CHAPTER V. HOW TO BUILD A STEP-DOWN TRANSFORMER FOR REDUCING THE 110 VOLT A. C. FOR EXPERIMENTAL PURPOSES.
A Step-Down Transformer will reduce the 110 volt alternating current to a low voltage. These low voltage currents are still alternating but will operate almost all sorts of direct current apparatus with the exception of permanent magnet type motors which are now practically obsolete.
A suitable step-down transformer may be used to operate electrical toys, such as small fans, motors and miniature electric railway trains, ring bells, light small lamps, operate spark coils, etc., in fact the different uses are practically unlimited, depending only upon the ingenuity of the experimenter.
[Illustration: FIG. 58.—Details of the two different Pieces of Sheet Iron used in building up the Core. Sufficient of each piece are required to form a pile of each three-quarters of an inch thick.]
A step-down transformer cannot be used to recharge storage cells or for electro-plating unless the current is first passed through a rectifier.
A transformer is both in principle and construction really very simple.
It consists simply of two coils of wire wound around an iron core. One coil consists of many turns of fine wire and is connected to the 110 volt alternating current. This coil is called the *primary*. The other coil consists of a smaller number of turns of larger wire and is called the *secondary*. The lower voltage currents are obtained from the secondary.
The first step to take in making a transformer is to procure some sheet iron of the sort used for making stove pipes. It is called stove pipe iron.
This iron will have to be cut into a number of pieces like A and B in
Figure 58.
The exact shape and dimensions are most easily understood from the illustration. A somewhat resembles a three-pronged fork, while B is simply a strip 2 5/8 inches long and 1/2 inch wide.
[Illustration: FIG. 59.—The Method used in piling up the Strips to
Assemble the Core.]
A sufficient quantity of each piece will be required to make a pile three-quarters of an inch thick when tightly compressed.
The best method of procedure is to lay out one piece like "A" very accurately and then cut it out. Hammer it out perfectly flat and use it as a pattern to lay out the other pieces with. The cutting can be done with a pair of ordinary tin snips. The pieces should all be cut very accurately and truly so that they will match when piled up. The rough edges of the iron can be smoothed up with a file.
Figure 60 shows how the core should look when it is assembled. It should form a rectangle three and three-quarters inches long and two and five-eighths inches wide.
The method of piling up the *laminations*, as the pieces forming the core are called, is illustrated in Figure 59. One of the strips B is placed against the open end of A. The next set of strips is arranged in the same manner except that the position is reversed so that the strip B is at the opposite end of the pile. This method is continued, reversing each alternate set of strips until the pile is complete as shown in Figure 60. After the core has been assembled, examine it carefully and make certain that none of the strips are of the wrong size so that some protrude farther than others.
The next step in the work is to make the windings, The windings are each made on a cardboard tube having an inside diameter of about 1 1/16 inches. The tubes may be made by rolling a strip of wrapping paper around a piece of broom handle or some other round object of the proper size. The edges of the tube should be glued so that it cannot unroll.
[Illustration: FIG. 60.—Assembly of the Core.]
The *secondary winding* is an inch and nine-sixteenths long and one and thirteen-sixteenths of an inch in diameter.
It is composed of 120 turns of No. 17 B. S. gauge single cotton covered wire wound in six layers.
A tap is led out at the end of the second layer.
The *primary* is one and nine-sixteenths inches in diameter and one and one-sixteenth long. It is composed of 1040 turns of wire wound in 13 layers.
[Illustration: FIG. 61.—Details of the Primary and Secondary Windings.]
[Illustration: FIG. 62.—Showing the Core completely assembled with the Primary and Secondary in position. P, P are the Primary Terminals. 1, 2 and 3 are the Secondary Terminals.]
The windings must be carefully made and every care taken to see that they are properly insulated. It is a very good plan to insert a layer of paper between each two layers of wire.
After the windings are finished they are ready to assemble on the core. In order to put them in place, the core must be taken apart first. The primary and secondary go along side of each other on the centre tongue of the A strips.
The core is put together again with the windings in position by slipping each alternate "A" strip through from the opposite direction so that when it is all assembled the result is like that shown in Figure 62.
The transformer is now ready for mounting. The usual method of mounting a step-down transformer is to place it in an iron box or case. This is of course the best way because the windings and core are then protected. The experimenter who wishes to go to the trouble of building an iron box for his transformer will find it worth the time.
For the benefit of those however who may wish to mount the transformer on a switchboard or as part of some apparatus, the scheme shown in Figure 63 is suggested. The base consists simply of a rectangular piece of hardwood which has been given a coat of shellac.
[Illustration: FIG. 63.—The Step-down Transformer mounted on a Wooden
Base.]
The transformer core rests on two wooden strips marked "S" and "S" in the illustration. It is clamped to the base by two strips, "T" and "T," held in position by round head wood screws at each end.
The terminals of the primary winding are led to the two binding posts, marked "P" "P." The secondary terminals are connected to three binding posts, marked 1, 2 and 3.
The 110 volt A. C is connected to the primary posts P P by means of a flexible wire and plug which can be screwed into the nearest lamp socket.
The low voltage is secured from the binding posts 1, 2 and 3.
· Binding Posts 1 and 2 will give 2 volts
· Binding Posts 2 and 3 will give 8 volts
· Binding Posts 1 and 3 will give 12 volts
[Illustration: FIG. 64.—A detailed Drawing showing how the Sides of the
Case are formed by bending a long strip of Sheet Iron at four points.]
The drawings in Figures 64, 65 and 66 will be of assistance to those who wish to enclose the transformer in an iron case instead of mounting it on a board.
The case is made of sheet iron. The sides are composed of a strip of sheet iron fifteen and seven-eighths inches long and two and three-quarters inches wide. The strip should be bent at four points so as to form a rectangular box, the sides of which are four and three-sixteenth inches long and the front and back, three and nine-sixteenths.
The ends of the strip should overlap about three-eighths of an inch and be riveted or soldered.
The bottom may be fastened to the sides with a couple of rivets or by soldering at one or two points.
The bottom and top of the case should both be the same. They are made from a rectangular shaped piece of sheet iron four and seven-eighths inches long and four and one-quarter inches wide.
A notch five-sixteenths of an inch square is cut out of each corner and the sides and ends then folded down along the dotted lines as indicated in Figure 65.
[Illustration: FIG. 65.—Details of the Top and Bottom of the Case.]
The transformer is laid in the case with a small block of wood under the core at either side so as to raise the windings up off the bottom. The secondary terminals are connected to binding posts marked 1, 2 and 3 in Figure 66. These binding posts are mounted on insulating bushings which pass through the holes marked No. 1, No. 2 and No. 3 in Figure 64. The insulating bushings consist simply of fibre washers so that the binding posts will not short circuit on the metal case.
One end of a piece of double conductor lamp cord is led through a hole in the back of the case and soldered to the two primary coil terminals. The other end of the lamp cord is connected to an attachment plug which may be screwed into a lamp socket.
[Illustration: FIG. 66.—The completed Transformer.]
The transformer case is then poured full of molten coil wax so that the space around the transformer is entirely filled. After the wax has cooled, fasten the cover on and it is complete.
Arts and Science Series No. 8
Home-made
Electrical Apparatus
A Practical Handbook for Amateur
Experimenters
In Three Parts
*Volume II*
Second Edition
*BY*
*A. M. Powell*