Audio Choke Construction

Radio Record, Volume IV, Issue 17, 7 November 1930, Page 17

Audio Choke Construction

Detailed Account for Amateurs

[N response to applications from many correspondents, we are republishing a section of the 1930 "Listener’s Guide," dealing with choke censtruction. We regret that so many of our readers have been unable to obtain copies, and are doing our best to meet the situation by reprinting the most wanted sections in the "Record." Assembly of chokes differs from that for a transformer on account: of the "gaps." which are ‘breaks in the continuity of the core introduced to prevent the core becoming ‘macneti-

cally "saturated,’ as even before saturation point is reached the inductance lowers considerably. On account of this’ it will be seen that a choke holds only. its nominal inductance so long. as the specified mills carried is not exceeded. Saturation is avoided when increased mills. are carried, by increasing the width of the gaps, but this also lowers the inductance, therefore it must be recognised that the earrying capacity of a choke-is fairly limited if the nominal inductanee is to be maintained, that heavy carrying capacity can only be obtained by the employment of a massive core and ample gaps-sometimes up to din. The pieces of stalloy that’ pass through the centre of the winding are cut to project the width of the stalloy at each end. ‘These: are packed in tightly, after which the remainder of the core is assembled. Squares of cardboard of suitable thickness to form the gaps are stuck to. the sides.of the projecting central core, and the short pieces of stalloy assembled against these. Clamps, which may be of wood, are provided at each end and bolted together. Four sizes of stalloy. are used in building a choke core, one heap the thickness of the core for each size.. That is, for a 1in. core, each pile would be i}in. high. Hnamel is the only. practical insulation for wire to be used .in small chokes, because the inductance depends

largely upon the number of turns that ean be put into .:given space, or, in other words, upon how near to the core the turns may all be placed. Inductance varies as the square of the turns, and proportionately to the cross-section of the core. If the number of turns is halved, the inductance drops to one-quarter the value, but if

the cross-section of the core is halved the inductance will be halved. It is thus seen that an alteration in the number of turns has a much greater effect than an alteration in core cross-section. Causing the turns to occupy too great a space is equivalent to reducing their number, as the distance from the core renders them less effective. This un- . Lhoke Construction

necessary spacing may be caused either by using too heavy gauge of wire, or by using wire with bulky insulation. In gauges of wire smaller than 26 ‘s.w.g. double cotton covering occupies more space than the conductor, and of 26 only half the number of turns of d.c.c. can be put in the space occupied by a given number of turns of enamel led. Coming down to 36 gauge, it is found that d.c.e. occupies over 4 times as much space as enamel, therefore d.c.ec. Wire cannot be used for. small chokes if high inductance is required, without unduly increasing bulk and direct-current resistance on account of. the excessive number of turns required. , The foregoing also shows that in order to keep the inductance high, the gauge of wire should be no larger thau,

will safely carry the current to be passed. * ‘Chokes toicarry not more than 10 milliamperes are sometimes made without gap, giving a very high inductance, This type can be utilised for impedance or choke. coupling in audio. amplifiers, The ends of the winding are, usually brought out to two" terminals in a strip’ of ebonite, which is attached to the wooden clamps in‘a suitable: posiion. ‘ aa A table is given of particulars $f core and number of turns of wire for small audio?frequency chokes for eliminators, choke coupling, suppres: sion of "motor-boating," etc. Dimen' sions are also given of a core suitable for chokes wound with wire not heavier than 34. The smaller dimen: sion of the window may be altered to suit ‘the requisite turns. A further table shows approximately how many turns of a given gauge of wire may. be put into a "window" of given size, whilst the following tables enables the approximate resistance to be found. by multiplying the figure there given by the number of thousand turns to ,be used. whilst at the right-hand of the game table the weight of wire. per thousand turns is given. Thus the weight to be purchased for a given size may easily be calculated, although a ‘small amount in excess should al--ways be allowed, also bearing in mind that not less than lb. can be. pure chased. wines S If a choke is constructed ‘to give"a _ certain inductance when .the maximum specified current is passing, then themore current is reduced, the higher will the inductance value rise. ' Chokes for "A" Eliminators. A GOOD low inductance choke is es . sential to an "A" eliminator, and . the one here outlined will be found to answer the purpose well. -Details: of choke. construction -will be found -in (Concluded on page 29.)

Choke Construction TS

(Continued from page 15.) another article, so only the specifications. will be given here. The choke is constructed of 1}in. stalloy built to 1}in., and with a ‘winding of 300 turns of 18 s.w.g. dice. will have an inductance of .5 henry when passing 2 amperes, the maximum current. The spool ends are 2%in. square by 1-8in. thick, preferably of fibre, with an inside length of 24in., which will accommodate 38 turns per layer if the wire is fairly straight. Hach layer should be shellaced when wound and allowed to dry for a few minutes before covering with brown paper and proceeding with the next. The "window" in the core will measure 2% by $, which is sufficient to accominodate the eight layers necessary. An important point in this type of choke is that the direct-current resistance shall be low, and in this case it is less than 1 ohm, .91 to be exact, so that when one ampere is passing the drop will be .91 volt, and with two amperes 1.82, or less than two volts. If a lower resistance than this is required, 10 s.w.g. wire could be used. The amount of stalloy required will be 24 dozen 3ft. strips. Increasing the dimensions of the core raises the inductance, but as each turn is then longer, the resistance is not greatly reduced, although fewer turns will give the same inductance. A smaller choke of 4 henry inductance may consist of 360 turns of 18 s|w|g| on a lin, core, 12 layers, 30 turns per layer, the inside of -spool being 2in. longer, ends full 2% square. The larger choke takes 1lb. 90z. of wire and the smaller 1lb 180z. Considering the smaller inductance obtained from the smll choke, and the fact that its resistance is slightly greater than the layer, there is no advantage in constructing the small choke except in a case where space occupied is an object. The two gaps in the large choke should be 38-82in. each if the full 2 amperes is to be carried. The gaps in the small choke should be not less than 1-16in. each. These chokes are suitable for smoothing the output of an "A" battery charger in conjunction with an electrolytic condenser of high capacity.