A Trickle Charger for both A and B Batteries

Radio Record, Volume II, Issue 13, 23 November 1928, Page 26

A Trickle Charger for both A and B

Batteries

By

Pentode

may ELE} commercial tendency Maal in the design of modern receiving seems to be for the operation from the mii electric mains, doing away 43 entirely with all batteries. . The prices of these A.C. sets are being reduced and on inquiring from radio dealers, they are selling for little more than’ battery operated sets. But it will be a long time, if ever, before batteries are done away with entirely, and so for the present we must be content to heave half a hundredweight of lead, or so it seems after quartermile, down to the service station every fortnight, unless of course we charge them at home. ‘To buy a five ampere charger costs anything from seven to ten pounds. The average man would sooner put this money in the other pocket and suffer a little inconvenience by taking his battery for its fortnightly walk. A trickle charger can be purchased for £3 or under, but even this price could charge the battery a few dozen times. No one, however,! could say that to invest 15s. and an evening’s labour in making a home charger is a bad way of saving money, after the price of one or two visits to the charging station. The trickle charger to be described is one that will give years of satisfactory service providing it is given a little attention periodically. As it employs a chemical rectifier, water will -have to be added every now and then, terminals will have to be kept clean, ete. When working efficiently it will charge at about } ampere, so that with the average set using four or five low consumption valves, one or two nights per week, should keep the battery up to scratch. Also, by the inclusion of a lamp in the circuit, this charger can be utilised to charge B accumulators at varying rates, depending on the resistance of the lamp used. The Electrodes. ‘THE eluminium for the electrode should be as 1. rly chemically pure as possible. Very often it is obtainable in rods 4-inch diameter while again, sheet metal is all that can be obtained. If in strips, a piece 6in. long and #in. wide will be suitable The disadvantage of the impure element is that it tends to be corroded in use and form a white deposit on its sides. This white deposit is not a rectifying medium, and must be scrap-

ed away periodically. It is advisable to fix the aluminium strip in such a way that it is easy. to replace by a new one, Owing to impurities in the commercial ammonium phosphate, the lead electrode assumes a brown appearance after a few weeks’ use. This should ‘also be wiped away periodically, as this brown deposit is a bad conductor of electricity.

Assembly. A POUND glass jam jar is quite suitable for this charger, and should measure about 5 to 6 inches high and 3 to 4 inches in diameter. A wooden cap must be made to fit in the top of this jar. This can be either turned in a lathe or made by fastening two wooden circles of wood together-one small enough to slide in the neck of the jar, the other large enough to form a shoulder, so that it will not slip right into the jar. Next, two slots are cut in the wooden lid, about 2 inches apart and parallel with each other. These slots need be just large enough to let the aluminium and the lead electrode slide through. An angle is made in both electrodes, so that they hang down into

the jar within about an inch of the bottom. Another bend is made in each strip, so that when two terminals are fastened at the top of each, ,connections can easily be made to the various parts inside the box. A small 3-8in. diameter vent-hole' should be drilled in the centre of the lid to allow all gas to readily escape. The wooden top is now either painted or soaked in hot parafiin wax and allowed to cool. This prevents the wood from absorbing the

moisture when thrown off as spray. and to a certain extent prevents creeping, The dimensions of the wooden box depend on the size of the jar, assuming that the jar is 3 inches diameter and 4% inches high. BOX has to be constructed, with inside measurements 84in. long, 4in. wide and 44in. deep. This can be made out of half-inch timber and nailed together except one side, which has to be removable and is better screwed into place. In one end, an inch from the bottom is cut a slot 38#in, long by lin. wide, on which is screwed the terminal block. In the top at this same end a circle is scribed, 1-8in. greater than the diameter of the jar. This hole is cut so that the jar slides into the box with the glass lip to prevent it from going right through. If the jar is. higher, then the bottom will rest on the base of the box inside. A 5-16 inch hole is drilled in the middle the end opposite to the one in which a slot has been cut. The whole box can now be either painted or stained and varnished. While this is drying the terminal strip can be prepared. ; . A piece of ebonite, 4in: by 14in. is filel square and four holes drilled to take the four terminals from the A ar" B batteries. If these terminals are placed in a row, an inch apart, the terminal strip can be screwed over

the slot so that the nuts at the bac can be used to fasten the leads insid¢ the box. alae By glancing at the diagram it be seen where the lamp holder and the tumbler switch are screwed. ‘These are on the top and-cirectly underneath is fastened the small bell-ringing transformer. One should be obtained with the primary windings to suit the supply and the secondary voltage about ten to twelve. These small transformers are quite inexpensive and will stand a load of about 20 watts without unduly heating. Wirin, Up. K\VERYTHING is now ready for wiring up. Insulated wire must be used and lengths of rubber-covered flex or single copper wire with an insulated sleeving will prove quite suitable. About 6 feet of twin flex should be allowed with a lamp adapter at one end. This flex should be passed through the small hole in the end opposite the slot for the terminal strip, and a knot tied so that no strain will be put on the terminals of > transformer, if the flex happens to be pulled. The circuit diagram shows all the connections, but each wire should be checked over as it is put on. One end of flex supply wire goes rectly to the switch, while the other suply lead joins to one of the 230 vy. terminals of the transformer and also’ to one side of the lamp socket. The. other terminal on the 280 v. side of the transformer joins to three points, the lead electrode in the jar, the free side of the tumbler ~ switch and also to one of the terminals on the low voltage side of the transformers. The terminal strip now requires joining up, and the connections run as follows: Terminal marked A goes to the free terminal or low voltage side of transformer; B_ terminal to free side of lamp socket; terminals A plus and B plus join together and also to the aluminium strip in the rectifying jar. Check over all connections carefully before turning on . the power. The end terminal strip should be distinctly marked on the outside so that no mistakes can occur when connecting up the batteries for charging. The Solution. Now prepare the solution for the jar, This is a saturated solution of ammonium phosphate in distilled water. Put the $lb. into a jug and pour on about 12 ounces (just over half a pint) of the distilled water and stir well. When the liquid has dissolved all it can of the salt, pour off the clear liquid into the rectifying jar. This should be filled to within one inch of the wooden cap. To keep this at the correct level it would be a good plan to drill two inch diameter holeg. on each side of the box on a levet with the level of the liquid, and it can easily be seen. if any water has evap‘orated off. Forming the Electrodes. [tHE charger is now complete and all that remains is to form the electrodes. This is done by putting a lamp, a 75-watt lamp is quite suitable, in the lampholder on the charger.

4 Short circuit the B plus and B minus ~ terminals with a piece of wire and turn on the current. The lamp will be found to light up brightly at first, and gradually it will grow duller until it is just half brilliancy. In this condiition the charger is rectifying properly Crit can be connected to the battery. When in use always remember to disconnéct the battery unless the power is switched on. As a rectifier, the chemical type is not 100 per cent. efficient. It relies on its properties of being able to pass more current in one direction than in the other. A little current will, however,.pass in the opposite direction and unless the power is turned on to more than counterbalance the flow in the charging direction, the accumulator will gradually discharge through the jar and the transformer windings. Another point worth noticing is the fact that both the A and B charger terminals are connected to the mains, and therefore both A and B batteries must be disconnected from the receiver, which is earthed, before they are connected up for charging. The operator must also stand on some insulating material, otherwise he is liable to get a shock when the terminals are touched. In connecting a battery up to be charged, remember the positive terminal of charger joins to positive of the accumulator-plus to plus, and negative to negative.

As a means to show whether the charger is functioning properly when in use, the following polarity tester will be a great help. Full constructional details appeared in the "Radio Record" of June 8: A small glass pot is fitted with two copper electrodes similar to the rectifying jar already deseribed. These electrodes are brought out at the top and short lengths of wire fastened on. This pot is then filled about half-full of a saturated solution of common salt, with. a pinch of phenolphthalein added. As much as will be on a threepenny piece is enough. It is only slightly soluble in water, so there is no point in using toa much. This compound is used in chemistry to detect the presence of an alkali, and if a current is passed through a solution of common salt, caustic soda is formed on one of the electrodes, which immediately turns the phenolpthalein a deep, red colour. This property underlies the principle of this polarity indicator. On passing a current through this piece of apparatus, a deep red colouration will be formed on the negative electrode. To use it with this charger, break the negative lead from the battery to charger, and insert this indicator. If the accumulator is being charged, a bright red colouration will be seen on the electrode that is connected to the charger A terminal. If red is noticed on both the electrodes, no charging is taking place, and the rectifying cell must be formed again.

List of Components

1 Bell-ringing Transformer 10-12 volt secondary. Lead Strip, lin. x 6in. Aluminium Strip, or rod, 6in. long. 1 Tumbler Switch. ) 1 Lamp Adapter. Glass Jar. Half Ib. Ammonium Phosphate. Distilled Water. Six Terminals. Length of Insulated Connecting Wire. Box Container (as specified).