How to wind Toroids
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In this chance we want to post how to wind Toroids may this explanation will help you to understand. The effective inductance of a toroids coil or a transformer winding depends in part on the distributed capacitance between the coil turns an...
In this chance we want to post how to wind Toroids may this explanation will help you to understand. The effective inductance of a toroids coil or a transformer winding depends in part on the distributed capacitance between the coil turns and between the ends of the winding. When a large number of turns are used (for example 500, 1000), the distributed capacitance can be as great as 100 pF. Ideally, there would be no distributed or parasitic capacitance, but this not possible. Therefore, the unwanted capacitance must be kept as low as possible in order to take proper advantage of the Al factor discussed earlier in this section. The greater the distributed capacitance, the more restrictive the transformer of the inductor becomes when applied in a broadband circuit. In the case of a narrow –band application, the Q can be affected by the distributed capacitance.
Fig. 70 – (A) Illustration of a homemade winding shuttle for toroids. The wire is stored through the center hole of the toroid, again and again, until the required number of turns is in place.(B) it is best to leave a 30° gap between the ends of the toroid winding. This will reduce the distributed capacitance considerably. (C) Edgewise view of a toroid core, illustrating the method for counting the turns accurately. (D) The low-impedance winding of a toroidal transformer is usually wound over the cold end of the main winding.
The pictorial illustration in Fig. 70B shows the inductor turns distributed uniformly around the toroid core, but a gap of approximately 30 Celsius is maintained between the ends of the winding. The closer the ends of the winding are to one another, the greater the unwanted capacitance. Also, in order to closely approximate the desired toroids inductance when using the Al formula, the winding should be spread over the core as shown. When the turns of the winding are not close wounds, they can be spread apart to decrease the effective inductance (this lowers the distributed C). Conversely, as the turns are pushed closer together, the effective inductance is increased by virtue of the greater distributed capacitance. This phenomenon can be used to advantage during the final adjustment of narrow-band circuits in which toroids are used.
The proper method for counting the turns on a toroidal inductor is shown in fig. 70C. The core is shown as it would appear when stood on its edge with the narrow dimension toward the viewer. In this example, a four-turn winding has been placed on the core.
Some manufacturers of toroids recommend the windings on toroidal transformers be spread around all of the core in the manner shown in Fig. 70B. that is,k the primary and secondary windings should each be spread around most of the core. This is a proper method when winding conventional broadband transformers. However, it is not recommended when narrow-band transformers are being built. It is better to place the low-impedance winding (L1 of Fig. 70D) at the cold or grounded end of L2 on the core.
This is shown in the pictorial and schematic format Fig. 70D. The windings are placed on the core in the same rotational sense, and L1 is wounded over L2 at the grounded end of L2. The purpose of this winding method is to discourage unwanted capacitive coupling between the windings, which aids in the reduction of spurious energy (harmonics, and so on) that might be present in the circuit where the transformer is used.
In-circuit that have a substantial amount of voltage present in the transformer windings, it is good practice to use a layer of insulating material between the toroid core and the first winding. Alternatively, the wire itself can have high dielectric insulation, such as Teflon. This procedure prevents arcing between the winding and the core. Similarly, a layer of insulating tape (3-M glass tape, Mylar, or Teflon) can be placed between the primary and secondary windings of the toroidal transformer (Fig. 70D). Normally these precautions are not necessary at impedance levels under a few hundred ohms for RF power levels below 100 watts.
Once the inductor or transformer is wound and tested for proper performance, a coating or two of high-dielectric cement should be applied to the windings of the toroid. This protects the wire insulation from abrasion, holds the turns in place, and seals the assembly against moisture and dirt. Polystyrene Q Dope is excellent for the purpose.
The general guidelines given for toroidal components can be applied to pot cores and rods when they are used as foundations for inductors or transformers. The important thing to remember is that all of the powdered-iron and ferrite core materials are brittle. They break easily under stress.
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