Secondary Coil

The secondary coil is responsible for generating the very high voltages we are after and must be constructed with care, to avoid disappointment . The coil can be split into four functional areas, the form, the wire, the insulation and the connections.

Coil Form:

The first thing to decide when constructing a secondary coil is what form to use. Ideally, a secondary consisting of just a coil of wire could be used but it's a little difficult to make without some sort of support to wind the wire on! A form of the desired dimensions is required. It can be made of many different material, from cardboard to acrylic plastic, as long as certain factors are taken into account. Ideally, the form should be as thin as possible while remaining strong enough to support its own weight and that of any discharge terminal placed on top.

The type of material is important too. Plastics are ideal because they are good electrical insulators whereas metals are totally out of the question! As well as being a good insulator it is preferable that the material's DF (dissipation factor) is low (see Tank Capacitor section for explanation of DF). This is of debatable importance. Polyethylene has a low DF, compared to PVC but there are many PVC tube based coils in operation to prove that it isn't too critical. A real killer of performance is water. A cardboard tube will make quite a good coil form as long as you dry it out (in some sort of oven arrangement) and seal it with polyurethane varnish, inside and out, before it has a chance to re-absorb moisture from the air. My recommendation is to use PVC tubing, if your chosen coil size matches the available tube dimensions (available at your local DIY builders merchants). It is recommended that PVC tubing also be dried and sealed before use.

The open ends of the coil form should be sealed, especially the top end, to prevent internal damage from streamer strikes. I used a couple of disks of acrylic glued in each end with PVC glue. For the wire winding process you need to support the form in some fashion which will allow it to be rotated. I am using a half-inch steel rod running through holes in the acrylic disks; the top hole will be sealed with some more plastic when the winding is complete.

Wire:

Enameled copper is used for the secondary coil, the same stuff as is used in motors and transformers. Getting hold of enough wire for one coil can be a problem though. Small 500g rolls of wire can be picked up from any electronics store or mail-order catalog but to get enough wire for say a 6 inch x 24 inch coil, you will need about 1.3Kg. It is not recommended that you join wire half way along the coil. With the high voltages involved, any sharp points or bumps in the coil are potential weak spots and targets for streamer strikes! Your best bet is to find a friendly motor or transformer rewind company and see if they have any end-of-reel wire you can buy (they buy 20Kg reels!). I got mine from a wire distributor called 'Hi-Wire'. They were able to wind off some reel-ends onto 6Kg spools (their smallest size) but their minimum order value ended up around £80!

After lots of experimentation, the modern pioneers of Tesla Coiling have come up with the following rules for determining the optimum size of a secondary coil (see table below). This gives you size of coil, so to get the form length add a couple of inches to the coil length.

Once you have chosen your coil size you can calculate the optimum wire size to use. It is recommended that you aim for between 800 and 1000 turns of wire. See Data Lists section for a table of wire sizes.

Coil Winding

Once you have dried and sealed your PVC coil form, and have glued the end disks in place, you are ready for a test of your self control! The Winding!

It took me 2 hours to wind approximately 816 turns of 0.71mm (double coated polyester) enamelled copper wire. I knocked together a motor driven jig to make the winding and varnishing processes less prone to accidents! I also put together a motorised wire feeding/tensioning system, providing constant tension to the wire as it came off the spool (it was functional but not pretty, therefore no photo!).

The wire was secured to the form with a couple of turns of strong tape (I used Kapton tape). The bottom end of the wire was cleaned and soldered to an earth tab (1.5 x 4.5 inches) which I epoxy glued and taped down. At the top I left a couple of feet extra length, ready for connection to the discharge terminal.

Varnishing

I have used a clear gloss polyurethane varnish (for interior use) for sealing the PVC tube and for insulating the coil after winding. If you keep the form rotating while you apply the varnish, a coat of greater thickness can be achieved than if the form was stationary. My coil form (28" x 6") took about 1 litre for 16 thick coats.

To seal the PVC coil form, give it 1 to 3 coats; one coat may be all that's needed but I like a little in reserve when I'm sanding down lumps and bumps, prior to winding. After you have wound the coil, apply as many coats as you can manage. A good guide is when the grooved pattern from the windings beneath has gone.

Force drying the varnish with radiant or fan heaters can cut down the overall time it takes to make your coil but don't make the mistake I did. Too much heat can cause air trapped under the wire to expand and cause the varnish to blister!

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