Audio-frequency Choke Coils

Radio Record, Volume II, Issue 47, 7 June 1929, Page 28

Audio-frequency Choke Coils

Their Construction and Uses

By

MEGOHM

HE chief uses of audio frequency chokes are for smoothing out the ripple in the rectited current of A or B eluminators, or in conjunction with a fixed condenser as an output filter, or as the "impedances" in chokecoupled amplifiers, and in valve plate leads with the object of preventing feed-back through battery or eliminator. This stops the tendency to "motor-boating." This type of choke coil has invariably an "iron" core, or so it is usually called, though the actual core material now generally used is stalloy. For this purpose it is superior to iron in every respect. Stalloy carries a much greater number of lines of magnetic flux to the inch than does common iron, and this property is of great importance in the construction of chokes, especially those which are to pass current amounting to more than a few milliamperes. The purpee of an audio choke is to allow the passage of direct current such as that in a valve plate circuit, and at the same time prevent’ the passage of alternating current such as the speech frequencies imposed upon the plate current. The "choking’ ’of these alternating impulses depends upon the suitable construction of the coil for the purpose, though the latitude in this respect is very great, and offers no stumbling-block in construction. An audio choke should offer a very high impedance to frequencies between 100 and 5,000, but the resistance of the wire composing the winding must not be so great as to reduce unduly the voltage of the direct current traversing it. It should be stated here that "impedance" sums up the opposition offered to alternating current, by combining the effects of resistance and "react-

ance," or "inductance," which latter depends upon magnetic effect, this being greatly increased by the presence of the core material. Reactance varies according to the frequency applied to the coil, and the lower the frequencies to be held back, the greater must be the impedance of the coil. Thus it is seen that the coil must be large enough to hold back the lowest frequencies (lower notes) handled, otherwise reproduction will be impaired. The inductance of a choke is expressed in units known as "henrys." Saturation of Cores. T is a well-known fact that a direct current flowing in an insulated winding round a bar of iron will.cause the iron to become magnetised whilst the current is flowing. This is known as an "electro-magnet." The strength of the magnetic field in the iron increases as the number of turns of wire are increased in number, the current remaining the same, until a certain point is reached, when the iron is said to be "saturated" with magnetism, and no increase of turns or current will further strengthen the electro-magnet. Exactly the same action takes place in a choke coil. Usually the current used is comparatively small, but the number of turns is large, quite sufficient to "saturate" a core of unreasonably small dimensions. And when the core of a choke is even approaching saturation, its inductive value begins to drop. When saturation is reached, it is seriously impaired. This shows that if a choke is to function properly, means must be taken to prevent magnetic saturation of the core.

The inductance of a choke is creased(a) By increasing the number of turns. (b) By increasing the cross-section of the core. (c) By reducing the "gap" (if any). Saturation is produced by(a) Passing more than the specified current through the windings. (b) By using too thin a core. (c) By too many turns of wire. (d) By too small a "gap" or none at at all.

Function of the "Gap." {[t will be seen from the above that to obtain a high value of inductance by increasing the number of turns, there is a danger of approaching the saturation point. A much safer plan is to increase the cross-section of the core, but where much current is to be earried, another means -. of avoiding saturation is adopted. This consists in having one or more distinct breaks through the core, which is in two or or more separate sections of piled-up laminations. At these breaks or "gaps" the two adjacent sections of the core are not allowed to come into metallic contact, a piece of cardboard or thick paper being introduced, which allows of pressure between the two sections. The "gap" is usually given in specifications in one measurement, but if there is more than one gap, this measurement must be divided equally between the two or more. If 1-32 of an inch is specified for the gap, then two gaps would be made 1-64 each. If a choke is found to be saturating when tested by holding a piece of iron near the gap, the latter:can be increased, but at the same time it must be remembered that this decreases the insuctance, and cannot be carried very ar. Chokes can be constructed without gaps, but only when the maximum direct current to be passed is known. Such chokes are small and compact, as the absence of a gap greatly reduces the number of turns required for a given value, and if the required inductance is not high, will also allow of a reduction in the size of the core. The reduction of turns is a good feature, because it reduces the direct-current resistance. It is not recommended that the home

constructor should make eliminator smoothing chokes except in the case of a small eliminator for a two-valve crystal amplifier. Output filter, amplifier plate and grid chokes, plate feed chokes, and those for other suitable purposes may be constructed without gaps if made and used according to specification. Inductance varies as the square of the turns and proportionately to the cross-section of the core. That is to say that if the number of turng is

halved the inductance drops to onequarter the value, but if the crosssection of the core is halved or doubled, the inductive value will be halved or doubled, as the case may be. With this short survey of the main points affecting the inductive value of chokes, we will proceed to deal with their construction, especially with a view to the special purpose for which they may be required. . General Construction. "THE general construction of a choke coil is much the same as that}! a transformer, and so far as the col. Ps concerned, if no gap is provided, the laminations are built up exactly as in a transformer. If a gap, or usually two gaps are to be provided, then the

\ pieces of stalloy comprising the core are in four different sizes instead of two, and the assembling is different. Outting of stalloy strips is usually carried out by the home constructor with a pair of snips, and this process unfortunately puts a slight curve in the end of every piece, which necesistates tapping the ends flat on an iron surface, The size is easily gauged by holding a piece of the required size on the long strip, and cutting close against the end of the gauge. In both Auckland and Wellington there are firms which supply stalloy cut to sizes ready for assembly. If the stalloy is not coated with insulation, it must be thinly coated with shellac dissolved in

methylated spirits and applied with a brush, each piece being treated separ- . ately after cutting. A-set of very useful stalloy stampings is now available for constructors at Thos. Ballinger’s, Wellington. These stamping consist of 'T’s and U’s which fit together,.each pair forming a complete layer. They can be assembled brick-wise without gap, or all one way, which leaves three gaps. A diagram is given of these stampings, showing the dimensions. They have the usual

insulating composition on one side and oxide on the other, and require no shellac coating. They are absolutely flat and truly cut, and pack together very snugly. .Besides being very suitable for eliminator chokes, they will also make good transformers-a matter which will very shortly be dealt with in these pages. A diagram shows the appearance of a home-built choke coil, whether with or without gap. A pair of wooden clamps of suitable thickness secures the laminations at either end of the Spool by means of brass or iron bolts of correct length. Threaded brass rod s handy for bolting, as it can be cut with a hack-saw to any length. ‘The actual construction of spools to hold the winding, and the process of winding is fully dealt with on page 57 of the "Tisteners’ Guide." In the case of a choke coil there is only one winding, usually of enamelled wire, which is wound on without much insulation of various portions, in fact the winding appears about the same as that upon the spools containing the wire when purchased. A layer of adhesive tape wound with

overlapping edges on the outside makes a good protective covering. Double cotton-covered wire is used only when it is important that the selfcapacity of the coils should be kept down, but this is not usually an important point, so enamelled wire is used on account of the great saving in space thereby effected. In very thin sizes of wire the double cotton-covering increases the total cross-section. to several times that of the same gauge of enamelled, so that on account of the great increase in bulk, its use would in many cases be impossible. Reference to the table on page 57 of the "Guide" shows that 27,889 turns of 38’s enamelled wire will occupy a square inch, only 5,625 turns of the same gauge can be put into that space if the wire is double cotton-covered. This table will be found particularly useful in the construction of chokes, as by its aid the space to be occupied by any number of turns can be decided. Supposing that the window of a choke core measures two inches by half an inch, its area is one square inch, Deduction must then be made for space occupied by spool ends, for insulation round the core and outside, and for the fact that the turns will not pack as closely as the mathematical reckoning indicates. It would be safe to deduct 80 per cent. and reckon 2-3 square inch available for the wire. Then this space wound with enamelled wire would take 5000 turns of 32’s, 6700 84’s, 10,000 36’s and so on. In most eases the window size will be decided after the winding has been carried out, so that the table will be used to determine the dimensions of the spool, especially with the object of preventing the length from being too short. From 14 to 2 inches is the usual length, in proportion to the thickness of wire. For 30’s and larger, the length may be over 2in. if the number of turns is large.

N spite of the advantages to be gained by the use of a choke-condenser output filter following the last valve of a receiver, a great many listeners still neglect to instal this comparatively small and inexpensive addition to the circuit. Its use on a short-wave receiver cuts out quite an amount of capacity and other trouble. Several good makes can be purchased ready made, but constructors can easily make the coil and purchase the 2mfd. condenser, which should be ef high test to stand up to the plate voltage of the power valve. A neat 20-henry choke to suit the average receiver with power-valve is made with a core Hn. square, spool 2in. long inside, and wound with 8000 turns of 36’s enamelled wire. Window, 24 x lin. Gap each end, thin card in each. One dozen 3ft. strips of stalloy required.. This will have a direct current. resistance of about 700 ohms. Space does not permit of further treatment of this subject, but the article will be concluded next week, when two useful tables of chokes with and without gaps will be included.

TF one of the joints in a crystal detector is suspected of being imperfect (for instance, the joint between the outer terminal and the cat’s-whis-ker), a distinct improvement will often be noticed if a flexible lead is soldered between these points. ee

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