Radio Broadcast (May 1928-Apr 1929)

M^aking an A-Power Unit From Your Battery* Charger FIG. I. HOW TO ASSEMBLE AN A-POWER UNIT AT HOME A battery-charger, an A-filter, a rheostat, and a voltmeter are combined to produce a source of filament current directly from the light socket. In this illustration we show the use of a complete Tobe A-filter consisting of choke coils and condensers, all combined in a single container; the input terminal of the filter connect to any good battery-charger of not less than 2-ampere rating and the output of filter goes through a 10-ohm power rheostat to the filament terminals in the radio receiver. The voltmeter reads the output voltage which should be adjusted to 6 volts. A unit such as this can supply up to 8 or 10 201-A tubes or their equivalent, and makes it possible to do away with the storage battery, and obtain complete a.c. operation of the radio receiver COHERE are probably many among our readers J. who have a source oj uo-volt a.c. and who are still hesitating to supply the filaments oj their tubes directly or indirectly with a.c. We described in recent issues the ease with which the adaptor kits with cable and attendant filament transformer may be used to avnd the A-battery. Now, the faithful battery-charger occupies a new place and the present article by Mr. Burnham describes how a very serviceable A-supply may be constructed. A power unit was constructed in the Laboratory in accordance with the data given in this article. We used a Tungar 2-ampere charger in conjunction with A-filter condensers and the proper chokes. With this form oj filament supply for a standard storage battery set, the hum audible in a high-quality loud speaker was quite low. — The Editor. ten te* THE owner of a battery-operated radio receiver who desires to convert it to a lightsocket operation has before him a definite problem. How shall the conversion be accomplished? To many, converting a receiver to lightsocket operation means only revising the set for the use of a.c. tubes. This is one way to do it. Another method of making a set light-socket operated, which in some cases may prove cheaper and easier, is illustrated in this article. In the first place we should realize that the part of the power equipment of a radio receiver -which-in most cases makes the set not light-socket operated is the storage battery. The B and C potentials for the set are now generally supplied from the light socket through the use of a B-power unit. Therefore, any device which enables us to obtain filament current from the power mains without the use of a storage battery, supplies the missing link in the a.c. chain. It should be realized that "light-socket operated" does not necessarily imply the use of a.c. tubes; any method whereby a receiver is operated with power from the light socket is a method of socketpower operation no matter what types of tubes are used in the set. THIS A-POWER CIRCUIT IS SIMPLE THE method used in B-power units to obtain B and C potentials from the light socket is quite familiar to our readers. For these potentials -we take the current from the light socket, rectify it, filter it and then apply it to the radio receiver. It seems reasonable to suppose that the same method might apply equally well for the A supply. So it can, and the various A-power units now on the market make use of a rectifier-filter system to deliver sufficient current at six volts for the operation of radio receivers containing up to about 8 or io storage battery tubes. An A-power unit consists, as we have mentioned above, of a rectifier and a filter system. ! The rectifier may be any of the types ordinarily used in battery chargers capable of supplying enough current (about 2 amperes) and the filter system should consist of a combination of a choke coil and two special condensers designed for use in A-power units and containing a large amount of capacity — several thousand microfarads. Condensers for use in A-power units are now available as are the choke coils and it follows therefore that by making use of these two units any owner of a battery charger can make himself an Apower unit. The storage battery can be discarded and your present radio receiver — with no other changes — in the future operated directly from the light socket. This is the particular subject of this article — how to make an A-power unit using your battery charger. Thus the millions of batterychargers in use around the country can be used with an A-filter to give the owner of the receiver a.c. operation requiring no rewiring, harnesses, or new tubes. Certainly this is worth considering. The essential parts required are the batterycharger and A-filter, a power rheostat to control the output voltage of the unit, and a voltmeter to read the output voltage. Most of the companies making these various parts are listed in the table accompanying this article. The story of how to do it is told quite completely in the illustrations accompanying this voltmeter A C FIG. 2. HOW THE PARTS SHOULD BE CONNECTED This picture-diagram shows clearly how the units are wired. The various parts can all be mounted on a single baseboard and then located at some point convenient to the radio receiver. Be sure to use a power rheostat, for ordinary types will not safely carry enough current. With the receiver turned on, adjust the power rheostat so that the voltmeter reads six volts. If the line voltage varies it may be necessary to readjust the rheostat in the course of an evening 137