Making your own toroidcore inductors and rf transformers – circuit design gas 1981


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.

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. gasbuddy 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. 2 bul has higher Q over range 20 to GO MHz

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*. electricity notes class 10 pdf 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

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

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. gas pedal lyrics 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. gaston yla agrupacion santa fe 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-

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

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.

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. o goshi 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.

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. electricity billy elliot broadway 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.