What to use to create correct diameter for an air core inductor? - Page 1
Forum section: electronics / beginners

author: novicefedora
author_rank: Contributor
timestamp: July 30, 2020, 05:12:24 am
content: What to use to create correct diameter for an air core inductor? Most of the content online seems to suggest wrap the coil around screwdriver or some rod, but this doesn't allow one to create air conductor inductor the desired diameter.I'm wondering how to create something of any diameter I want.

author: Circlotron
author_rank: Super Contributor
timestamp: July 30, 2020, 05:16:35 am
content: PVC pipe?

author: novicefedora
author_rank: Contributor
timestamp: July 30, 2020, 05:24:38 am
content: Quote from: Circlotron on July 30, 2020, 05:16:35 amThey are of preset diameter, if the air-core inductor I'm making requires a diameter of those sizes, I can use it, but what the diameter I need is not found on screw drivers, pipes, etc?

author: IanB
author_rank: Super Contributor
timestamp: July 30, 2020, 05:28:34 am
content: Make a former of the diameter you want, and then wind the coil around it.

author: bob91343
author_rank: Super Contributor
timestamp: July 30, 2020, 05:32:56 am
content: There used to be something you could buy called a Morris Coil Winder. I never saw one but they were in all the catalogs for many years. If you can find one maybe it will do what you want.The PVC idea is a good one, and while not every size is available, you should come close enough to what you want. Or start with the nearest smaller size and over wrap it with paper or cardboard or plastic until you reach the desired size. Or take a roll of duct tape and unwind enough to reach the desired diameter. That stuff isn't too expensive for that.What sizes might you want?

author: Circlotron
author_rank: Super Contributor
timestamp: July 30, 2020, 05:33:22 am
content: Buy something that comes on a roll and unroll it till it gets to the right diameter.Edit -> Like bob91343 says. ^^Start off with a smaller size tube and wrap over it to increase the diameter.

author: Berni
author_rank: Super Contributor
timestamp: July 30, 2020, 05:40:30 am
content: Quote from: novicefedora on July 30, 2020, 05:24:38 amIn that case make your own custom former on a lathe if the diameter is so critical. The same lathe can also be used for neatly winding very thick wire.I don't see a reason for needing an air core inductor of like exactly 12.445 mm in diameter (Other than mechanically fitting into some other thing with tight tolerances). An air core inductor with a 10mm or 15mm diameter still has pretty similar inductance anyway. And if its too far off you can still add or remove a turn or just stretch it out a bit to fine tune the inductance in fine increments.If you want a really precise inductance you need to measure it anyway since just winding it by hand will not be super repeatable

author: fourfathom
author_rank: Super Contributor
timestamp: July 30, 2020, 06:33:20 am
content: For small-diameter cores I use a drill bit of the appropriate size. I've even wound nH coils using an unfolded paper-clip wire as a form. Berni is right though, you usually end up tweaking the finished coil anyway. Of course that depends on the particulars of the circuit.

author: David Hess
author_rank: Super Contributor
timestamp: July 30, 2020, 05:14:31 pm
content: I do what fourfathom suggests for small coils; I use a drill shaft. For larger ones, I use wooden dowels and recompute the number windings based on the diameter and wire size that I have.Note that Q is highest when the length is twice the diameter.

author: Vovk_Z
author_rank: Super Contributor
timestamp: July 30, 2020, 09:10:41 pm
content: We may use other diameter and recalculate number of turns to this diameter you have. It is much convenient.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 30, 2020, 09:54:29 pm
content: Use a cone and form the wire at the appropriate location.What, you wanted straight, even helices? Git gud..I jest; practical for small radio coils though.Tim

author: Berni
author_rank: Super Contributor
timestamp: July 31, 2020, 05:11:10 am
content: Quote from: T3sl4co1l on July 30, 2020, 09:54:29 pmSometimes cone shaped coils are even better. Wideband bias T designs love to use them.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 31, 2020, 09:35:32 am
content: Yup, awfully pricey though, if you go shopping for 'em.Tim

author: bob91343
author_rank: Super Contributor
timestamp: August 01, 2020, 03:00:37 am
content: You make conical coils by using a sharpened pencil as a form.

author: TimFox
author_rank: Super Contributor
timestamp: August 01, 2020, 03:08:59 pm
content: Years ago in QST, I saw an interesting method of making an adjustable coil. A conical winding (of somewhat springy wire, not dead soft), not very sharp cone angle, with a standard nylon nut and bolt down the axis. Tightening the screw changes the inductance, without use of iron.

author: wizard69
author_rank: Super Contributor
timestamp: August 01, 2020, 06:37:45 pm
content: It is just an issue of using your imagination. There are all sorts of things that can be used coil formers. For example just looking at my desk there is a can of Easy Cheese, med bottles, a Coke can, pens of various diameters, Micro Phones, various cords, light bulbs, barbells and other items.If you have a tool box (you should), it will have all sorts of round items in it from sockets, to screw drivers, to punch sets and whatever else might reside in a toolbox. Interestingly pin punches can be had in an array of diameters to cover many smaller inductors. In the same way a set of drill bits can cover the range of 1/32" to half inch rather cheaply and larger diameter drills can be had in specific increments. If you are up to ordering form industrial suppliers (exmples: MSC, McMaster-Carr, Travers & etc), you can get "drill rod" in very specific diameters that don't have the flutes actual drill bits have. Those same suppliers can also supply various plastics in rod form. You can also source this sort of material from material suppliers.If all else fails you can buy a lathe and make any form you want. Further that lathe can then help with actual inductor forming or build an inductor manufacturing machine. A lathe is the ultimate machine for creating cylinders of a specific diameter.Quote from: novicefedora on July 30, 2020, 05:12:24 am