A FULL-WAVE B BATTERY ELIMINATOR FOR HOME CONSTRUCTION

Radio Record, Volume I, Issue 14, 21 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

(Second Instalment.)

‘the description of this full-wave B eliminator commenced last week with a description of making the bobbin of transformer and winding the same with the three necessary different windings. Itlustrations of the former on which the bobbin is formed are given below, aud also a sketch of a rough stand abont seven inches long with two sides in which are slots to take spindle of spool when winding. Before going further it is necessary «o call attention to the circuit diagram published last week. ‘Through the wrong continuation of a line, the detector and LF outputs appeared to be connected together. There is a 2 mfd condenser joined to the detector output, and the line continuing up above the right-angle turn to the detecier terminal is to be marked cut. Lock up the diagram and mark this. THE LAMINATED CORE. The cutting of the laminations can now be proceeded with, and for their construction six sheets of ordinary tinplafe 20 by 28 inches are required. Give each side of the sheets a good coat of shellac before cutting up. When this is dry, mark out as shown, which plan gets fourteen pieces out of one sheet without waste. These could be cut up by the tinsmith on his guillotine and all wonli then be even in size The laminations consist of T’s and U's, one of each being made from each 7 by 6Bjin. piece as shown in the diagram, the two strips which come out of the ‘windows’ being waste. The cutting of the laminations should hot he done antil the transformer winding has been completed and finishsd off. The exact size of the lamina-

tions is then to be regulated to fit. The Zin. leg of the T pieces must be just an easy fit through the centre of the bobbin, and the length of this leg must equal the inside length of hole through bobbin, which should be 45 imches. One piece should be cut to make a 4 and a U, and with these the sizes can be tested. The U must be large enough inside to clear the outside of the windings by omne-six-teenth inch. When the exact size has been determined, a cardboard template should be cut as shown, and when this is laid upon the lamination pieces they can be quickly marked out by scratching round the cardboard and in the straight part of the two notches, which give the two points for cutting in to separate the two parts. The actual depth of the windows should be just under Zin, by 44 long. If .. is necessary to make them any deeper, too much space has been wasted in putting on the wire and insulation. Cutting is best started at X marked on diagram of lamination, then cutting down right round inside edge of U ‘The first leg of U must be bent down a little to assist cutting, but bend the tin as little as possible. A sharp pair of snips will soon get through this work. When the cutting is complete, the bends must be flattened out, and this is most easily, done with the back edge of an old-fashioned flatiron, the plate laid on the bench, and stroked with the back straight edge of the iron, its point being held up in the air. Stroking the tin of either side with fair pressure quickly straightens it, The next process is to give all another coat of shellac with a brush. There will be 84 of each, T’s and U’s, and these should |

~ be ample to fill the lfin. thickness, and probably there will be a few over. The ordinary tin runs about 80 sheets to the inch, and the shellac coating will make up the difference All laminations have to be electrically insulated from each other, and shellac forms the handiest method. ASSEMBLING LAMINATIONS AND TESTING WINDINGS. The laminations are now ready for assembling. Each layer consists of a T and a U, each placed the opposite way round to those in the last layer,

so that the joints do not come opposite. These should be assembled in a flat position. When most of them have been put in, and they begin to feel tight, the windings nf the transformer can be tested Care must be taken that everything is well arranged for this test, as a "short" of the high-tension winding might prove disastrous, The two.

wires HT1 and [FIT centre tap are to be properly connected to a 40 or 60-watt 230-volt lamp and then the in and out primary ends are to be connected to the main supply of 230 volts a.c. The second wire is only ‘to be touched on momentarily at first,

(and if the lamp lights up at nearly full brilliancy everything is correct, and the lamp may be lit again for a moment. Then cut off main supply and connect the lamp _ to HT2 instead of HT, leaving the centre tap connected. Now connect up the primary to main supply, and if the lamp lights up, all is well with the high-tension and primary windings. On no account be tempted. to connect anything across the HT1 and HT2 taps, as their combined output is over 400 volts, and not safe to take any liberties with. The filament winding can now be tested, an old 6-volt valve being suitable. This should be connected across the two taps marked 45 turns, which give six to seven volts, according to the load. ‘To save fusing the filament, a short piece of ‘resistance wire could be put in the circuit. Now connect primary to the mains, and the valve should light up. Tf vou have a voltmeter, the voltages across each pair of taps can be tested. All being correct, the remaining laminatfons can be put in. ‘The sharp corners should be cut off the leg of the remaining ‘I’s at a sharp angle so that they will not cnt the inside casing when being forced in. Use care and patience in this work and get in as many as possible. The wood -clamps can then be put on end screwed up with din. brass bolts 2in. long, a small square 22’s brass plate being placed under the head of each bolt, and a similar plate under each nut in order to protect the wood from damage. These clamps, four in number, are 9in. Jong, 1}in. wide, jin. thick. Quarter-inch holes, centres drilled about 7iin. apart to allow bolts to clear laminations. When bolts are in they must be covered with tape to prevent touching and connecting the ends of laminations. FINISHING THE TRANSFORMER. The last item for the transformer is the small patel containing the a.c. and high-tension fuses. This panel, of ebonite, measures 3}in. by 1jin., and is supported by being screwed through the centre to a strip of wood, in turn screwed to the top clamps of transformer in the position shown. All the fuse contacts in this are spaced one inch apart, centre to centre, ‘in both directions. ach hole is drilled jin. and is fitted with jin. bolt with a washer and nut on the, top surface of panel. ‘The fuses are slipped under the washers, For the a.c., 40’s copper wire, ot thinner if handy, will answer well. Tor the high tension special fuses are required, the same being used on the front panel for outputs. The making of these fuses lias already been described, but to make the article complete the description must be given, again. _ The fuses are made by pasting a strip of cigarette tinfoil upon a piece of writ-ing-paper, and then cutting this into the finest shreds with sharp scissors. The best euttings about an inch or three-quarters long can be stuck with seccotine to a strip of strong paper, trimmed to size, atid the ends slipped under the washers. The fuses will act so long as they contain a place where the tinfoil] is narrow. (To be continued.)