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A new 8.5 x 34.5 Toroid for the 'Phoenix'
Firstly I made a wooden former to hold the ducting. Some simple maths will give the overall diameter needed to achieve your desired finished size.
I used a 3.5 inch spacing between the two discs, so my discs needed to be 18.8 inches diameter with 8.5 inch ducting to achieve my overall size of 34.5 inches. The 3.5 inch spacing ensured that the duct once mounted, could not 'slip out' too easily before it was glued in place.
The two disc are easily cut with a jig-saw from some thin flat MDF (Medium-density fibre-board).
I then clamped the two discs together, and drilled six evenly spaced holes through both pieces, with a drill that was considerably smaller than the fixing nails I was going to use. For the six spacers in the middle I then used round wooden rod (old broom handle).
I cut each rod accurately using a lathe, so they were both exactly square at the ends equal in length. Then using the lathe to again ensure accuracy I centre drilled a tiny locating hole in both ends of each of the six rods.
Then with some nails tapped through the tiny previously drilled holes in the discs I could locate them in the centre drilled holes of the wooden rods before tapping them home. Plenty of wood glue formed the main fixing though, as the nails were primarily only used for location. This method ensured that both discs are mounted true to one another.
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I then found a piece of Acrylic tube that fitted nicely inside the duct, and using a hot glue gun I sealed the inside edge into the duct. This must be a good strong bond, as the initial forces when mounting the duct on the wooden former will want to try to pull this joint apart.
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This type of duct comes in a compressed form and will stretch to about twice its uncompressed length, so by applying a bit of stretch before assembly, you will find it easier to slip onto the wooden former. Too much stretch though and it will be loose, whilst not enough will mean it is impossible to mount. I knew from the past that it's easy to stretch, but it's fairly difficult to compress back again without leaving it misshapen afterwards, so I took it slowly this time.
Next I applied a very slight curve to the two ends that will butt together otherwise they will meet dead straight and look wrong. I then pushed the free end of the duct onto the previously glued Acrylic collar and applied plenty of hot glue.
The trouble with hot glue is that it is hard to apply beforehand as it sets so quick, so I had to carefully inject hot glue into the butt joint I had just made, doing just a small section at a time. You also need to make sure you apply the hot glue to the duct's end faces, and not on the outside, as the coating of filler that I later apply, won't stick to it.
Because of the stress applied to this joint during the mounting process, I also applied a big blob on its inside curve (where it won't be seen inside the wooden former) and also opposite on the outside curve of the duct, this last blob, being just a temporary measure as it would be seen otherwise. You will to cut it down level afterwards (needs a really sharp blade) once the duct has been mounted and hot glued against in place.
You should now have a floppy tyre made of ducting, ready to be popped onto your wooden rim that you made first of all.
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Once the duct was on the wooden former I carefully levelled and squared it up before applying a good solid bead of hot glue around the edge of the former's circumference. This needs to be a good joint as the fully finished toroid weighed 16 pounds (10Kg) once all the filler had been applied.
This area of hot glue is exposed and will need to be roughened with sandpaper to help the filler stick.
Once both sides were glued and had completely cooled I then cut away with a sharp knife the 'blob' of temporary glue that I had applied to the outside curve. The main task of holding the tube in place having now been taken by the two beads around the wooden former's circumference.
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Here you can see the unfinished 8.5 inch and my current 6 inch alongside one another.
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You often see a nice well equipped workshop in coiler's photographs - well here is the opposite. This is my entire work area which tends to get rather crowded. As there is also a small bench mill tucked out of the way to the right hand side, it means that a lot of work inevitably gets done by working on the floor!
Shown here is the messy stage of applying several coats of filler to give a smooth surface for the Aluminium foil.
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Here we have the new toroid after several coats of filler have been applied and sanded down and a coat of varnish applied. The varnish is needed to ensure the Aluminium foil will stick.
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As well as the new 8.5 x 34.5 inch, I am also constructing a smaller 4 x 18 which may be needed underneath the main toroid to help shield the top of the secondary. (Although a 3x16 might be a better size for an 8 inch secondary)
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Here is the last, and most tedious stage.
The three inch wide foil I used is a nightmare to apply without any creases as your applying a two-dimensional flat foil onto a curved three-dimensional surface. Result: creases! One inch wide would have been ideal, but taken for ever, so with hindsight two inch would have been a good compromise on larger toroids. In the end I was cutting the three inch tape down to two inches!
Fortunately it will smooth out quiet well with something like the back of a spoon being rubbed over it. You must though use a piece of paper between the foil surface and the spoon otherwise the foil will tear. I find the discarded backing paper of the foil itself is excellent for this.
You can still achieve good acceptable results with smoothing away creases, even if when you initially apply the foil it looks awful, so don't get too disheartened with the initial creases.
You will not however get a nice smooth surface like that of a professional spun aluminium toroid, but the finished product will be smooth enough to avoid having the lots of little break-outs that normally occur with just bare duct.
Toroid Height & Size
The toroid not only acts as a capacitor to hold the HV charge, but also serves the purpose of offering some electrostatic shielding to the top part of the secondary. Without shielding you would get excessive corona or even break-out from the top of the secondary coil windings. So placing it too high will mean you loose the benefit of any shielding.
On the other hand placing it too low will mean it will act like a shorted turn at the top of the secondary, which will waste power. So getting it the optimum height is fairly important.
Basically you want to place it as high as you can without the top of the secondary breaking out.
A good upper starting point would be with the toroid positioned so that height 'X' = 0.8 of diameter 'X' - see below.
The final position (probably a bit lower) is only really found by trial and error. This is best done by running the coil in the dark and photographing it to detect any breakout from the top of the secondary.
My own 8 inch 'Phoenix' coil running on a 6 x 24 inch toroid suffered break-out from the secondary at a recent Teslathon, and following some advice I lowered it and quickly solved the problem. When I measured it later though I found I hadn't even followed my own advice above, as the height 'X' prior to adjusting had been about 1.2 times diameter 'X'.
The actual size of the toroid will depend on the size of the secondary coil. There is it seems no hard and fast rules, though it is generally accepted that the minor diameter will not be much larger than the diameter of the secondary coil, in general though on smaller and medium coils the ratio is usually about 0.8, while on 8 inch secondaries and higher it will usually be 1:1. The overall toroid diameter would normally be 2.5 to 4 times the secondary diameter. Other people may aim for a maximum diameter that is about 80 to 90% of the actual winding height though.
If the minor toroid size is too small it will not be able to build up and hold sufficient charge before breakout occurs, so the streamers will be shorter and weaker. A correct sized is one will allow a healthy charge to build up before break-out occurs. You will then get a nice solid streamer that is longer than that produced by a smaller toroid.
A toroid that has too big a minor diameter, will not have any breakout at all unless you add a 'breakout point' (a small rod or tack sitting on the toroid's surface). However because it can't break-out, it can often cause what are called 'racing sparks'. This is where sparks shoot vertically up and down the secondary coil surface. The usual culprit behind racing sparks however is that the coupling is too tight, so always check that aspect first. To alter the coupling you would need to either raise the secondary (& toroid) in respect to the primary, or else lower the primary in respect to the secondary.
My own coil needs about 80% power before breakout occurs, when not using a break-out point, which I consider to be a good goal if you want long streamers.
Update 5th July 2010:
With my old 6 x 24 toroid I had a secondary resonant frequency of around 88khz. The new 8.5 x 34.5 toroid gave me an incredible, and obviously wrong, frequency of 97khz!!!
How? Well to cut a long story short after a lot of head scratching I found the central mounting post was not in contact with the toroid's foil surface.
Without a toroid the secondary resonates at 138khz. The new toroid was sat so close to the top of the secondary (but not in electrical contact with it) that it lowered that 138khz down to 97khz just by its proximity.
The correct frequency is now 78khz.
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