The Advantages of the B Battery Eliminator

Radio Record, Volume I, Issue 41, 27 April 1928, Page 12

The Advantages of the B Battery Eliminator

Its Action and Use Fully Described

By

MEGOHM

‘At the present time owners of bat-tery-operated sets, if they have been using dry B batteries for a while, will be looking for some imeaus of obtaining a more dependable source of high-tension current thau they have becn employing. ‘The wet B battery or accumulator is undoubtedly gcod and reliable, and its only drawback is the fortnightly or weekly charge and the cust of the necessary charger, or a trip to the charging garage at frequent intervals. ‘The latter method is a workable expedient where uo electric current is available, but where such power can be utilised the B climinator is a most satisfactory solution of the problem. Ounce the voltages are regulated by means of the variable 1esistances to suit the several stages of the .eceivers, there is no further trouble. First the A battery is switched on, then the eliminator, and it will give high-ten-sion current for as long a time as required, and at a trifling cost. ‘fhere is not much in an eliminator to cause trouble, and being valve operated, there are uo liquids to deal with or spill. Some of the more recent elitninators also provide C or grid-bias voltages up to as high as 40 volts, which suits a large power-valve. The grid-bias is most satisfactory wlen obtained from a separate rectifying valve for the purpose, but which need only be halfwave, and as there is no actual current flowing, the snioothing conden:ers may be of comparatively small capacity. TYPES OF RECTIFYING VALVES. B ELIMINATORS may be diviled into two classes, accurding to the ivpe of valve uscd for rectificatics. of the current after it has been stepped up from mains voltage by means of a transformer, where alternating current is concerned. ‘There is the filament valve, which necessitates a separate Jowvoltage winding on the transformer to supply a 4 or 6-volt current to heat the filament. Several different types of such valves are on the market. ‘Then there is the type of valve that rectifies without the assistance of a heated filament. ‘The Ravtheon tube is the most prominent in this class, the LH type being more generally used, as passing a large amount of current suited to the average multi-valve set, and giving a large marcin above actual requirements, The life of a rectifying valve is usually from one to two thousand hours, depending to some extent upon the amount of current drawn from it, and in the case of filament valves, dependent upon the filament t+ing supplied with the correct voltage, with no tendency to overload. The Raytheon tube fits the UX socket, the two filament connections representing the two plates, which are connected to the outer ends of the respective high-tension windings of the

transformer. ‘Ihe positive output is the usual plate connection on the sock‘t ud represents the cathode or metal cap within the valve. This connection i1uus to the chokes and smoothing condensers, and is the B positive output for each valve, voltage being suitably altered by variable resistances. B POWER SUPPLY CIRCUITS. EFERRING to tie diagrams, two circuits are presented one showing the connections for a filament valve giving full wave rectification, and the other showing connections when the Raytheon Bii tube is employed. The smovthing system is identical in both, except that in the Raytheon circuit a resiscance R8 is shown across the output, but this resistance is "not indispensable, and may be omitted. In the case of the filament valve there is a low-tension winding on the transformer to supply filament current, and this is adjusted by a suitable rheostat. In the filament valve circuit the B positive is frou a tap in the centre of the filament winding, the reason for this being thet in the process of rectification, curreat can only flow from plate to filament in the valve, so that it has to be drawn off at an equally balanced point in the filament circuit, and this point, if correctly determined, is the electrical centre. In actual practice, it works quile efficiently and without introducing hum, if the tap is at the central turn of the winding, though it may not be the exact electrical centre, a In the Raytheon circuit no filament Witmling is necessary on the transform¢r so far as the rectifier is concerned, but in the case of converting a filament yalye climinator to use a Raytheon, the discarded filaincut winding may be profitably utilised to heat the filament of the last power-valve, aud although this current is only raw a.c. at low voltage, no hum is introduced into the receiver, In the Raytheon citenit two by-pass condensers are placed one across each higlhi-tension winding of the transformer. ‘Tliese assist the action of the valve, and may also have the effect of decreasing any radio-frequency cur: rents induced from the mains. Mains operated sets are frequently said ta be less selective than those operating from batteries, but the writer has found that by inserting radio-frequency chokes in a suitable positicn in the circuit, that climinator operation can be just as selective as when batteries are uscd. The presence of tlicse RE. currents is accouuted for by the fact that the mains conveying the current act as efficient acrials and pick up the transmission of a station near the wavelength of that being received, passing these currents iuto the receiver through the power circuit, ignoring any selective tuning arrangements that may be in the aerial circuit proper. Raaio chs 6 consisting of 1000 turns of fine wire will stop these currents, and all that is needed is to insert a choke in each plate circuit as shown at XX in the standard circuit.

ACTION OF THE CIRCUIT. . BREIWEEN the circuit diagrams is one showing the action of the Alternating current is made up of circuit in smoothing out the current. small impulses changing at the rate of 50 to 80 per second. In the case of 50 per second, or ‘50 cycles," there will be 25 impulses in each direction every second. A few of these impulses are shown in the diagram alternating on either side of a central or ucutral line, the short arrows showing the direction of the current for one-half of a complete phase or vibration. This kind of current enters the transformer, and on account of its alternating firm, it is able to induce in an adjacent winding a similar amount of current tieasured in watts, dependent upon the

ratio between the turus on the second ary winding or windings and the turns on the primary windings. If the number of turns is equal on both primary and secondary, then the same voltage and amperage less a conyersion loss, wil? oe obtained from the secondary, and the same pauge of wire would be used on each, unless as in the case of an climinator, the required output is extremely small, But the number of primary turns must be accurately determined if the transformer is to work efficiently. If secondary turns are more than on the primary, there is a propor tional step-up in vollaye, but fewer amperes may be safely drawn, so the secondary wire can then be of thinner gauge. If very few turns are on the serondarr, we get low voltage, and may draw more auipcres to make up the total watts, so that heavy gauge wire must be used, as is the case for fllament windings to give say six volts at one or two atniperes,

When the current has bcen- transformed, either stepped up or down, it 18 still alternaung, and then passes through the rectiymg valve, which only passes current from plate to filament. With =the tapped high-teusion secondary wuiding cot nected as it is, the alternating pulse in one direction travels in the valye from plate to filament from one hall of the winding. ‘Che neat puise 1s in the. cpposite direction, through both windiugs, but the construction of tle valve only allows it to take an impulse from plate to filament, which is now the pulse in the other half of the winding, so that we now have the alternating jmpulses that were formerly gotng in opposite directions, now travelling through the output circuit all in the same direction. The current next encounters in turn smoothing coudensers, and one ot two choke coils with iron cores. The sinoothing condensers have the picperty of storing up a charge of current momentarily and giving this charge back to the current when its voltage is below that of the stoved-up charge. — Ry this means the low-voltage gaps_ between the ‘himmnps" of voltage shown in the diagram are filled m ‘rhe action of the choke coil is to stop thc passage of fluctuating current, so that only fairly smooth direct curren: can pass. ‘The Jast condense: (C3) stores up a charge which 1s chiefly utilised to provide for any extra demands that may be made on the audio side, and if the placing of a latger power valye in the receiver causes any indication of hum, it is this condensr that should be increased in capacity, the benefit being especially noticeable on the low notes. Output voltages of ditferent values are obtained by reducing the main voltage by means of variable resistances, and in order to prevent any tendency to hum, a fixed coadenser is placed across each different output. TROUBLE FINDING. A® remarked above, there is little to give trouble in a well-construct-cd eliminator, but if trouble does develop, it may readily he located and remedied. More often the trouble is in the receiver, and it should be decided by proper tests that the trouble is vot in the recciver before commencing upon the eliminator. The logical place to begin the hunt for trouble in a radio nower unit is at the resister bank, and then work backwards through the filter, rectifier tube, and finally the transformer. It is assumed, of course, that ile alternating current igs known to ke flowing through the transformer. and that the rectifier tube is not visibly damayed in any Way. An open-circuited or burnt-out resister will result ta no voltage from the tap it controls. If the 10,000-olim fixed resister beccmes open, in the case of the B-power unit, the detecter

voltage will inmmediately imcrcase 60 that in the tuned radio-frequency receiver, the signal strength will be greatly diminished, while in tiie regenerative receiver there will be constant oscillation. ‘he simplest methed tu lovate a defective resister is by means of a highresistance voltmeter, conuected to each tap im turn. In fart, this deve is essential in adjusting B powe1 voltages to any receiver,in place of the cut-and-dry method. In the absence ct this device a 16-watt @80-volt incardescent lamp may be employed. It should glow a dull red on the full output and on the intermediaze tap of the l-power unit. If it lights equally bright on the detecter tap, it is an indication of an open or defective 10,600 olim fixed resister. If the tap voltages ate found to be satisfactory, and the recetver still doc not operate well, the troubl« may be due to an open or an omitted by-past condenser. A short-circuited Dby-past condenser will act the same as a shortcircuited resister. No Voltage at all ‘ferminals. -This condition can be caused by at open circuit in the wiring tratisformer, choke coils, or by a broken-down filter coudcuser. With power disconnected from the Lpower unit and tle Rayileou tube removed, a click should be heard in the testing telephone when commected in series with battery between plate ternunal of rectifier socket and tie positive B of the power unit. A click should also be heard between either filament terminal of the rectifier socket and the negative 8 of the B-power unit. These clicks should be of equal strength. If one filament terminal gives a much louder click than the other, it gencrally indicates a defective buffer condenser. If no click is leard on either filament terminal, then the transformer secondary is open-circtit-ed, or the centre tap of the transformcr does not connect to the negative Ii side as it should. The circuit continuity of the transformer itself mav be tested by the click between the twd filament lerminals of the rectifier socket, with the tube removed. If the transformer seccndary tests O.K. on the foregoing procedure, there must of necessity be an open circuit in the negative B lead. A short-cireuit in the secondary of the transformer can be most casily checked by connecting a 25-watt, mains voltage lamp in the scries with the primary. The current is now turned on in the usual way, but with the tectifier or Raytheon tube removed from the socket. The incandescent lamp should glow dull, if at all. If it glows bright either the transformer secondary or one of the 1 mfd. condensers is broken down. With the lamp still in the primary, the rectifier tube is inserted in its socket. If the secot.dary connections are O.R. and the Raytheon is operative, the lamp will increase in hrillianey.