This is chapter 10 of the DRSSTC design guide: Topload
The topload of a Tesla coil serves more than one purpose. The most important and why it is there in the first place is that it acts as a capacitor that can store the high voltage charge and it will also lower the resonant frequency of the secondary circuit.
The magnetic field around a topload will prevent sparks from forming on the top winding of the secondary coil and also in some degree prevent sparks from going inwards towards the secondary coil, primary coil and strike rail.
It also adds more distance out from the center and height to the entire assembly to avoid spark length being limited if it would always strike to the ground because of a short distance.
Round and smooth surfaces are preferred to get the longest possible sparks when corona losses is avoided. A uneven surface will result in many small sparks, barely visible to the naked eye where energy is dissipated and this means there is less energy to be put into the spark you are aiming to get.
Round and smooth surfaces almost only leaves us with two shapes, spheres and toroids.
Spheres can result in problems with sparks going downwards to strike the primary coil or result in racing sparks along the secondary coil. This is caused by the electromagnetic field shape from the sphere. If a Sphere is used it is best to place the break out point on the top of it so sparks would be forced as far away from the secondary coil as possible.
Toroids gives the best electromagnetic field shaping where sparks are much less likely to travel inwards towards the secondary coil. If the diameter of the toroid is very large a secondary smaller toroid can be used underneath to ensure that there is no breakout from the top winding of the secondary coil.
Solid vs skeleton
From an efficiency point of view it doesn’t really seem to matter whether you use a solid metal toroid, or a piece of flexible ducting bent round into a ring, or a skeleton frame made out of tubing, they all work much the same. However, if you are wanting to run the coil without a breakout point, you’ll find that a smooth surface gives a single large streamer that wanders around the topload, whereas a rough one can generate several shorter streamers.
Do use these materials
Copper: One of the best conductors in terms of price/resistance ratio.
Aluminium: About 2 times the electrical resistance of copper.
Corrugated aluminium air duct is made entirely from aluminium that is twisted to form a continues lock seam.
Two frying pans mounted opposite against each other can form a small, uniform and cheap topload.
Styrofoam torus/donut/toroid shapes from a hobby store covered in aluminium tape makes for a light weight and cheap toroid, it can however be hard to get a very smooth surface.
Two bottom parts from beer or soda cans can form a tiny topload.
Brass: About 4 times the electrical resistance of copper.
For connections through a threaded rod or bolts to mount the topload on, brass is the prefered material as its resistivity is still good compared to how rigid and strong it is.
If possible, avoid these materials, they are not optimal
Metallized polyester flexible air ducting: The polyester film is very thin and only rated for a maximum operating temperature of 71 C. A break out from the surface of the film could easily damage or melt the film. The spiral is made from spring steel.
Stainless steel: About 40 times the electrical resistance of copper.
The stainless steel salad bowls from Ikea are cheap, almost round and easy to build a sphere from.
Current and losses in toploads
Udo Lenz at the 4hv.org forums made some very specific measurements on topload and arc current on a DRSSTC on three different power levels, where the highest was producing 80 cm arcs. His measurements for the DRSSTC was inspired by the measurements by Greg Leyh from the worlds largest SGTC Electrum.
The current through the topload is
The current through the topload is given (at a given operating point of your coil). If you increase the resistivity, the dissipation in the topload goes up (see equation above) and so your Q factor goes down. If the topload was superconductive, it would not dissipate anything and won’t do anything to your Q factor.
Choosing minor and major diameter
A rule of thumb for choosing the minor diameter for a DRSSTC topload toroid is to take the secondary coil diameter and use the same value.
Kind of the same rule can be used for the major diameter, take the length of the secondary winding and that is your toroid major diameter.
These rules apply if you follow the general design guidelines for sizing of the secondary coil.
The minor diameter will affect the breakout voltage, so if the minor diameter is too large then the coil will not break out without a break-out point. Too small a minor diameter will cause it to break out at a lower voltage depriving the coil of some of the voltage build up on the toroid, usually resulting in multiple smaller streamers.
The major diameter affects the field control under the toroid, so too small and you will get strikes to the primary coil / ground rail or if way too small strikes to the secondary coil which might destroy it!
Choosing distance to top winding of the secondary coil
The underside of the toroid should align with the top winding of the secondary coil or be raised a distance at up to secondary coil diameter.
One or two toroids
If you want to raise the topload higher above the secondary than the above figure, it will be a good idea to add a second smaller toroid for field shaping. It can also be necessary to add a second smaller toroid if the main toroid is very wide so field shaping around the secondary coil top is weak.
Calculating the capacitance of a sphere
K is the dialetric constant 1.01, R is the radius of the sphere. This formula is using inches for measurements, 10 mm = 25.4 mm. Result is in pF.
Calculating the capacitance of a toroid
D1 is the major diameter of the toroid and D2 is the minor diameter of the ring of the toroid. This formula is using inches for measurements, 10 mm = 25.4 mm. Result is in pF.
A breakout point should be a good conductor with a even surface to avoid a large corona spray all around the wire, rod or what is used for it.
The tip of the breakout point can be tungsten if heating or even melting damage to the tip is a problem.
The breakout point has to stand out far enough from the very close and heavy magnetic field that is close to the topload surface. At least 10 centimeter would be a good start, even longer and upwards pointing can be used to get arcs further away from striking down in the primary coil, earth rail or just get longer arcs to ground from the bigger distance.
Running a DRSSTC without a breakout point can be very stressful on the IGBTs as the Tesla coil have to build up a large potential on the surface on the topload for it to break down from the very even surface. The IGBT inverter is much happier when it can just keep feeding energy into a arc.
Tales of running a DRSSTC without a breakout point tells that nothing happens until it almost hits full input voltage from f.ex. a variac. Suddenly large arcs will start lashing out in unpredictable and random directions. Half of the time the sparks would either be from topload, racing sparks on secondary or even inside of the secondary.
The art of making a metal toroid is called metal spinning. A sheet of metal is formed into shape with a metal rod against several different wooden master models mounted in a lathe.
Aluminium ducting is easy to mount around two circular pieces of wood with spacers between them.
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