening 39, and is threaded through the coil 5 and through the access opening 40. Its shoulder 33 ensures that the coil. 5 is accurately placed both relative to the moving core and relative to the connection pads, as can be seen in Figure 5. The mould is then ready for filling with a plastic potting material whic is poured in under gravity.

The mould to right of Figure 4 is shown at the end of a moulding operation. The plastic potting material has set, the plug 31 is removed from the cavity 30 by unscrewing it, and the coil together with its pair of connection pads potted in a block of plastic material are lifted out by means of the remaining portions 21 of the metal tape.

The miniature variable inductor is finished by breaking off the remaining portions 21 of the metal tape and by mounting the ferromagnetic rod and its grub screw in the void lefu by the plug 31.

It is preferable to use a flexible mould to facilitate unmoulding. The mould should then be placed in a ίο holder during moulding as is shown in section in Figure 5.

This figure shows a flexible mould 50 with a cavity 30 lodged in a rigid holder 51. Notches 52 and 53 are provided in the rigid holder 51 to enable the plug 31 to be introduced into the mould. The rigid holder 51 is provided with lateral stop 54 to ensure that the plug 31 is inserted to correct depth for its shoulder 33 to engage the coil 5 inside the cavity 30, thereby ensuring reproducible electrical characteristics for different miniature adjustable inductors of the same nominal value.

Claims (12)

1./ A method of manufacturing a miniature variable inductor potted in a plastic material and having a ferromagnetic core, said method consisting in: 5 - taking conductor wire that is coated in thermo-adhesive insulation, winding it under hot air into a coil on a temporary maundrel, and then removing the temporary maundrel; - tinning and bending the ends of the coil; - providing a ferromagnetic rad of smaller diameter than the 10 coil and moulding a grub screw of plastic material onto one of the ends of the rod; - providing a pair of connection pads, one for each end of the coil, the pads being formed in opposite sides of a frame cut out in a tape of metal, said sides having their edges folded 15 upwardly from the metal tape, and said pads being held in their final positions relative to each other by the rest of the frame to which they are joined by isthmuses to enable their subsequent separation therefrom; - placing the coil on the frame approximateiy in its final 20 position, and fixing the ends of the coii to the connection pads; - placing the winding in a mould having a cavity of parallelipiped shape and having an open faca of smaller eize than the frame, two opposite aides of the cavity being the same 25 distance apart as the connection pads on a frame, and the frame holding the coil inside the mould cavity through the open face thereof with the connection pads lying inside the cavity along said opposite sides thereof; - threading a plug through the inside of the mould cavity via a 30 side opening lying on the axis of the final position of the coil, said plug having a threaded portion of the same diameter and thread as the grub screw that is moulded onto the end of the ferromagnetic rod, a shoulder, and a smooth end portion of greater diameter than the ferromagnetic rod but of smaller 35 diameter than the coil, the smooth end portion being threaded through the coil and the plug serving to hold the coil accurately in position and reserving empty space for the ferromagnetic rod and its grub screw; - filling tha mould cavity with a plastic potting material; - removing the plug by unscrewing it after the plastic potting 5 material has hardened; and - unmoulding the block of hardened material and inserting the ferromagnetic rod and its grub screw in the void left by the plug.

2. / A method according to claim 1, wherein the coil is placed on 10 the frame so that its axs is perpedicular to the pads.

3. / A method according to claim 1 or 2, wherein the mould used for potting is a flexible mould.

4. / A method according to claim 1, 2 or 3, wherein the mould used for potting is placed in a rigid holder with notches to 15 enable the plug to be inserted into the cavity of the mould and provided with a aide stop serving to determine the depth to which the plug is inserted in the mould.

5. / A method according to any preceding claim, wherein the mould cavity is gravity filled. 20

6. / A method according to claim 1 of manufacturing a miniature variable inductor potted in a plastic material and having a ferromagnetic core, said method being substantially as herein described with reference to figures 2 to 5 of the accompanying drawings. 25

7. / A miniature variable inductor of parallelipiped shape potted in a plastic material, obtained using the method of any preceding claim and wherein the inductor comprises: - two electrical connection pads each formed from a thin strip of metal folded into a channel section shape with two flanges 30 of differing profile and height interconnected by a web, the larger flange terminating in a finger folded towards the other flange, said tabs being symmetrically disposed opposite each other on two opposite sides of the parallelepiped, the web of each pad being against the lower edge of the side on which it is found, with the larger flange being flush with the surface of said side to provide a flange surface accessible from outside the parallelepiped, and with the smaller flange penetrating into the base of the paralleliped and anchoring the pad therein; - a coil disposed inside the parallelepiped with its ends connected to the fingers of the connection pads; - the remainder of the parallelepiped being filled with plastic potting material, except for the inaide of the coil and for two cylindrical access openings, one of which is tapped and of greater diameter than the coil; and - a ferromagnetic rod of smaller diameter than the coil, with a plastic grub screw moulded on one end, the diameter and thread of the grub screw corresponding to the said one of the access openings.

8. / An inductor according to claim 7, wherein the larger flange of each connection pad is flush with a side of the parallelepiped, and occupies the lower half thereof over its entire width.

9. / An inductor according to claim 7 or 8, wherein the profile of the larger flange of each connection pad is substantially rectangular with a wide rounded notch in the middle of its edge.

10. / An inductor according to claim 7, 8 or 9, wherein the smaller flange of each connector pad has side notches to improve its anchorage in the plastic potting material.

11. / An inductor according to claim 7, 8, 9 or 10, wherein the coil is disposed inside the parallelepiped perpendicularly to the connection pads.

12. / A miniature variable inductor of parallelepiped shape potted in a plastic material, obtained using the method of any one of claims 1 to 6 and substantially as herein described with reference to and as illustrated in the accompanying drawings.