The "A" Battery Charger.

Radio Record, Volume II, Issue 10, 21 September 1928, Page 28

The "A" Battery Charger.

A CORRESPONDENT writes asking how the charger outlined on September 7 cun be adapted to use on 110volt (Wellington) supply. The charger in question is intended for use on a supply of 230 volts, 50 cycles, the New Zealand standard, but for use on 110 volts the primary is wound with the specified turns, with the addition of a tap at the half-way point. In actual practice it would be advisable to make two taps, at the 580th and 550th respectively, so that the one giving the best result may be used. Connection is then made to the commencement of the primary, and to one of these taps. The remainder of the winding is not used, but is there ready for the changeover to 230 volts, at which time the whole of the primary winding will be used, and the half-way taps discarded. Secondary and filament windings will be the same for either mains supply. Another plan is to arrange that the two portions of the primary have the same number of turns, and to connect the two halves in parallel for 110 volts. This would mean bringing out both ends of each primary winding separate- ly, and arranging that any taps taken were over the same number of turns In each half. ‘The two beginnings would be connected to one mains lead and the two ends to the other. ‘

Extra Primary Taps. OWING possibly to slight modifications in construction, error in counting primary turns, and other causes, there have been cases where constructors have not obtained full voltages expected from the secondary windings. To compensate for possible differences, constructors are advised in every case to take two or three taps in the primary winding at say. 970, 1000 and 1050 turns, as by employing a lesser number of turns the output voltage may be increased, as a greater magnetising current is used, but this must not be carried too far, or the charger will not be economical of current. Some experimenters will prefer to fit a switch to vary the number of turns, otherwise the best position may be found by trial, and a permanent connection made. "THE original charger was designed with the maximum number of primary turns for economical working, but a considerable number of turns may be cut out with very small increase in current consumption. The extra trouble of taking taps as recom~ mended will be found worth while by experimenters. For ordinary charging purposes the secondary taps shown in the original charger are not an absolute necessity, put were provided so that experimenters could make use of the transformer for other purposes. For charging only, the whole of the two secondary windings may be permanently connected up, the resistance lamps. automatically regulating the output to suit the battery being charged, from 2 to 12 volts. Ordinary iron cannot be used in place of the stalloy prescribed for the core, as the turns specifications are quite different for black iron, which is not nearly so efficient as stalloy. If, through some fault in construction, a transformer does not give at least a total of 40 volts over the two secondary windings, it is possible for the primary turns to be reduced and the output voltage raised by the following method: Take out the stalloy strips and remove the spool end at the side where the primary ends. Carefully pull out the wire, removing about 50 turns, cutting off the surplus wire, and passing out the end for the finish of the primary. ‘Then secure the spool end in place. If found necessary, the other spool end may be removed, the beginning of the last layer pulled out, and a tap soldered on. If this does not etfect the necessary improvement, the transformer should be rewound and a careful check made when counting turns. Unless 4 Pell nenseretaacipere is used, the turns on each layer shoufd be carefully counted after the layer is ‘wound, and noted on paper, so that when the last layer is reached the total may be added up and the exact specified number of turns put on. ONSTRUCTORS must recognise the importance of correctly counting turns. The constructor of an eliminator transformer, writing recently to

say that it was not delivering the expected voltage, frankly confessed that he did not know how many turns were on the primary. Pap constructors have had success with*the charger built as specified, and these latter remarks are for the assistance of the odd ones who from some cause or other have not obtained the expected results. Shellac and Laminations. pD° not be tempted to dispense with shellac on the laminations, for a great deal of energy can be wasted if there is electrical connection from plate to plate across the section of the core. Incidéntally, too, the shellac deadens any mechanical rattle that may be present through looseness of laminiitions. —