making inductors on T50-2 (and other) cores

Home-made inductors wound on T50-2 toroidal cores are used in a lot of ham radio circuits that can be adapted to mediumwave and shortwave broadcasters' needs. So, a bit of info here for those not familiar with such inductors.

The cores can be obtained from mail-order electronics suppliers. They have an outside diameter of one-half inch and are made of a ferrite material. The most common way of winding the inductor is shown in the drawing; the windings cover about 80% of the circle. Small-diameter insulated wire is used to wind the inductor. #26 enameled wire is a common choice. The enamel insulation must be carefully sanded or scraped off the ends; then you tin the exposed conductors with solder before you try to install the inductor into the circuit. If the turns of wire around the core are too moveable for your liking, small dabs of glue or silicone sealant can be applied near the first and final turns.

The toroid can be laid flat on the circuit board or stood up on its edge. A small blob of silicone sealant or glue can be used to hold the coil to the circuit board after the circuit has been finished and tested. If the circuit is going to be subjected to a lot of jolts and vibration, it can be laid flat and then held in place with a nylon or fiber washer on the top held down by a plastic screw and a plastic nut.

The table below gives the approximate inductance of simple inductors wound on T50-2 cores.

of turns
14 1.0
20 2.0
25 3.0
30 4.5
35 6.0

Transformers can be wound on toroidal cores also. When a toroidal transformer is called "bifilar" that means two wires are held side-by-side and wound around the core together. There are several ways this kind of transformer can be represented in schematic diagrams; big round dots are normally included to help distinguish one "end" of the windings from the other.


Toroidal transformers can get pretty complicated, with trifilar and even quadrifilar windings that are connected to each other at certain points. Considerate circuit designers will include a pictorial diagram for such transformers to help the builder understand how to make the connections.

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