D.C. Eliminators, Small and Large

Radio Record, Volume IV, Issue 12, 3 October 1930, Page 27

D.C. Eliminators, Small and Large

How to Run P! ate and Filament from D.C. Mains

By

MEGOHM

FROM time to time inquiries are sent in for particulars of battery eliminators for direct current mains. As there are quite a number of areas where the supply is direct current, this article has beén compiled to show different methods of dealing with these conditions, so that constructors and experimenters, large and small, are catered for. The article, beside giving specific circuits, is intended to illustrate principles so that constructors may make up a circuit to suit their own needs. Constructors are récommended to read the whole article through, because, in some cases hints are given regarding one circuit that may also be useful in another.

NFORTUNATELY suitable direct-current supply is not available in Wellington’ for _ experiment of any kind, but "for some time the writer has been gathering literature on the subject, from various sources, until he has been: enabled to design the "full plate and. filament supply : eliminator," described at the end of the article. This is designed to work on the standard 230 volts. (or 240) as available in New Zealand. This will be found not too costly in running expense, and will perform much more satisfactorily than circujis designed to use .1 amp. valves, which unduly hampers the output stage. , Although in the use of direct current for radio work, there is not the necessity for rectification which accompanies the use of alternating current, yet the use of d.c. from the mains is not always the straightforward problem that it should be when considered on a theoretical basis. Whereas alternating current may be stepped up or down in voltage, rectified and smoothed, the direct-current supply has its limitation from the fact thut the voltage available is no greater than that actually provided by the mains. This is Jess of a disadvantage when the mains voltage is 230, than when it is only Most towns using d.c. are wired upon the "three-wire system,’ which must be briefly explained in order to acquaint readers with the reason why he supply in some houses has the positive main earthed, whilst in others the negative main is earthed. _ When the latter is the case, the problem of radio supply is a simpler one. 7 The Three-wire System.

a By Fig. 1, A and B are two generators at: the power-station, connected in series just as two accumulator cells ‘would be. Each generator gives 230 volts, and the connecting-wire between the two is earthed, a wire from this point being also led along every street. Along main streets there will also be a cable from the outer end’ of each generator, and a supply taken across these two gives 460 volts for factories. Up residential streets the earthed wire and only one of the outer wires is led, the aim being to get half the 230-volt load halved on each generator. Looking the diagram we can see why a living in "King Street" finds that "the negative main is earthed, while his friend living. in "Queen

Street," has to deal with the problem of an earthed positive main . The earthed main is found by connecting one side of a lamp to earth, and with a wire from the other side, insulated except:at the end, touching each main connection in turn, but only momentarily. The unearthed main gives a bright light, and the earthed main none, or a dull glow that indi-

cates a few volts above ground potential. Polarity is then formed by one of the tests that have been published from time to time. The regulations stipulate that any operation such as this where the mains are interfered with, a certificated wireman must perform the operation. General Considerations. "THE desire of the radio enthusiast will be to run his receiver entirely without batteries, and whilst it is quite possible to do this, it usually pays to bias the anode bend detector with a lry battery if there is any difficulty in getting rid of a small residual hum. _ The eliminator may be intended only to supply plate current, or it may sup-

ply filament current as well, and in either case grid bias for at least the last audio stage. For either style of eliminator the best procedure is to construct .a separate unit so that it may be placed a short distance away from the receiver, thus minimising any chance of the grid circuits, especially the detector, picking up hum from the smoothing equipment. A potential divider should be dispensed with, and plate current voltages reduced by a separate resistance for each voltage. These resistances are best placed in the receiver itself, with the accompanying by-pass condenser, in which case the eliminator gives only one plate voltage, which is connected to a suitable terminal on the receiver. ‘Filament Current. [HL usual method of filament heating with completely ‘"d.c. mains"

operated sets is by connecting all the filaments in series. This is better than parallel for several reasons. This arrangement, however, brings to light one of the limitations of complete. d.c. working. On account of the filaments all being placed in series, it is necessary that the amount of current taken by each should be equal and preferably not too great. The average a.c. mainsoperated receiver requires at least 100 watts from the mains, and in this respect the d.c. operator should not expect to obtain good results unless a reasonable amount of current is consummed. Current at 230 volts costs just twice as much per ampere as it does at 110 volts, so that the 110 volts circuit will supply a half-ampere at the same cost as a quarter-ampere on a, 230 volt circuit, though the watts consumed will be (practically) the same in each case. With a half-ampere available, filaments taking .25 amp. may be wired in parallel in pairs, each pair being placed in series, so that push-pull will then haye no complications regarding grid- bias. For the sake of economy it is well to restrict the consumption on 230 volt mains to a quarter-ampere, which is equal to the ordinary household 60 watt lamp, as there is little to be gained in practice by doubling the Circuits have been published to work with a consumption of only 0.1 amp., but this is unnecessarily low, and has

too restrictive.an effect upon the type of valve that may be used in the last audio stage. Anode Bend Detector. THe detector, as in A.C. sets, is liable to pick up a certain amount of hum if of the grid-leak type, and for this reason an anode-bend detector is recommended. Experimenters need have no apprehensions about converting to anode-bend detection on a multivalve set. This is now known in America as a "power detector," and is being widely used. ‘The writer has had it in use on the two r.f. BrowningDrake over a long period with marked suecess. If a grid detector is used, the grid condenser must be placed on edge, that is in a vertical position, and not horizontally, screwed down to the baseboard, as is sometimes the ease. Whether the grid-leak is

retained or not, the lead to the grid terminal of the valve-socket should be. kept as short as possible, and away from other leads. Plate Voltages. ANOTHER point to be borne in mind is that when filaments are in series, the plate current for all valves returns through the filaments, and if the plate current of the last valve is heavy, must be taken into account. For this reason some circuits are arranged so that the plate current of the last stage is returned without traversing all the preéeding filaments. When all plate voltages are taken from a common resistance such as @ voltage divider there is a chance of "motor-boating" developing, and in order to avoid this it is good practice to have a separate resistor for each plate, so that there is less chance of un" wanted coupling.