Air cores - real air or coil formers

[sky_123]

Hello,

For small inductors, (up to a couple of hundred nH), is there such a thing as empty coil
formers available (with grooves), to wind air cores?
It would be quite convenient, so that the coil is a bit more rigid than just a wire coil
on its own, and it would also be a bit more consistent if more than one coil was made,
because the grooves would keep the spacing the same.

I notice old radios used to use some waxy gunk to keep the coils in their shape.
Not sure how effective that was!

Coil formers would be quite convenient, especially for tapped inductors.

How do people keep their coils stable (apart from just using heavy gauge wire!) for
air cores?

Thanks!

 

[chuckey]

Use a perplex former and melt the turn on with acetone. If you put one end of .5mm copper wire in a vice and give the other end a REAL pull. The copper wire becomes fairly stiff, wind it around a round former tightly then remove former.
Frank

 

[sky_123]

Hi Frank,

That's brilliant info, I've never heard of this method. Just so I'm clear; the acetone will create the grooves, yet will still allow the wire to be unscrewed out of the former, and
then re-use the former (now with the grooves) to create new coils that have the same winding spacing?

Going to get some nail varnish remover tomorrow!

Thanks!

 

[chuckey]

If you use perspex*, acetone will dissolve it, so if you want to unscrew the coil, you must be extremely careful not to overdo the solvent. Years ago , in America you could buy airspaced coils where the coils were about 25 -100 mm diam, where the turns were held apart by four strips of perspex about 3mm square that ran down the length of the coil. Often featured in home made transmitters in the ARRL handbook.
Frank
* hard to get now?, most clear plastics are polycarbonate.