B BATTERY ELIMINATOR FOR D.C. MAINS

Radio Record, Volume I, Issue 32, 24 February 1928, Page 10

B BATTERY ELIMINATOR FOR D.C. MAINS

A UTHOUGH most places in New ‘ Zealand are supplied with alternating electric current, inquiries have at times been sent in regarding an eliminator to work off direct current. Usually this presents a much simpler problem than dealing with alternating current, and certainly entails less expense, Direct current from the mains, however, is not like direct current from a battery, as owing to, the method of its production by a dynamo, it carries an objectionable ‘‘ripple,’’ caused by the system of rectifying by means of a commutator on the machine. This ripple is not altogether the mere break that might be caused in certain cases by the transfer of the brushes from one segment to the next, but is in many cases a slight back-voltage, which necessitates the use of low-resistance chokes to assist in smoothing current for eliminator use. Roughly speaking, a d.c. eliminator is the same as one for a.c., minus the transformer and rectifying tube. Direct current does not allow of a stepup in voltage, as may he effected from alternating mains, but where the available voltage is 230 this need cause no concern. In the case of direct current, a certain amount of smoothing takes place in the mains, where the positive and negative leads run alongside, so that current that has travelled the greatest distance in this way will tend to be the snioothest, but at the same time is, perhaps, more liable to pick up interference from trams, etc. THE CIRCUIT DIAGRAM, MHL, diagram shows the general sequence of parts, commencing with a fuse on each main, which may be strips of tinfoil 1-l6-inch wide, clamped under washers at each end on a strip of fibre or ebonite, preferably with a strip of mica under each fuse. Then a sixty-watt lamp of the mains yoltage is placed in each lead. On the positive side one or two choke coils of 20 or 30 henries impedence are included, One choke should he

tried first, and if there is still hum owing to the nature of the ripple in the mains, another choke can be added. If only one choke is used the | condenser X will be omitted. The choke may be made.as directed in the issue of November 18 for use in the a.c. eliminator. ‘The 2 mfd. condensets may be the ordinary paper insulation type, Dubilier or T.C.C. make. It is important to note that with this eliminator in use a .5 mfd. fixed condenser must be placed in series with the earth lead from the set, and this condenser must be one of high voltage test and reliable make. The reason for its inclusion is that very often the positive main is earthed, and the condenser prevents the current from taking a short-cut return through the set. The output may be provided with variable resistances as shown, one for each voltage, a fixed condenser being connected across any found to require it, and a fixed resistor totalling, say 15,000 ohms across the last tap and the negative B. A fixed Dubilier or T.C.C, condenser of .5 m.f.d. is also placed across as shown. A POTENTIAL DIVIDER. LSSTHAD of providing variable resistance across the output, a better plan is to purchase a "potential divider" (climax) which costs about 12s. 6d, ‘This is an arrangement of a continuous resistance with ten taps, each of which may be connected to a socket on the panel, each socket giving a different B voltage. The voltages that may be expected from 2380-volt mains should not be less than 160 (max), 186, 112, 88, 64, 56, 48, 40, 82 and 24 volts respectively. Higher voltages may quite likely be obtained, but whatever _ the maximum voltage, all the tappings will be raised in exact proportion, 50 that if the maximum turns out to be 186, then the tappings will he 153, 126, 99, 72, 68, 54, 45, 36, aud 27 respectively, ‘The low potential end of the divider connects to negative B, which is provided with a terminal, or if pre-

ferred a socket, in line with the others, A tin case finished with black cycle enamel forms a neat container, and only a strip of ebonite is required to take the row of sockets and negative B terminal. The B positive voltages are tapped off by wires running to the tespective valves, and provided with a pin or wander-plug for making connection. A twin flexible wire of suitable length with an adapter, provides for coutection with the lighting circuit. If a lamp-socket is wired in. close to the adapter plug, then the latter can be inserted into any convenient ‘ight-socket, and the lamp placed in the extra socket, so that the light is not interfered with. WHEN POSITIVE MAIN IS EARTHED. {OMIi constructors of D.C, climinators have struck trouble, finding it impossible to get a satisfactory B supply. This is sometimes the case wheu the positive main is earthed, and one remedy recommended is to place an H.I. choke in each lead ou the inver side of the fuses. Such a choke was described in the A.C. eliminator article, and consists of 1000 turns of 86’s enamelled wire wound in a flat spool with j-inch wood centre, two fibre discs 2 1-8-inch diameter, 8-16-inch apart. Instead of placing a second choke in the positive lead, it may be placed in the negative, oposite the one in the positive. A high standard of insulation must be maintained between the A accumulator and earth. Iuses as shown niust on no account be omitted, as their inclusion prevents the house fuses from "blowing" if a short-circuit occurs in the receive?, The smaller the power supply installation, tle more likely is the D.C. to be "rough,’’ and the voltage erratic. The former trouble is reduced by having two chokes, each of high inductauce, up to 100 henries, which would however, cause a reduction in voltage output. Increasing the capacity of the 2 m.f.d. condensers to 4 or 6 m.f.d.’s will also help to smooth out any obstinate ripple.