How to Design an Iron Core Audio Frequency Choke Coil
By A. K. Aster, Ph. D., University of California
Radio for July, 1922
The design of a choke coil may be determined f r o m five factors, namely: the inductance of the coil, the length of the magnetic field, the area of the core, the permeability of the iron, and the number of turns of wire. As two or more of these factors are likely to be unknown at the outset, it is necessary to assume certain values for them, so that we may determine approximate values for the other factors. To design a coil for a definite inductance, we must proceed more or less by trial, making reasonable assumptions and trying them out in the formula. If the desired result is not obtained, we make some changes and try again, until after several trials the desired value is obtained.
Let us therefore proceed to work out the design for a 100 henry choke coil to be wound on a rectangular closed core, having a cross sectional area of one sq. cm. and a magnetic path of 15 cm. Thus we have three of the five required factors and have only to determine the permeability of the iron in the core and number of turns in the coil.
The permeability depends upon three things: the kind of iron, the magnetic flux, and the frequency.
Only the best kind of silicon steel should be used for the core. Curves or tables giving values of permeability for differences in magnetic flux are furnished by the manufacturers. For instance, Fig. 1 shows the magnetization curve for Apollo special electrical steel, made by the American Tinplate Co.
Fig. 1. Magnetization Cirves for Apollo Special Electrical Steel. Readleft column μ as permeability in lines per sq. cm.
Let us therefore assume a magnetic flux of 12 lines per sq. cm. From Fig. 1, it will be seen that the corresponding permeability is 1000. But this value is for low frequency and must be multiplied by a factor to give the correct value for a higher frequency of, say, 1000 cycles, which we will adopt as a standard, because it is the average frequency of the human voice and so is suitable for both telephone and telegraph work. For No. 29 gauge sheet steel, this correction factor is 5/8. Therefore, the effective permeability is 1000x5/8=625 for a frequency of 1000 cycles.
Consequently our problem now reduces itself to finding out the number of turns necessary for a 100 henry choke coil, having a one sq. cm. core area and 15cm. magnetic path, assuming that the iron has a permeability of 625 for 1000 cycles. This may be found from the equation"
where t-number of turns, L- inductance=100 henries, l-length of magnetic path =15 cm., a-cross sectional area of core =1 cm., and μ- permeability=625. Substituting these values in the equation, we have:
So now we know that 13,823 turns will give an inductance of 100 henries, if wound on a rectangular core 1 cm. in area with a magnetic path of 15cm., as shown in Fig. 2, if we have a magnetic flux of 12 lines per sq. cm.
Fig. 2. Choke Coil for Audio Frequency Amplifier
The next question that arises is how many amperes in such a coil will cause a magnetic flux of 12 lines. This may be found from the equation A=.8HL/t, where A= current in amperes, H= magnetic flux-12, L=--inductance= 100 henries, t= number of turns =13,823. Substituting these values and solving, we get A =.8x 12x 100 -:- 13823=.07 appromixately.
The next thing to determine is the size of wire to be used. The usual rule in transformer design is to allow 1000 circular mils per ampere. Since the rating of this coil is .07 ampere, 70 circular mils should be allowed. Therefore No. 36 B. & S. gauge wire will be ample, this value being determined from the ordinary wire table.
Fig. 2 shows the general details of the core discussed. All joints should be lapped. A butt joint must never be used, as the magnetic leakage of this type is very high. The details of the actual method of winding the coil have been omitted, as it was assumed that the reader is familiar with them.
There are probably some readers who have not the equipment or time to make their own coils, and who would like to know what standard apparatus is available on the market, as there are no choke coils actually being made especially for use in amplifiers. After testing a number of standard coils, the writer believes that the smallest size Wayne bell ringing transformer will be satisfactory for amplifier work. This coil has an inductance of about 30 henries. The 110 volt winding should be used as a choke coil, and the 12 volt winding should be left open. This transformer can be purchased from any dealer in electrical supplies.
