Electronic circuits that produce RF (radio frequency) signals at specific locations, such as in the IF (intermediate frequency) stages, require the use of toroidal transformers to provide stabilization of a given frequency needed. As an electronics technician, there are times where you will find that you must wind such a transformer manually. After you have spent the time determining the number of wire turns necessary for your given frequency, you must evaluate a method of winding the toroid. You can use the center of the wire to begin this winding process easily.
Cut two 1/2 inch lengths of blue vinyl masking tape from the tape reel. Stick one end of each on the edge of your work area so they are readily available when needed for securing the wire ends after the winding process has been completed.
Slide the wire through the center of the round core until the interior of the core is at the middle of the entire wire length. Fold the wire gently by holding each of the wire ends between your thumb and index fingers on each hand. Hold the ends up in the air so that the wire and core appear as a necklace with the toroid core hanging at the bottom. Grasp both of the wire ends between the thumb and index fingers of one hand, and pinch the wires where they form the "U" shape at the toroidal core using the thumb and index fingers of the other hand.
Let go of the two wire ends to free up the hand that was holding them. While still pinching the wires against the toroid core with the other hand, use the thumb and index fingers of the other hand to begin threading one of the loose wire ends back around, and through the center of the toroid core. Pull the wire through the center of the core and loop it upward until it comes close to the end of the other wire once again. Pull the wire until this first loop on the core is as tightly wound as possible. When winding this first loop, the portion of wire going through the center of the core must be perpendicular to the edge of the hole in the center, just as the wire on the outside of the core is perpendicular with the edge on the outside. The result should be more space between the outer wire loops than the inner wire loops.
Continue wrapping the first side of the wire around the toroid core until only 1 inch of length remains. This end length will be just enough to pass through one of the holes of the circuit board, and not quite enough to make another loop through the toroid core. Point this wire end downward so it is pointing away from the toroid core center, then use one piece of the blue masking tape to wrap around the loop preceding this "wire tail" at the end. Wrap the tape firmly so the end is held against the toroid core while you wind the other half of the wire.
Wrap the other half of the wire around the toroid core in the same way the first half was wound. Begin by first threading the end through the center, making sure that you wrap the first loop in the same direction as you did with the first loop on the first wire half. The wire loops must be going around the toroid core in the same direction from one end to the other for a properly functioning toroid coil. Tape the end to the toroid core once the second half of the wire winding has been completed.
Apply a light coat of fingernail polish around the wrapped toroid core so that the polish coats all surfaces outside as well as the interior of the hole in the center. Allow the fingernail polish to dry thoroughly to where it is no longer tacky to the touch. Once the polish has dried, all of the wire loops on the toroid core will now be permanently held into place.
Strip away about 1/2 inch of insulation from the tips of each wire end "tail" so those ends can be passed through the soldering holes in the circuit board.
Electronics industry practice, when winding toroidal coils, is to wind the coils tightly enough so only 330 degrees of the round core is covered with the windings, leaving the remaining 30 degrees free of any windings. The empty 30-degree arc is generally made to be the bottom of the core that sits nearest the circuit board.