TO ELIMINATOR CONSTRUCTORS

Radio Record, Volume I, Issue 19, 25 November 1927, Page 12

TO ELIMINATOR CONSTRUCTORS

SOME FURTHER HINTS

When constructing a B eliminator it is necessary to remember that all apparatus connected to electric supply mains is expected to conform to! certain regulations concerning electrical wiring. In a general way, the larger the amount of current being handled, the more stringent are the regulations, which are framed to protect life and property, and to ensure uninterrupted service. An eliminator only consumes a small fraction of the current used by an ordinary lighting bulb, and a possible shock from it would be quite harmless. ; The eliminator just described is housed in a metal case, and in the original a moveable sheet of tin is also fitted inside, a short distance behind the panel to afford complete metallic screening. Constructors should not neglect to provide a tin or iron case of some kind, and should also go to the small amount of trouble involved in fitting the four small fuses above the transformer. Two of these fuses are in the A.C. connections, and as the current passed is very small, they may be of fine tinfoil strips as used on the H.T. side. If any short circuit occurs one of the fuses will "blow’’ and automatically eut off the current supply. In the original also the condenser holder and metal lining of the partition are joined to the case by soldered wires, the former being shown at FE on the under-base wiring. Where there are A.C. mains very close to the eliminator, it is- sometimes a good plan to attach an earth wire to the metal case.

TRANSFORMER TESTING. An efficient transformer passes practically no current through the primary at "no load,’’ that is, when the secondary windings are not doing ‘any work. The primary winding and core must form a complete "choke" to alternating current at "no load," .and this effect can be tested by temporarily including an ordinary electric lamp in the primary circuit. This will show no glow whatever at "‘no load," and even when the eliminator is supplying a large set,

still no glow will be visible in the lamp. If the lamp lights up, some~ thing is wrong, and must be rectified, as without the lamp in series, a transformer containing insufficient primary winding might give quite a good secondary output, but would be pass-_ ing through the primary an amount of current that would cause the household electric light bill to as- . sume alarming proportions. The foregoing is a hint to ‘constructors to adhere rigidly to the gauges of wire and number of turns specified for the transformer, which .. will ensure a consumption of current. so low that it will pass unnoticed in the lighting account. The number of hours that an eliminator is run makes the current consumption an -important matter. Once this is as-/ sured there need be no stinting of H.T. in the set itself, as the cost of high tension current for an extra valve or two will be practically nil. CHECKING CONSUMPTION. The consumption of an eliminator can easily be checked upon the household electric supply meter during daytime reception when no other current is being taken. If current consumed in one hour does not exceéd one division . on the "hundredths" dial, then the consumption is satisfactorily low. At this rate the rotating dise of the meter will only be perceptibly moving. A SIMPLE ELIMINATOR, "Megohm" does not recommend eliminator circuits in which no transformer is included, as in such case the set is too intimately connected to the mains. Some readers who are running two and_ three-valve_ sets may wish to enjoy the advantages afforded by an eliminator, and yet would not care to expend the amount necessary to build the full-size article recently described. With this idea in view "Megohm" will shortly describe the simplest reliable singlewave eliminator. Construction will be cut to the minimum, and the cost kept as low as possible consistent with service.

Mainly About Construction (Continued from Page 12.)

Charging B Accumulator, G.T. (Levin). -It will not take much over a unit a month to charge the B accumulator for reception of daily broudcasting, Donble-wave valve rectification is more economical and satisfactory than chemical methods. For the latter the electrodes should be kept small, and two cells are preferable to one, as the extra one adds a certain amount of necessary resistance, asict also helps to clean up the rectification. , Lead peroxide for positive and litharge for negative plates. [ff peroxide cannot be obtained, ordinary red lead is a substitute. These are separately mixed into a stiff »aste with dilute sulphuric acid of /|attery strength. ; Rheostat Query. 'G.T. (Levin).-If yeur rheostat appears to give no more current when full on than when only at startingpoint, it seems to indicate ‘hat its resistatice is too Jow. A 30-ohm rheostat is suitable for a P.A1.3. In working these dull emitters filament voltage should always be kept es Jow as possible, to give required volume. After 2 certain point uo increased volume is gained by increasing voltage, and the life of the valve is shortened. Charging A Battery. C.S. (King Country).-The solution of salt and water is useless in a chemital rectifier. A strong or saturated

solution of ammonium phosphate is the usual liquid. Before charging the battery, the plates, which must be of ample size, must be ‘formed’? by running for an hour without the battery in cirenit. As you are charging an 80 amp-hour A battery, the greatest eare will have to be taken to see that the rectifier does not get overheated, as if the temperature of the liquid tises above 70 degrees Fahrenheit, its rectifving properties are impaired, and therefore the amount of current passed mest not be too large, and then the rate of charging is slow. ‘‘Megolim’’ does not consider it very wise for an inexperienced person to attempt to charge an A battery with a chemical rectifier. With a suitable double-wave valve and correctly wound transformer there is no risk.