Power Unit and Battery Charger

Radio Record, Volume IV, Issue 44, 15 May 1931, Page 20

Power Unit and Battery Charger

Suitable for "Differential" Series and "Home and Country’’ Portable

By

Cathode

N designing this combined "B" power unit and "A" battery charger, we have borne in mind the constructors of both the "Home and Country" portable and ‘Differential’ ae series. In the "Home and Country" receiver, something of the kind is essential in order to economise on. the drain from small portable batteries, while the unit will be found .at once an economy and a convenience by all those readers who have constructed one of the * ‘Diiferen tial’. sets. "The same power transformer is used for: both the "B" eliminator and the battery. charger. .The number of windings is, therefore, somewhat larger than usual, and the transformer perhaps a trifie more difficult to construct. : However, every effort has been made to simplify the construction as far as possible, while if, after 2 perusal of the details, the reader still feels that the transformer is heyend his abilities, he can have it made up by one of several firms who undertake this work at a very reasonable charge.

The Transformer. HE construction of the transformer should be the first tusk. This has been simplified somewhat by the use of two bakelite bobbins on one of which: the primary and the windings for the filaments of the high voltage rectifier valve and the charging valve are wound, while on the other bobbin are wound the centre tapped high voltage secondary and the two 16-yolt plate avindings for the charging rectifier. The bobbins used give a core area of fin. x i4in., so that the primary will have to be of about 1850 turns. A suitable gauge of wire will be 32 s.w.g. ‘There is no unlimited room on the bobbins, so that either enamelled wire or double silk covered wire must be usec. 4f enamelled wire is used it is absoJutely essential that shellacked paper should-be put between each layer, and if this is put on while it is still tacky it will help to hold the winding firmly together,

If double silk covered wire is used, and each layer well shellacked, it is permissible to dispense with the paper interlerving. Wire of this kind, howeyer, is somewhat more expensive than is enamelled wire of the same gauge.

When the primary is wound. a couple of layers of oiled silk should be wound on for purposes of insulation, to be followed by the filament winding of the high voltage rectifier. The 280 rectifier is probably the best available and the tilament winding for this consists of 80 turns of No. 20 diac his winding must be tapped at the 15th turn, the tap being brought out between a couple of scraps of oiled silk. On top of this again, and separated from it by another two layers of oiled silk, should be wound the filament winding for the battery charging rectifier. A Philips Type 451, the filament of which is rated at 1.75 volts, has been chosen for the charging rectifier. The winding should consist of 11 turns of 18 d.c.c. tapped as near the centre as can be managed. On the other bobbin the high voltage secondary is the first winding to go on. and this can consist of 2000 turns of

36 s.w.g. This may be either. enamelJed or donble silk covered, the same remarks applying as in.the case of the primary. ~On top of this winding, and separated from it by oiled silk, as before, the plate windings for the charging valve may be wound. There are two of these, and each consists of 96 turns of No. 24:s.w.g. double silk covered wire being really best to use. As each winding is started and finished if is advisable to tag the end of the wire, so that no confusion will arise among the many tappings. ~The winding will he rendered very mith easier if a little winding jig is mad# to fit the babbin, and opportunity can be had also to bolt small seraps of aluminium or formica on to the jig to prevent the cheeks of the bobbin spreading. ‘ This is not absolutely essential, however, as the bobbins are fairly stiff and only spread a fraction of an inch. The individual turns need not be counted in the primary and high voltage secondary. The turns in one layer should be counted, and if, then, the layers are

counted instead of the turns, this will- é be accurate enough for all practical purposes. Assembling the Laminations. WITH the bobbins wound, it then remains for the laminations to be assembled. The bobbins are made to fit the standard.No. 4 stalloy Jaminations, and the T pieces should be inserted from alternative sides, so that no two joints will come above one another, Each layer. comprises, of course, a’ piece and a U piece. The laminations must be clamped tightly to * prevent hum, and suitable clamps, ag

. Well as bobbins, are ayailable com‘mercially. No terminal strip is méunted on the transformer, as it is more convenient simply to lead the ends of the wires to Appropriate points through pieces of insulating sleeving. Tor this reason the ends of the wires should be left reasonably long to make the connection, or if it is found that they get in the way. too much, then they may be cut fairly short and wires subsequently soldered to them for the purpose of making the connection, In the case of the fine wire windings, these wires should not be too heavy or they will place a strain on the fine wire which comes from the transformer itself. The Smoothing Chokes. "Pu smoothing chokes may also be constructed (it desired) from similar materials to those used for the pdwer transformer, . Full instructions for the construction of smoothing chokes. are given in the "Guidé," so that there is no need to. give any further information on the subject here. The chokes should be constructed to specifications giving an inductance of

30 henries or thereabouts when pissing a current of 25 or 30 milliamps. Choke D. It is, as a matter of fact, somewhat questionable whether the saving ef.

fected by constructing smoothing chokes is sufficient recompense for the labour inyolved in their manufacture. They can be-bought quite cheaply. The Smoothing. The slightly unusual arrangement of the smoothing condensers in the power pack is the outcome of a considerable amount of experiment with the object of obtaining maximum smoothing from a minimum expenditure on condensers. From measurements made, it was found that the improvement affected by increasing the size of the first condenser aboye 2 mfd.. or that of the second condenser above 1 mfd., was so inconsiderable that to provide bigger condensers at these points was clearly uneconomical, The third condenser, however, shows 2 progressive improvement in smoothing, as its size is increased until 2 capacity of 5 or 6 mfd. is reached. The improvement is Jess marked, how-

ever, When the size of the condenser is increased above 4 mfd., so this value has been chosen in the present design, These three filter condensers ‘should have a working voltage of at least 300 (.e., or a test voltage of at least G00 d.e. The remaining condenser connected between the tapping for the screen grid voltage and earth (for the "Differential" receiver) is not subjected to any very high yoltages and the ordinary small-sized condenser will normiully be quite safe in this position. The precise arrangement shown in the diagrams is intended more particularly fo fit the requirements of the "Differential Four." It is, however, eminently suited for: other reccivers. including the "Ifome and Country" portable, and may be used for any such receiver without any modification whatever. It is permissible, however; in the case of a receiver employing but a single "B" yoltuge (such as the "Ifome and Country" portable) ro

---e dispose of the tapped resistance acros# the output of the power unit and with the condenser from the tapping point on this resistance to earth. In the case of the "Home and Country" receiver, of course, the terminals are connected to two lengths of twin flex terminating in phone plugs to fit into the special jacks provided in the receiver. Battery Charging. PERHAPS the most unfamiliar part of the apparatus to most readers Will be the battery charging valve and its associated equipment. ' ‘The Philips Type 451 valve is a small _ mereury vapour rectifier having a hot cathode

and two plates to each of which an nc. voltage of approximately 16 is applied. The filament of this valve draws a current of 3.5 umps at a volte age of 1,75, so that the wire connecting the transformer winding to the yalreholder must be fairly heayy. The reader will be wise to follow the previous recommendation that the loose end of the transformer winding itself should be used to make the connection between transformer and valye-holder. The output of the 454 rectifier is 1:3 iinps, and this charging current is iniintained constant for any battery up to 6 volts by the ballast resistance bulb (Philips No. 452). which serves the double purpose of protecting the rectifier and regulating the charge. Usually, the ballast bulb does not slow with a 6-volt battery on charge, but inay glow slightly for a 4-volt bate tery, and fairly brightly for a 2-volt battery. It may be noted that both {he rectifier and the ballast bulb are supplied with the Inglish type of valve base. The rectifier bulb has the normal connections for the filament with the two plates connected to the grid and plate terminals, ‘The resistance lamp has its: two outside connections connected to the filament pins, while the centre tapping of the resistanee ig connected to the plate pin. This bulb has only three pins. No ammeter or charge indicator of any kind is res quired, as the output of the charger, if correctly made, is absolutely cone stant and reliable. With these few notes it is not con sidered that the constructor will ex« perience any difliculty in completing the apparatus, as the theoretical and praes tieal diagrams show clearly the eons nections and arrangement of the components. The connections -from the eliminator and charger to the plates for the "Ifome and Country" receiver are shown separately in the small dia« gram, and this should render this little point quite simple and straightforward, The only point is that ihe utmost are must be taken not to insért the plugs into the wrong jacks, as an evror of this kind may lead to serious results, °