Making your own toroidcore inductors and RF transformers

A lot of construction projects intended for electronic hobbyists and amateur radio operators call for inductors or radio-frequency (RF) transformers wound on toroidal cores, A toroid is a doughnut-shaped object, i.e., a short cylinder (often with rounded edges) that has a hole in the center (see Fig. 4-4). The toroidal shape is desirable for inductors because it permits a relatively high inductance value with few turns of wire, and, perhaps most important, the geometry of the core makes it self-shielding. That latter attribute makes the toroid inductor easier to use in practical RF circuits. Regular solenoid-wound cylindrical inductors have a magnetic field that goes outside the immediate vicinity of the windings and can thus intersect nearby inductors and other objects. Unintentional inductive coupling can cause a lot of serious problems in RF electronic circuits so they should be avoided wherever possible. The use of a toroidal shape factor, with its limited external magnetic field, makes it possible to mount the inductor close to other inductors (and other components) without too much undesired interaction.

(A) TrifHar wound transformer circuit; (B) actual windings (0) glue or silicone sea] is used to hold the ends nf the windings.

Materials used in toroidal cores

Toroid cores are available in a variety of mat erials that art1 usually grouped into two general classes; powdered iron and ferrite. These groups are further subdivided.

Powdered-iron materials

Powdered-iron cores are available in two basic formulations: carbonyl irons and hydrogen-reduced irons. The carbonyl materials are well-regarded for their temperature stability; they have permeability (uO values that range from Ijx to abovit 35|i. The carbonyts offer very good Q values to frequencies of 20t) MHz, Carbonyls are used in high-power applications as well as in variabie-frequency oscillators and wherever temperature stability becomes important. However, notice that no powdered-iron material or ferrite js totally free of temperature variation, so oscillators using these cores must be temperature compensated for proper operation. The hydrogen-reduced iron devices offer permeabilities up to 9<V but are lower Q than carbonyl devices. They are most used in electromagnetic interference (EMf) filters. The powdered-iron materials are the subject of Table 4-2,

Table 4-2. Powdered4ion core materials

Material Permwsability (p,) Commento d i Used up to 200 MHz; inductance varies with method of winding

1 20 Made of carbonyl 0; similar to mixture no. but is more stable and has a higher volume resistivity

2 10 Made of carbonyl E; high Q and good volume resistivity over range of 1 to 30 MHz

3 35 Made of carbonyl HP; very good stability and good Q over range of 0.05 to 0.50 MHz

6 8 Made of carbonyl SF; similar to mixture ni>. 2 bul has higher Q over range 20 to GO MHz

10 6 Type W powdered iron; good Q and high stability from 40 to 100 MHz

12 3 Made of a synthetic oxide material, good Q but only moderate stability over the range 50 l a 100 MHz

15 25 Made of carbonyl GS6; excellent stability and good 0 over range 01 to 2 MHz; recommended for AM BOB and VLF applications

17 Carbonyl material similar to mixture no. 12 but has greater temperature stability but lower Q than no. 12

26 75 Made of hydrogen reduced iron; has very high permeability; used in EMI filters and DC chokes

Ferrite materials

The name ferrite implies that the materials are iron-based (they are not), but fer-rites are actually grouped into nickel-zinc and manganese-zinc type*. The nickel-zinc material has a high-volume resistivity and high Q over the range 0.50 to J 00 MHz. The temperature stability is only moderate, however. The permeabilities of nickel-zinc materials are found in the range 125 to 850^, The manganese-zinc materials have higher permeabilities than nickel-zinc and are on the order of 850 to 5000|ju Manganese-zinc materials offer high Q over 0.001 to 1 MHz. They have low volume resistivity and moderate saturation flux density These materials are used in switching power supplies from 20 to 100 kHz and for EMI attenuation in the range 20 to 400 MHz, See Table 4-3 for information on ferrite materials,

Toroid-core nomenclature

Although there are several different ways to designate toroidal cores, the one used by Amidon Associates is perhaps that most commonly found in electronic hobbyist and amateur radio published projects .

AJthough the units of measure are the English system, which is used in the United States and Canada and formerly in the UK, rather than SI units, their use with respect, to toroids seems widespread. The type mimber for any given core will consist of three elements; xx-yy-zz. The "xx" is a one- or two-letter designation of the

Making your mmt toroid-corc inductors and RFtransformers 79 Table 4-3, Ferrite materials

Material Permeability (ft)

43 61

68 72

J/75

850 850

40 40

30 2000 5000

2000

3000

Remarks'

M-Z; used over 0 001 to 1 MHz for loopstick antenna rods, low volume resistivity

N-Z; medium-wave inductors and wideband transformers to 50 MHz; high attenuation over 30 to 400 MHz; high volume resistivity N-Z; high Q over 0.2 to 15 MHz; moderate temperature stability used for wideband transformers to 200 MHz

High Q over 15 to 25 MHz; low permeability and high volume resistivity

N-Z; high Q operation over 10 to 80 MHz; relatively high flux density and good temperature stability; similar to Type 63, but has lower volume resistivity; used in wideband transformers to 200 MHz

N-Z; excellent temperature stability and high Q over 80 to 180 MHz; high volume resistivity High Q to 0.50 MHz but used in EM] filters from 0 50 to 50 MHz; low volume resistivity

Used in pulse and wideband transformers from 0.001 to 1 MHz and in EMI filters from 0.50 to 20 MHz; low volume resistivity and low core losses

0.001 to 1 MHz; used in wideband transformers anrt power converters and in EMI and noise filters from t»J> to 50 MH2

Is similar to Type 77 above bfet offers a higher volume resistivity, higher initial permeability, and higher flux saturation density; used for power converters and in EMI/noise filters from 0.50 to 50 MHz

1 N-Z nicket-iUvi M-Z: manganese-zinc general class of mat erial, i.e., powdered iron txx = "T") or ferrite (xx = "TFT The "yy1 is a rounded-off approximation of the outside diameter (o.d. in Fig. 4-4) of the core in Inches; "37" indicates a 0.375" (9.53 mm) core, while "50" indicates a 0.50" (12.7 mm) core. The "zz" indicates the type (mixture) of material, A mixture no. 2 powdered-iron core of 0.50" diameter would be listed as a T-50-2 core. The ( ores are color-coded to assisi in identification.

Inductance of toroidal coils

The inductance of the toroidal core inductor is a fmiction of the permeability of the core material, the number of turns, the inside diameter (id) of the core, the outside diameter (o.d.) of the core, and the height (/¿) (see Fig. 4-1) and ean be approximated by:

This equation is rarely used directly, however, because toroid manufacturers provide a parameter called the AL valuet which relates inductance per 100 or 100 turns of wire. Tables 4-4 and 4-!> show the Rvalues of wmmon ferrite and powdered-iron cores.

Table 4-4. Common powdered-iron Ai values

Core material type (mix)

Table 4-4. Common powdered-iron Ai values

Core material type (mix)

Core size

3

15

1

2

6

10

12

0

12

60

50

48

20

17

12

7

3

16

61

55

44

22

19

13

8

3

20

90

65

52

27

22

16

10

3.5

37

275

120

90

80

40

30

25

15

4-9

50

320

175

135

100

49

40

31

18

6-4

68

420

195

ISO

115

57

47

32

21

7.5

94

590

248

200

160

34

70

58

32

10.6

130

785

350

250

200

110

96

15

200

896

425

250

120

100

_

Table 4*5. Common ferrite-core AL values

Material type Core _

Table 4*5. Common ferrite-core AL values

Material type Core _

size1

«43

61

63

72

75

77

33

188

24.8

7.9

396

990

356

37

420

55.3

17.7

884

2210

796

50

523

68

22

1100

2750

990

50À

570

75

34

1300

2990

1080

50B

1140

150

48

2400

2160

82

557

73.3

22.8

1170

3020

1060

114

603

79.3

25.4

1270

3170

1140

I14A

146

2340

240

1249

173

53

3130

6845

3iyo

Winding toroid inductors

There are two basic ways to wind a toroidal core inductor: distributed (Fig. 4-5A) and close-spaced (Fig. 4-5B). Jn distributed toroidal inductors, the turns of wire that are wound on the toroidal core are spaced evenly around the circumference of the core, with the exception of a gap of at least 30° between the ends (see Fig. 4-5A). The gap ensures that stray capacitance is kept to a minimum. The winding covers 270* of the core. In close winding (Fig, 4-5B), the turns are made so that acjjacent turns of wire touch each other. This pratice raises the stray capacitance of the winding, which affects the resonant frequency, but can be done in many cases with little or no ill effect (especially where the capacitance and resonant point shift are negli-

240 Inductor

Toroid core

Toroid core

4-5 Tbroid winding styles: (A) distributed, (B) close wound gibîe), In general, close winding is used for inductors in narrowband-tuned circuits, and distributed winding is used for broadband situations, such as conventional and balun RF transformers. The method of winding has a small effect on the final inductance of the coil. Although this makes calculating the final inductance less pre dictable, it also provides a means of final adjustment of actual inductance in the circuit as-built.

Calculating the number of turns

As in all inductors, t he number of turns of wire determines the inductance of the finished coil. In powdered-iron cores, the^lL rating of the core is used with fair confidence to predict the number of turns needed For powdered-iron < ores:

where

LyM = inductance in microhenrys (>H)

LmH = inductance in millihenrys (mH)

At is a property of the core material.

Building the toroidal device

The toroid core or transformer is usually wound with enameled or formvar-insulated wire. For low-powered applications (receivers, VFOs, etc.) the wire will usually be no. 22 through no, 36 (with no. 26 being very common) AWG. For high-power applications, such as transmitters and RF power amplifiers, a heavier grade of wire is neeued. For high-power RF applications, no. 14 or no. 12 wire is usually specified, although wire as large as no, 6 has been used in some commercial applications. Again, the wire is enameled ot forcnvar-covered insulated wire.

In the high-power case, it is likely that hi£h voltages will exist. In high-powered RF amplifiers, such as used by amateur radio operators in many countries, the potentials present across a 50-il circuit can reach hundreds of volts. In those cases, it is common practice to wrap the core with a glass-based tape, such as Scotch 27.

High-powered applications also require a large-area toroid rather than the small toroids that are practical at lower power levels. Cores in the FT-15Q-z2 to FT-240-az or T-130-zz to T-500-zz are typically used. In some high-powered cases, several identical toroids are stacked together and wrapped with tape to increase the poiver-handling capacity. This method is used quite commonly in RF power amplifier and antenna timer projects.

Binding the wires

It sometimes happens that the wires making up the toroidal inductor or transformer become loose. Some builders prefer to fasten the wire to the. core using one of the two methods shown in Fig. 4-6. Figure 4-6A shows a dab of giue, silicone adhesive, or the high-voltage sealant Glyptol (sometimes used in television receiver high-voltage circuits) to anchor the end of the wire to the toroid core.

For ferrite cores:

4-6 Methods for fssieningthe wire on a toroid winding: (A) glue spot and (B) "tuck under" method .

Other builders prefer the method shown in Fig. 4-6 B, In this method, the end of the wire is looped underneath the first full turn and pxUled taut. This method will effectively anchor the wire, but some say it creates an anomaly in the magnetic situation that might provoke interactions with nearby components. In my experience, that situation is not terribly likely, and T use the method regularly with no observed problems thus far.

When the final coil is ready, and both the turns count anil spacing are adjusted to yield the required inductance, the turns can be anchored to the coil placed in service, A final sealant, method is to coat the coil with a thin layer of clear lacquer, or "Q-dope" (this product is intended by its manufacturer as an inductor sealant)

Mounting the toroidal core

Toroids are a bit more difficult to mount than solenoid wound coils (cylindrical coils), but the rules ^that one must follow arc not as strict. The reason for loosening of the mounting rules is that the toroid, when built correctly, is essentially self-shielding, so less attention (not no attention!) can be paid to the components that surround the inductor. In the solenoid-wound coil, for example, the distance between adjacent coils and their orientation is important. Adjacent coils, unless well shielded, must be placed at right angles to each other to lessen the mutual coupling between the coils. However» toroidal inductors can be closer together and either coptanar or adjacent planar can be placed with respect to each other. Although some spacing must be maintained between toroidal cores (the winding and cnre manufacture not being perfect), the required average til stance can be less than for solenoid-wound cores.

Mechanical stability of the mounting is always a consideration for any coil (indeed, any electronic component). For most benign environments, the core can be mounted directly to a printed wiring board <PWB) in the maimer of Figs. 4-7A and 4-7B. In Fig, 4-7A, t he toroidal inductor is mounted flat against the board; its leads vmrmrinmrm

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Responses

  • heath
    How to make rf transformer?
    2 years ago
  • Maria Zajicek
    How to build low power rf transformers?
    1 year ago
  • donnamira
    What makes a capacitor polarized?
    1 year ago
  • veijo
    Why are toroid matching transformers not used for 144 MHz?
    1 year ago

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