A Diagnosis of Radio

Radio Record, Volume V, Issue 1, 17 July 1931, Page 16

A Diagnosis of Radio

‘The Technical Editor. CHOKES ‘AND BATTERIES

R.F. and A.F. Chokes. [ HAVE been taken to task by .one of our readers for not making clear ‘the function of radio and audio chokes,: The fact of the matter is I tried to. pass over a rather intricate point by generalising, but got into deep water. ° Unfortunately a couple of words were transposed and the sense of this kernel’ ‘sentence was quite lost. However, [. shall take the opportunity to explain ai little more fully the work of r.f. andl a.f. chokes. The general function of a choke is to restrict certain impulses to any section of a radio circuit the constructor wishes. "A radio frequency’ choke, which is a number of turns of fine wire, wound on an air-core former, allows direct and audio-fréquency cur rent to pass, but stops the radio fre‘quency current. The function of an audio frequency choke, which is a larger number of turns. of heavier-wire on. an iron core, is to allow direct and radio frequency to pass, to stop audio. frequency. ° It will be remembered that a condenser passes radio frequency current, but stops direct current. Its be haviour to audio frequency current,which is current alternating at a rate slower than radio frequency current. depends upon the capacity of the condensér. A small capacity. will pass only radio frequency current, but _ the higher capacity will pass both radio frequency and audio frequency current. It can bé seen now that one of the most important factors influencing the design of the radio frequency choke is its self-capacity. If it is wound insuch.a fashion that there is a.large capacity present between wires, then it will not block the radio frequency current, but they will pass through the

vondenser part of the choke. for this reason, it is essential to see, when purchasing a radio frequency’ choke, .° tf the self-capacity ‘is. very’ low. The accompanying diagram shows more than can ‘be in a page of words. It depicts a detector valve followed by a stage of choke capacity coupling. _Now we shall start with the plate of the valve. Thé direct current flows toward us through the valve to the filament, and so round to its source.

it ‘is shown by> the thick black line. Just for the time. being we shall imagine that it is flowing the other way. Tt will, not make any difference to. our example, -but it will make things clearer. Current passes from the plate of. the. valve and goes as far as fhe join markéd "A." It now separates, and can go through the coil Li or .to the choke. Let us trace it through the coil. , It can all go through the coil, but the condenser C2 is a very serious obstacle to some of the current. It is of ‘Small eapacity, usually a 0001 to .00035, so that it can pass the radio frequency, and will block the audio and direct current. Thus a certain amount, of energy can be allowed to eseape to: earth, but it must first pass through the regeneration: coil, the field of which ‘transfers energy .to the secondary. coil I.2,.and thence to the valve. This is called regeneratiou.’ The amount of current held in the coil is determined by the setting of the condenser; the greater the capacity the more current we allow to pass back through the tickler and so to the secondary coil. Now we will go back to the radio frequency choke. The audio frequency and direct current can pass through it, but the radio cannot and this is just what we want. The current is not wanted in the rest of the circuit. If it passes the detector stage it is likely to cause instability, so a choke is put there to keep it in its right place. Furthermore, the choke has the effect of forcing it back to the. regeneration coil. We are now past the r.f. choke and come to. the audio frequency choke, which -will not allow the audio fre. quency current to pass, only the direct,.so it has only one path, and that is to the condenser. This coupling condenser is of a high value (.001 to £5 mfds.), and ‘consequently will allow, the; audio frequency :current pass to the grid of the next valve, If

we have a fairly high value coupling condenser, we shall get through the low frequency as well asthe higher frequencies’ with the result that the bass notes will be preserved. If we have a low value condenser reproduction will be tinny for the’ condenser will not pass the low frequency or bass notes. Just to make sure that no radio frequency current is left to interfere with the audio system of our wireless set, 2 condenser is often’ shunted across the audio frequency choke, or transformer, as the case may be. A.transformer primary acts in-.exactly.the same way as does an audio frequency choke, eset The same correspondent also alas that we go into the mathematics of radio a little, and also that we explain the working of a detector valve. While not making any promises, we shall see if we can possibly do this before our series close. We must not be too technical, and explaining detection is a very technical subject. Batteries ‘THE function of ‘a battery is to supply current to the radio set. There are two types of battery, the wet and the dry. '»We-shall deal with the wet battery first. A wet battery cousists of plates of soft lead immersed in a solution of-sulphuric acid. If these plates are: connected in a circuit one set of plates. being regarde:] as positive and one negative, current, will flow between the two points, The’ energy for the current is drawn from the sulphuric acid, certain elements being absorbed by the: lead. For this reason the sulphuric acid gradually becomes thinner. The specific gravity falls and we say the battery requires charging. When we change it we connect a supply of current to the battery and force it through the opposite way, driving back some of the goodness that was taken out of the sulphurie acid. Wet batteries can ‘supply usually a greater. amount of current than dry batteries so, that. they are

used for the "A" supply. Some are used for "B" supply, but they are smaller and cannot supply the same quantity of current as can the "A" battery. It is important when charging wet batteries not to exceed the charging rate. specified by the manufacturers. If this is exceeded, the battery will be come warm and damaged. Never charge so that your bat‘tery is warm when finished. If you require to charge a "B": wet battery. from an "A" battery charger connect up the individual cells of the "B" battery:as shown in ‘the diagram, The point marked with a cross indi-

. Salton where the negative of the "A" battery charger is to be connected. In other words, there should be four columus of cells. Never let a wet battery Tun anywhere near flat, ay considerable damage will be done. Dry batteries work: on a principle entirely. different from -wet batteries. ; It has been found that certain chem-' icals and metals if.combined in a certain way will force an electric current through a circuit, This knowledge has been made use of in the dry battery. Dry batteries are a type of wet cell, but different from the storage battery. ; The individual cells of a dry battery. each deliver one and ‘a half‘volts. The amount of current which such a cell or collection of cells can deliver depends upon the size of the cells themselves. _/"C" batteries then are made up ofl Xsmall cells, "B". batteries of larger cells, super "B" batteries have larger cells again, In the layer built battery the cells have been done away with and the chemical elements are arranged in layers. By doing this much space can be saved. There is another type ‘of battery which we shall only touch upon, and that is the air cell. This works upon an entirely new .principle, and these: batteries ‘are admirably suited to low voltage valves requiring a small current. They last for some two years Batteries are grouped in three classes -A," "B," and "6." We shall explain in the paragraph . dealing with valves the function of each one of these, so we shall not anticipate that section. Here are a few. hints concerning batteries :-

1. Treat wet batteries with respect and care. The plates are made up of very small pieces of lead which can be shaken from the grill holding them. If a piece of lead comes loose it is likely

to cause an internal short circuit and ruin the battery. 2. Always see that the "B" battery is large enough to supply. the current it will be called up to do.. Rather, bd over-sized than a little under-sized: When your "B" battery begins to sweat

4 = | it is an indication that it is nearing the end of its useful life and must be discarded. 8. Do not connect an old battery with a new one, hoping to drain the last out of the old battery. ‘You will surely spoil both reception and the new battery. ; 4. If you hear a crackling in your speaker and the battery is old, you can take it for granted that it needs to be renewed. Sometimes.a 1 mfd. condenser connected between B_ plus and B minus will cut out some of this noise, and ‘so extend the life of the "B" battery. 5. Do not let the level of the solution in a wet battery fall below the level of the tops of the plates. If this happens

‘fill the battery up again with. distilled water, or, if this is not obtainable, with rain water collected in an earthenware vessel. . ' vette