A Toroid Tutorial

By Charles Moizeau W2SH

Winding the Inductor

     Over the years there has been no shortage of groans and moans over the winding of inductors on toroidal forms. Kit producers sometimes state how few toroids there are to be wound, and at least one ham offers to wind your toroidal coils for a fee. In recent months there seem to be fewer complaints over winding toroidal inductors. Perhaps this bespeaks frustrated QRPers who have thrown in the homebrewing towel (please say it ain't so!), or maybe they've discovered a talisman about toroid winding which, like stealth antennas, they feel must be tightly safeguarded.

     I'd like to break ranks, eschew stealthy practices, and open the kimono on my preferred technique for winding toroids.

     The problem I faced was keeping each and every turn wound under tension while continuing to wind more turns. It just seemed that the coil forms were too small, my fingers too big and my two hands too few. Luckily, I believe I've found a solution.

     During its construction, holding the already wound turns of a toroidal winding rigorously in place while new turns are being added is achieved here by using a two-inch or greater length of tapered wood to plug the core. I favor sawed-off bamboo chopsticks, if only because waiters at Chinese restaurants are amused when I pocket my eating instruments after the meal. Those I acquired are about nine inches long and 7/32" in diameter, with a tapered section (the part that goes in your mouth) that reduces to 1/16" diameter over 1 1/2". Larger diameter chopsticks (with slightly rectangular handles), going for as little as one dollar for 20 sticks, are available at some supermarkets and all oriental food stores. The latter may also stock a king-size chopstick (I imagine this is what the cook uses to dip into the kitchen pot for tasting), costing one dollar for a pair; these measure 18 inches long with a maximal diameter of 3/8 inches that tapers over about five inches to a minimal diameter of about 1/8 inch.

     When the conically shaped portion of the chopstick stub is jammed into the toroid, it holds all the turns securely with fingers totally absent. Additionally, the upstream end of the chopstick stub provides a very useful handle with which to hold, maneuver or put aside the entire assembly, thus freeing one or both hands for other tasks.

     It is much more efficient to start winding at the middle of the chosen length of wire. Pair up the two ends of the cut wire to establish the midpoint.

     Before each new turn is made, the toroid is held firmly, with thumb and forefinger positioned over the most recently wound portion of the inductor. The chopstick is removed, the wire is passed through the core, pulled tight, and then the chopstick is quickly reinserted. The chopstick remains there until the wire has been wound tightly around the three outer surfaces of the core and is ready to once again pass through the core. Immediately after its removal, and the passing through and pulling tight of the wire, the chopstick is again jammed into the hole to tightly pin the latest and all previous turns. Note that the chopstick always points in the same forward direction as the leading end of the wire.

Securing the Turns and Mounting the Inductor

     When the winding is complete, the turns are held firmly in place at the core's inner circumference with the chopstick. Then I secure the other three surfaces-top, bottom and outer periphery -by bandaging them with high quality masking tape (I only use the blue masking tape used by painters and manufactured by 3M). With the inductor's turns thus doubly secured, I then remove the chopstick.

     For any inductor, distributed capacitance needs to be held to a minimum. Indeed, capacitance may be viewed as negative inductance, and this is obvious from the fact that the formula for capacitive reactance is a reciprocal whereas that for inductive reactance is not. Putting a dielectric other than air between the turns of any inductor will only increase the distributed capacitance and reduce the Q (quality) of the inductor.

     If the toroid has been well wound only a very small amount of doping material need be applied to permanently hold all the turns in place. The doping material must be applied in minimal quantity and at the right location. Using Q-Dope (polystyrene dissolved in acetone and heretofore manufactured by the General Cement Company), or, lacking this, a high quality colorless nail polish, is applied with a toothpick. I paint ONLY the inner circumference of the inductor, for this is where the inter-turn spacing is minimal and consequently where the air dielectric will be least compromised by a thin layer of goo. Such a sparingly applied coating is enough to keep the wound turns in place.

       After the Q-Dope or nail polish has dried, the masking tape bandages may be removed. The start and finish leads may then be anchored to the outside of the form with a tiny dab of the same stuff.

     I digress. Just about all the kits I've seen put the two pads to connect to the start and finish leads of a toroidal inductor directly adjacent to one another. Doing this implies that the inductor should be wound over just about 360 degrees. NO! I'm not about to digress into stuff that was written seven or more decades ago, yet now is pretty much forgotten. Stopping short, I'll just say that with inductors, be they solenoidal or toroidal, really bad things happen to the inductor's Q when its head gets close to its tail. (With a solenoid, this happens when there are multiple layers of windings going back and forth on the cylindrical form). Readers are probably familiar with the admonition that toroids be wound over no more than 330 degrees. Observing this caution, while it does provide a convenient wire-free portion of the toroidal core to plop on the circuit board, is really important because it helps keep the dark side of inductive reactance at bay.

     Stripping the enamel insulation from the wire can be accomplished by laying the wire on a flat surface and scraping it with an X-Acto knife. However, there is some danger of nicking the wire with the knife if it is pressed too hard and/or held at the wrong angle. Therefore, I prefer using sandpaper of varying grits, cut into rectangles no larger than 1 1/2 x 3/4 inches. Each of these is folded in half to form a square and dragged a few times on the portion of wire to be stripped. In order to completely remove the insulation, the toroid needs to be turned between drags so that all of the wire meets the sandpaper. The finer grit squares are used for finer gauge wires, for too coarse a grit will grab the wire and break it, whereas too fine a grit will require multiple passes to do the job.

Tin the bare portion of the inductor's leads

     Reinsert the chopstick to serve as a handle, and with a toothpick pry individual wire turns so that they are equidistantly spaced from each other. In order to obtain this equidistant spacing on all three exterior surfaces of the toroidal core, remove the chopstick and hold the inductor in one hand with the thumbnail positioned next to a turn on one surface to hold it while, with the other hand, use the toothpick, or the other thumbnail, to pry that same turn on an adjacent surface. Besides the toothpick, another useful implement is a wooden cuticle stick sold in the cosmetic section of drugstores, and the upstream end may be filed to a smaller chisel tip. Metal implements, which could pierce the insulation are to be strictly avoided. Arranging the turns' spacing in this manner does improve the inductor's appearance, but more importantly it minimizes the inter-turn capacitance. While tweaking individual turns, try to not to extend the overall winding to the point where it would cover more than 330 degrees of the core's body.

     To reduce capacitive coupling to a circuit board's ground plane, toroidal and solenoidal inductors should not be mounted flush against the board. Try to get them 1/16 inch, or a bit more for larger inductors, above the circuit board. Most likely, the wire leads will not provide enough support to achieve this elevation. When this is so, fashion a mounting pad with the smallest possible footprint to do the job. I favor tiny pieces cut from a wine bottle cork with an X-acto knife, and these are secured to the board and to the coil with thin coatings of glue (I use Goop). Cork has rigidity, provides a measure of shock absorption, and doesn't seem to add much capacitance. Dense styrofoam might work just as well. Having toroidal inductors that flop around is not at all desirable, especially in a rig destined to bounce around inside a backpack.

     Some of the foregoing techniques may not be applicable to a particular kit. Nevertheless, I've included them with the thought that they would be useful for purely homebrew undertakings.

     Properly done, hand winding of toroidal inductors can, and should be, quickly accomplished and a joyful experience.


Charles, W2SH

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