Constructing Iron Cored Choke Coils

Radio Record, Volume II, Issue 13, 23 November 1928, Page 28

Constructing Iron Cored Choke Coils

By

Megohm

TBON-CORED or audio frequency choke coils play an important part in radio apparatus such as eliminators, and filters of various kinds, either to prevent direct current flowing through the windings of the loudspeaker, or to smooth out the ripple in the rectified current of an eliminator. In deciding upon the proportions for a choke coil, the purpose for which it is to be used must be carefully considered, so that the earrying capacity of the wire is sufficient, though not greatly in excess of requirements, and its direct current resistance kept down to permicsible limits. The gauge of wire haying been decided, the number of turns and cross-section of the core, which should be preferably of stalloy, must be decided so that the combination will give approximately the inductance required, which is stated in units known as "henrys." iene

The chief trouble with the amateur constructor is to obtain particulars of a combination that will yield just about the required inductance for a particular purpose, and it is chiefly the objéct of this short article to give a few leading dimensions and particulars that will enable the con-. structor to make up a choke coil of given inductance. Main Considerations. N designing a choke coil there are conflicting factors that. must be taken into account. The smaller the cross-section of the core, the greater will be the number of turns required mew sasinee

to produce @ given inductance. Ok the other hand, if the number of turns is too great, there will be magnetic saturation of the core, and even when this state is being approached, the inductive value of the coil begins to lower. This possibility of saturation is greater as the current traversing the windings increases, and is guarded against by providing one or more airgaps across the section of the core, such air-gaps being usually filled with a piece of cardboard of suitable thickness. The above shows how the number of turns must be balanced against the current to be carried and the liability of the core to saturation. Another governing factor is the direct-current resistance of the winding, which in many cases must be kept down in order not to unduly decrease the voltage of current passed through. This factor has to be balanced against the advisability of using a reasonably small gauge of wire in order to prevent the necessary turns from occupying too uch space, as the average choke will contain 5000 or more turns, and for this reason enamelled wire is almost invariably used. Insulation is not usually an important item, because the voltage drop across the whole of the winding will be comparatively small, so that the difference of potential between layers is very slight. Inductance varies about as the square of the number of turns. ‘This means that if we have a given choke of say 20 henrys and take off half the number of turns, the inductance will then only be one quarter of the original; that is, 5 henrys. This shows that a high inductance value can only be obtained with a large number of turns, and by increasing the crosssection of the core, the inductance value of any given number of turns is proportionately inereased. Thus if a one-inch square core carries turns giving a certain inductance, twice the inductance will be obtained theoretically if the section of the core is doubled by increasing it to just over 1 3-8inch square, Core Material.

TALLOY strips are the best and most convenient eore material, and all specifications to follow are based upon it. The actual construction of the chokes will not be dealt with here. The core material of burnt-out audio transformers is often used, and if the transformer is not of too small dimensions, a very good choke results when the windings are replaced by 36'S s.w.g. enamelled wire, aS many turns as possible being put on a spool constructed to fit. If the core of the audio transformer is less than 5-8in. square, its inductance would be too low to be useful for ordinary purposes. If the laminations are in the form of E’s they are all assembled in the same direction without staggering, and a pile of narrow stalloy strips is cut to equal the total thickness. These strips then butt across the ends of the three limbs of the E’s with paper interposed to form the gaps, and are clamped by four strips of wood, secured by brass bolts: through the ends. This arrangement allows the new spool to occupy the full length of the

central bar of the BH, thus giving room for the necessary number of turns. If 6000 turns of 86’s wire are wound upon a core 5-S8in. square, a choke of not more than 10 henrys will result. A Sin, square core willgivea higher inductance proportionately, and a further increase on account of probably} giving space to accommodate a greagér number of turns, and such a choke coil would be suitable for an output filter, B Eliminator Chokes. N this type of choke the direct-cur-rent resistance must be taken into account, and this is easily ascertained from a table giving "ohms per pound." A suitable gauge of wire would be 32’s s.w.g., Which has a d.c, resistance of 247 ohms per pound, or 262 ohms per 1000 yards, this latter quantity weighing a little over one pound. ‘The usual inductance is 20 henrys, andifailxl core of stalloy is used, 6250 turns, should be put on a spool 14 to iin. long. The gap of a choke is determined by measuring round the centre of one layer of laminations and allowing 0.005in. for each inch measurement. The correct gap for the above choke would be 3-128, or, if there are two gaps, half this in each. General Design. " the turns of wire are most effec: tive when near the core, the spool should not be made too short with increased depth, giving a pancake formation. The shape to aim at for the coil is a cylinder with the length equal to or slightly greater than the diameter. Increasing the gauge of wire greatly reduces the inductance, all other fac» tors remaining the same. Particulars of Chokes. "THE following particulars will be @ guide to obtaining given inductances. The length of spool must be proportioned as above. Where the maximum current that the wire will carry is to flow through the winding, the gap can be increased to prevent saturation. The amount of magnetisation of the core may be tested by holding a piece of iron near the gap. , 4 e

Current-carrying Capacity of Wire. The current-carrying capacity of wire should be taken as follows :- 30’s s.w.g. carries 150 milliampered, 32’s gs.w.g. carries 120 milliamperes. 34’5 g.w.g. carries 90 milliamperes, 36’s s.w.g. carries 56 milliamperes. 89’35 s.w.g. carries 38 milliamperes, 40’s s.w.g. carries 24 milliamperes,

#-core lin. core Henrys. turns..- turns. 5 3000 — 10 5000 8800 15 6300 4800 20 7600 5700 5O 14000 11000 100 — 18000

36's S.W.G. Enamelled Wire.

tin. core ®in. core Henrys. turns. turns. 5 2600 -- 10 3800 1900 1d 4800 2400 20 6250 2900 50 -_ 15300 ©

32's Enamelled Wire.