A FULL-WAVE B BATTERY ELIMINATOR FOR HOME CONSTRUCTION

Radio Record, Volume I, Issue 13, 14 October 1927, Page 14

A FULL-WAVE B BATTERY ELIMINATOR FOR HOME CONSTRUCTION

A CONSTANT AND PLENTIFUL SUPPLY OF HIGH TENSION AT LOW RUNNING COST

This B battery eliminator has been designed to provide a constant and reliable source of high-tension power to suit the average receiving set, The full-wave principle has been adopted om account of the greater output it gives, aud also for the greater smoothness of working, which is an important point for DX reception. The principle of using two separate rectifying valves has been adopted because @ burn-out of one filament onlv costs the price of a single rectifier, which is considerably less than a double valve. The filament secondary to tfie transformer is tapped in such a way that several voltages can be obtained to suit various makes of rectifier valves, so that it is not mecessary that only one particular make should be nsed. On the other hand, if it is desired to use a double valve, only a very slight alteration will make it possible at any time. Then, again, using the full-wave puts less strain upon the two valves than would be placed upon a single one, so that filament life is longer, as they need not be run so bright, vet giving greater output. Three different voltages are provided for, ranging from about 160 volts to 35 volts. The eliminator is designed to work off alternating current of 50 eyeles at 230 volts By providing a tap at the 1300th turn of ‘the vrimary winding, and using these turns only, it would work off 105 volts as 4 temporary expedient until changed ovet to 230 volts, as wonld he the case at the present time in some parts of Wellington, The inclusion of a air of radio chokes is of special value in subduing strav noises caused bv radio-frequency interference. Fach voltage output is protected bv a fuse. Bo that there is no danger of destrovine a filament in case of an accidental "short" in the set. And ‘n order to protect the fine wire of the transformer secondary windings, a fuse is also provided. on each ontput tead close to its lead-out from the tratisformers THE TRANSFORMER. This is the most important part of the eliminator, so it will he described first. The laminated core is built up of crdinary tin plate, which is quite good soft iron for the purpose. The onlv difference between using this and stailov is that rather more wire must be used in the windings in order to compensate for the lessened magnetic flux densitv in the core ‘There ate three separate windings, that for the filaments consisting of two lavers of No. 18’s double cotton-covered wire. 60 to 64 turns in each laver, tapned im nine places. The primary winding is next, and thronch this the alternating current supplv flows Tt consists of 2600 turns of 30’s enamelled wire,

fo taps being required, except that at the 1300th turn already mentioned, in case it is desired to work temporarily off 105 volts. The third winding is that supplying, the high-tension current, which passes to the valves for rectifi cation, consisting of 5000 turns of 34’s enamelled wire. tapped at the 2500the turn. Fach 2500 turns gives about 260 volts, which leaves a_ good working voltage after the dron of about 30 volts in the rectifving valves. The filament tappings are brought out to a small distribution panel which allows of anv available voltage heing utilised for the rectifier filament, and also provides for the insertion of a short piece of resistance wire by means of which the voltage supplied to each filament will be equal, thus allowing of contro] by a single rheostat for the two filaments. THE CENTRAL FORMER, The tuilding of the transformer is eommenced hy making a wooden core upon which to build a spool and wind the wire. This wooden core measures 54 inches long by 2 inches wide and Li inches thick, and must not exceed these dimensions, This core is built up of pieces as will be shown in a

| diagram, a piece of three-eigliths dowel 9 or 10 inches long running through the centre and secured by screws, This is to act as a spindle when winding the thin primary and secondary wire, so it must be arranged that the dowel is fairly centrally placed. A handle is provided as shown. ‘This former is now covered with two layers of thick manilla paper such as that used for folders in offices, and this is secured with glue or seccotine in such a manner that it may be slipped off at a later stage when the winding is completed. ‘The square edges of the former may have just the sharpness taken off. Now for the ends of the transformer spool. They are made of strips of heart rimu or other good wood, shaped as shown in the diagram, the long pieces five-eighths wide, three-eighths thick, and the short pieces just over half that thickness, the ends of the long pieces being ‘halyed" so that the short pieces will be let in flush. The short pieces are drilled at each end to take a_ small brass screw. These spool ends are assembled by glueing the inner edge and sticking them to the manilla, which must be made to stay nice and fiat. When the four sides are in position and glued at the halved corners also, the four small screws are put in carefully on the inside surface without splitting the wood. The long end pieces should first be drilled for taps as shown and marked. The spool ends are so placed on the manilla that their inside surfaces are from 4 1-16th inches to 4 1-8th inches apart. About two dozen strips of good writing paper 4 1-16th inches wide and about 11 inches long will he required to place between layers of wire, and if these are accurately cut by a printer they can be wrapped round the core and used as a gauge for getting the position of the second spool end, with the advantage that the paper strips will fit accurately at the ends when used. When the glue has properly set, the snool is ready for the winding of the filament turns, which is done by hand, owing to the 18’s s.w.g. double cottoncoveted wire to be used. THE FILAMENT WINDING. The first layer of this is commenced by passing tle end of the 18’s wire from inside through the hole marked "Centre" in first layer side of spool end, leaving six inches projecting, then winding on the wire as shown with solid line for the first turn. Take care to make every turn sit snugly against the previous one, and nice sharp turns at the corners. A flatfaced hammer is of great assistance in making the turns lie flat by tapping them with sufficient force. When the 80th turn is reached, the first tap must he attached in the position shown in the diagram. ‘These taps are required to lie as flat as possible in order not to

throw the winding out of shape, so three 22’s enamel wires are cleaned at the ends, hooked over the 18’s, the cot}ton being scraped away at that place, and the three wires soldered in place. The joint is now shellaced and a piece of tissue paper wrapped round, then a strip of 3-inch adhesive tape is taken, folded at the joint so as to tun above and below the tap wires, right up to their ¢xit through the hole marked 30, Ist layer, a few inches being left projecting. Taps on all the windings are treated in the same way as regards insulation with the double strip of tape. This insulation of taps must be carefully done so that the enclosed wires are unable to contact anv others, especially at the inside of the spool ends. | Proceed with the winding, tapping as shown in the diagram at 40, 45, and the last turn, which may be held with thin twine passed round and tied. When this layer is complete, the whole is well shellaced, and when dry a layer of empire cloth is carefully placed all round with an overlapping joint and no spaces at the ends. ‘Then the’ winding of the second layer is proceeded with, but (a yery important point must he noted in comunencing this layer. The two layers

teally forin one continuous winding with the same effect as if a tap were soldered on at the centre and led out. ‘This is not done, as it is more convenient to make each layer separate, and connect the two windings outside the spool. ‘fhe important point is to remember in which direction the first winding goes when followed in through the tap. If the first winding goes to the right, the second must go to the left, and if the second goes to the left, the second must go to the riglt-that is, each wire must go the opposite way from the inside of the centre tap hole. When 30 turns have been put on the second layer, a tap is taken out on the opposite side of the spool to where thie first layer taps are taken, and directly in line with the lead-out hole marked 80. Other taps follow in the same maniter. The total number of turns should be the same as on the first layer. If this would leave a space, fill it by winding in twine; if it comes a turn short, ‘let it go at that. Trom 60 to 64 turns will be put on each layer, according to the closeness with which they are wound. The second layer is given a goad coat of shellac, and when drv is covered with two layers of empire cloth coming well

. . ° ? up at the ends, and then a layer of paper. THE PRIMARY WINDING may now be proceeded with, and in order to be able to rotate the spool, a stand must be made as shown, collars of some kind being slipped over the dowel ends to keep the spool running easily without end-play. . The wire to be used is 30’s s.w.g, enamelled, run on as evenly as possible with a layer of paper between each layer of wire. The end of the wire is passed out through the hole marked "In P," and winding is proceeded with by turning away from you with the right hand, guiding the wire on with the left Very soon it will be found that if the wire is held a few inches away from where it meets the spool it can be run on very evenly at a good speed, provided the reel containing the supply is running on a spindle in a convenient position. Tf it is mecessary to make a joint in the wire, it must be carefully ed and covered with sheliac and tissuepaper or other insulation. If possible, get the assistance of a friend to turn the spool during winding, so that attention may be concentrated on the wire alone; this will add to the speed at which the winding may be done. This wire winds about 70 to the inch, so making a small allowance for loss of space at ends, about 270 turns will go on a layer, so that ten layers will be necessary to take the 2600 turns. "When the required turns have been wound, a lead-out of thicker wire, say 24’3 d.c.c., should be soldered to the 30’s, given a turn round the spool, and then led out at "Out P."’ This attaching ‘of a strong lead-in and out wire should be carried out on both the primary and high-tension secondary windings. The primary winding is then covered with two layers of empire cloth and then a layer of paper to give a smooth surface for the hightension wire

Whilst winding is proceeding, all projecting tap wires are to be coiled up round the spindle and tied, so that they do not get caught and damaged or broken off. THE HIGH-TENSION WINDING is the mext matter for attention. The wire to be used is 34’s s.w.g. enamelled, ‘There will be fourteen layers of this wire, each separated by a layer of good paper. At the seventh layer the 2500th turn will be reached, and must be provided with a tap, hich could be two 26’s cotton-covered wire twisted together and soldered on. The joint must be covered with shellac and tissue paper, or other insulation, The tap must go out through the hole marked ‘‘centre-tap’? on the high-ten-sion end of spool. In case the winding is not yet completed where the tap las to cross to the hole, the tap must be twisted up and secured until the layer is completed, when it can be placed betwen tape and led out. Great care must be exercised in making a good job of this tap joint, because the wire is thin, and getting to it afterwards is a matter of difficulty. ‘The remaining seven layers can now be put on, the wire led out through HY2. The winding may now be given a good coat of paraffin wax or resin and beeswax in equal parts, melted and put on with a brush. Two layers of empire cloth are then put on, and may be finished oft with an outside covering of dark book-cloth. During the winding of the fine wire great care must be taken at the ends of layers to prevent any wire sinking down to come in contact with wires in the previous layer. A good way to prevent this is to put on at the ends a round or two of fine twine, securing it with seccotine, and thus fill up any space into which th. wire might sink. THE DIAGRAMS. A view of the eliminator as it looks when completed is shown. ‘The front panel, on which are placed the three resistances, rheostat, fuses, and connecting terminals, is ebonite, whilst the remainder of the case is of tin, to which is imparted a smart finish with black cycle enamel. Another view gives a geucral idea of the interior arrangement, the transformer divided off from the remainine components bv a

wooden ‘partition lined with tin, to isolate the’ huin as much as possible. At the rear of the transformer is a sinall distribution panel, from which varying voltages may be’ taken to suit different valves, and provision is made here for the insertion of a piece of resistance wire to regulate the current for each valve filament. Behind the partition are the two rectifying valves, the bank of condensers, tall and narrow, and thie three «’ ~kes,

Nar he not in view. On top of the trausformer a small piece of ebonite or fibre holds the fuses for each side of a.c., and each output high-tensiou

lead, which will save a burn-out of the windings in case of an accidental ‘short.’ Further diagrams connected with the former, aud winding of the transformer, will appear next week. The finislied size of the case is twelve inches wide, ten high, and ten inches deep.