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Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Does anyone have a formula or even anecdotal data on the capacitance of a regular spun or duct-tube toroid versus the ring toroids. How does spacing between the individual rings or number of rings affect the overall capacitance?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
To a first approximation, the skeleton toroid has pretty much the same capacitance as an ordinary toroid of the same dimensions. I think everyone just assumes this.
Tesla believed the capacitance of a conductor was simply proportional to the surface area of metal, but the truth is somewhat more complicated.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I have tried modelling all kinds of toroid shapes in JAVATC and the math used in that calculator suggests the same as Steve says, it is very close results to the same amount of surface area, with little regard to how its formed.
Someone did post FEMM simulations of skeleton toroids a long while back, it might have been Kizmo, try to search for it :)
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Seems that thread may have gone missing - others looking for it too. For now I am ok with knowing it's close the JAVATC formula. I am curious which factors would have the biggest impact on the capacitance. Does number of rings or spacing have an impact? What is the smallest number of rings you can get away with without drastically affecting the capacitance? At some point I would imagine that it decreases with less rings, but by how much?
As I said, more academic curiosity at this point. Will probably try build one and see how it goes.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Reducing the number of rings will affect the breakdown voltage more than the capacitance.
The argument goes something like this: Since the rings are all at the same potential, there must be points in between them at the same potential as this. The set of these points is an "equipotential surface". The shape of the surface is constrained to be smooth by Laplace's equation or some similar math babble. If you could see it (FEMM can help) it would look somewhat as if the structure had been shrink wrapped. Now, because all points on this surface are at the same potential as the metal rings, it follows that the surface could be replaced with a real metal surface connected to the rings, without changing the capacitance.
Reducing the number of rings reduces the breakdown voltage because the "ribs" stick out of the "shrink wrap" more.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
That is interesting. Excellent description ! When you put it like that it makes a lot of sense - very easy to picture. I would think that a plastic framework covered with a collection of rounded-edge discs would have the same effect too. One could start getting creative with toroid construction. Aluminum and copper tubing makes ring toroids expensive (I think I read that Eric Goodchild spent $250 per toroid on his larger coils). I have heard of plastic rings being covered in aluminum tape to reduce cost, but not quite as nice as aluminum tubing.
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I found 7 rings of 25mm coax cable for mobile antenna installations. Only cost me cable scrap price from a junk yard. It has a hard plastic outer shell, then a 1mm thick, 25mm diameter aluminium screen, filled with PE foam and a 8mm copper tube in the middle.
Cheap, light weight, perfect. Get creative and you can build awesome stuff for nothing :)
Too thin tubing should be avoided. I once had a 24" diameter toroid made from quarter inch copper tubes. There were arcs going from the bottom of the toroid into the primary inspite of a big breakout rod on top.
Registered Member #6038
Joined: Mon Aug 06 2012, 11:31AM
Location: Salado, TX
Posts: 248
Uspring wrote ...
Too thin tubing should be avoided. I once had a 24" diameter toroid made from quarter inch copper tubes. There were arcs going from the bottom of the toroid into the primary inspite of a big breakout rod on top.
What was the gap between the rings? Using the shrink wrap analogy that Steve described, small diameter tubing would presumably have to be very close together to start approximating a solid toroid. I would imagine, in general the smaller the spacing the better. Could there be a minimum ratio of Diam:spacing that still achieves, say 95% capacitance of a solid toroid? Just speculating here...
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Toroid Types
If you could see it (FEMM can help) it would look somewhat as if the structure had been shrink wrapped. Now, because all points on this surface are at the same potential as the metal rings, it follows that the surface could be replaced with a real metal surface connected to the rings, without changing the capacitance.