Primary Coil Winding Technique

[alan sailer]

I have made about six primary coils for various coils using the helical form, usually using a lexan supporting cylinder.
Figuring out a way to keep the coils in place has always been a big pain. The wire I use multi-strand THNN house
does not want to stay in place. The coil shifts, unwinds and in general drives me crazy.

I recently found a new way to wind and stabilize these types of coil. It also creates a freestanding coil that is stable
and looks good.

The materials I use are wire, black electrical tape, plastic cling wrap, two coil forms and most important self-leveling
silicone seal.

The two forms are as follows; one is the final ID of the finished coil. The second is a significantly smaller form. The coil I wound has a final ID of 70mm. The smaller form is about 45mm.

The first step is to wind the wire onto the small form. Then wrap this wire with black  tape to hold the wire in place. I put this coil and form into a warm oven. My oven is old and has a pilot flame so it always stays about 60C. Leave it in the oven for a while. This allows the plastic insulation of the wire to relax into a new smaller size. Kind of like a mild heat treatment.

Remove from the oven and carefully pull of the black tape. If you don't stretch or bend the wire it will expand some but will remain coiled and will be smaller in diameter than the final form.

Now take some of the plastic wrap and cover the final form with a layer of plastic. At this point you can now carefully wind this smaller coil onto the larger final form. It will expand slightly to fit onto the new form but it will not be jumping free and unraveling like it would if you tried to wind it directly. It is a pleasure to wind the smaller coil onto a larger form.

When the winding is complete I used a few pieces of the black tape to hold the ends of the coil. Try to push the coil together as you tape the end so there are no big gaps in your coil. Friction will tend to keep the coil tight but it may need a little help.

The coil is now ready to coat with the flowable silicone seal. Normal non-flowing silicone should work but it will not leave a smooth surface like the flowable type. Put a bead of silicon across the windings and then use your finger to spread is around the outside of the coil. If you press, the silicone will tend to penetrate into the layers and bond well. If the silicone gets pass the windings and hits the form, the plastic wrap will keep it from bonding to the form. Silicone seal does not stick to plastic wrap.

Let it cure overnight and check to make sure all the winding are covered. Pull the black tape and slide the coil off the form. I then do a second silicone seal application to the inside of the coil for a better bond.

I was actually surprised at how structural the finished coil was. It was actually necessary to use a razor blade to separate winding to remove turns (I was aiming for a five turn coil but would seven).

Cheers.