Construction details (Coil Design and Construction)

Preparing a Bobbin.

Unless a bobbin of the correct size is available ready-made, it will be necessary to fabricate one. Details are given here of two types that are simple to construct without special tools.

Fig. 14 illustrates the method with the first type, which is suited for the smaller sizes, as it gives adequate strength to support a small winding, whilst taking up little of the available winding space. It should be constructed from stiff cartridge paper, or similar material.

Five pieces should be cut, according to the final required dimensions, one as at (a), and two each as at (b) and (c). A center block, as at (d), will be required to support the bobbin while winding, and may also be used in making up the bobbin. Great care should be taken in the construction of this center block, to ensure that all its faces arc "square", and to the correct dimensions (very slightly larger than the core cross-section, and a little shorter than the window length, about 1/64". Also, to see that the hole drilled through it is absolutely parallel to its sides. The method of bending each of the parts is clearly shown in sketches (c), (f) and (g) of Fig. 14. At each stage the parts of the bobbin should be carefully glued so that the whole bobbin is united by glue, but is not stuck to the center block, as this has to be removed after winding. After the glue has set, small holes may be drilled or punched in the cheeks of the bobbin, so that the winding leads may be brought out and properly anchored.

Finally, end support plates will be needed during winding. These should be made of metal or wood, and secured in position by the center spindle as shown at (h). The holes drilled in both the center block and the end support plates should be only just the required clearance hole for the size of center spindle to be used-say, 2 B.A.

Nuts on the center spindle are used to secure the whole assembly, and should be tight enough to secure that the spindle will not turn by itself inside.

Fig. 15 shows a method of construction for larger bobbins, where the fabricated bobbin would not be strong enough to support the winding. It is made of bakelized paper or cloth sheet, about 1/16" thick. It has the advantage that all the parts can be cut from sheet-no tube is required-and that the "tags" on two of the center pieces prevent the cheeks from falling off during or after winding.

Two pieces each are required as sketched at (a), (b) and (c). Necessary holes in the cheeks for lead -outs and anchoring may be made before the bobbin is assembled. The method of assembly is clearly shown by sketches (d), (e) and (f). A center block and end support plates will be required, to mount up for winding. The center block will be exactly the same in form as that shown at Fig. 14 (d), but the end support plates must be different, being only simple rectangular pieces with a hole in the middle, of such dimensions that they fit conveniently between the end "tags" of the center-pieces. A spindle will again be required, but for the larger sizes should not be relied Ron to turn the bobbin during winding. It is suggested that one or two additional small holes be made in the end plates and center block, through which steel pins should be inserted to provide means of obtaining a positive drive to the bobbin.

Improvising a Winding Machine.

Two fundamental arrangements arc necessary for successful winding: a means of rotating the bobbin, and some provision for holding the reel of wire. A lathe makes a very successful winding machine. For the smaller sizes a standard three or four jaw chuck can be used to grip the center spindle. For larger sizes. the pins already inserted into the center block may be arranged to take a drive by bearing against the jaws of the chuck.

If a lathe is not available, an ordinary wheel-brace mounted in a vice so that the chuck rotates in a horizontal position, will serve as a good substitute. It will be found rather laborious, operating by hand, if a great many turns are required.

A spindle should be set up in a horizontal position a little distance away to hold the reel of wire so that it is free to rotate as wire is required. The wire should be passed between the thumb and first finger of one hand to steady it and to apply the necessary tension.

It is a great convenience to provide some means of counting turns. If a proper turns counter is not available, a cyclometer can be used as a good substitute. The turns counter should be coupled to the machine so that it numbers upwards in the direction of rotation when winding. The winding direction should be such that the wire goes on to the upper side of the bobbin. If an improvised turns counter is used, it may not register coincident with turns--i.e., it may take 5 turns to register each 1. This should be checked up before it is used, and the required readings at start and finish worked out in advance so that all attention can be devoted to winding.

Methods of Winding.

Wire gauges of, say, 24 and larger may be brought out of the bobbin direct, and a length wound around either the center spindle or some convenient peg to keep it out of the way while winding, until it can be terminated after winding is complete.

Wire gauges, of say, 26 and smaller should be carefully joined by soldering to a piece of silk -covered flex, taking care that a neat, flat joint is made that will not take up too much room, and will not cut through and cause short-circuited turns. It should be insulated at the joint by means of a small piece of insulating material. The silk flex should come out through the hole in the bobbin, and should make about two turns round the bobbin before the proper wire gauge "takes over". The silk flex should be anchored conveniently to await proper termination after winding.

All windings should be wound so that one turn lays as close as possible to its neighbor, until a layer is full, when another layer should be commenced in the return direction. On larger size coils, a layer of paper insulation will be inserted every layer, or perhaps every few layers, to prevent a turn from a high layer from slipping down into contact with lower layers. On smaller coils, and particularly with the very small gauges (beyond, say 36), it is not possible to insulate the layers in this way, and a method known as "random" winding is employed. The turns still go on approximately in layers, but it is not possible to guarantee that no space is lost between adjacent turns of the same layer, and so later turns may fill spaces left previously. In winding by this method care should be taken that the winding builds up level along the whole width of the bobbin, otherwise useful winding space will be wasted. For this reason it is especially important, too, that the bobbin shall rotate "true". This means that when the center is spun before winding is commenced, the four sides of the center must turn parallel with the spindle, and not show any sign of a skew wobble, and further the cheeks must not show any sign of wobble from side to side.

At the finish of the winding, the end should he brought out in the same way as the beginning was, according to the gauge being used.

Insulation.

Between windings, as well as between the layers on larger sizes.

a layer or two of insulation must be provided. Two or three layers of very thin material are better than one layer of thicker material.

This should be cut to the exact width between the bobbin cheeks, and wound on carefully and tightly over the winding. The insulation may be of thin high-quality paper, or may be of one of the acetate substitutes. Before proceeding with a further winding, the insulation should be firmly secured in position by the use of a little adhesive.

For paper, some Chatterton's compound, or, as an alternative, some high-quality wax, may be used to fix the last turn of the paper to the preceding one by applying quickly after heating momentarily on the butt of a soldering iron. If one of the acetate films is used, a little acetate may be quickly applied to weld the film, but care must be taken to see that no acetate comes into contact with the wire if it is enamel insulated. as the acetate may dissolve or soften the enamel and cause turns to short.

Terminating the Windings.

After winding is complete, a layer of insulation should be wound on to cover the windings and insulate them from possible contact with the outside limb of the laminations, Then all the ends of windings should be properly terminated. The ends should each be threaded through the pairs of holes in one of the corners of the bobbin several times. Before so threading, the whole length that will go through the holes should he stripped of insulation. The threading should allow the portion of lead from the hole where it comes out of the bobbin to the corner where it is threaded to lie slack. If it is tight, it may break later, especially if the bobbin cheek is slightly flexible and may bend. After threading in this way, the end should be quickly tinned with a soldering iron, which will secure it, and also provide a form of tag to which the external leads can be soldered.

Laminating and Finishing

Care must be taken when inserting the laminations that they do not damage the winding. For a choke, or a transformer there is D.C. flowing, all the E -shaped pieces. or T-shaped pieces, should be inserted first from the same side, until the center of the bobbin is full of laminations. An equal stack of I- or U-shaped pieces should then be taken, and brought into contact with the E's or T's with the appropriate gap spacing. Some form of clamps will be necessary to bold the whole core together and keep the gap tight up to the spacing used. These can easily be improvised, using an appropriate width of strip metal which can be drilled for clamping bolts either side of the laminations, and possibly the end of the clamps can be turned over to form mounting feet for the completed component.

For transformers with no D.C. the laminations of different shapes should be inserted from opposite sides of the bobbin in pairs. A convenient way of doing this quickly is to arrange a small stack of each shape on each side of the bobbin. Then, by working with both hands, alternate pairs may he picked up and inserted quite quickly.

When laminating is complete, similar clamping arrangements to those suggested for the choke may be employed.

Ordinary transformer iron is very subject to attack by rust, so it is a good plan. when the component is complete, to paint the exposed edges of the core with a good quality paint as a protection against rust.

The clamps may be bent at one end to provide feet for mounting, and at the other end to take a bakelized sheet panel on which soldering tags or terminals are mounted to make a finished terminal board for the transformer or choke.