Topload design and selection for Tesla coils

Published on: Sep 24, 2015. Updated on Oct 03, 2018.

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. The goes for two savarin cake molds, mounted opposite against each other, they come in a wide variety and some even have a filled middle which makes them easy to mount on a secondary coil.

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 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.

UPDATE: these links are now dead, try to find a copy, pictures of the coil can be seen here:

Hydron at has made some other very specific measurements with a battery driven data recorder:

The current through the topload is

\mbox{Power dissipation}=\mbox{Current}^{2}\cdot\mbox{Resistance}

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.

C=\left ( \frac{K\cdot R}{9\cdot 10^{9}} \right )


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.

C=2.8\cdot \left ( 1.2781-\frac{D2}{D1} \right )\cdot \sqrt{\frac{2\cdot \pi ^{2}\cdot \left ( D1-D2 \right )\cdot \left ( \frac{D2}{2} \right )}{4\cdot \pi }}


Breakout point

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.


Previous topic: Secondary coil Next topic: Tuning

Good MMC Capacitors

Published December 26, 2013. Updated February 24, 2021.

Here is a list of capacitors tested by the high voltage community to be known to withhold the use as primary capacitor in Tesla coils.

Capacitor specifications are taken from data sheets at 100 kHz and some values for peak current, RMS current, ESR and dv/dt are estimates(* marked) from similar capacitors and graph read outs.

Product  IpeakIrmsESRdV/dTRth
data sheetVμFAAV/μS°C/W
Aerovox RBPS20591KR6G1000285422742715*
Kemet Arcotronics

Capacitive reactance Xc = 1 / ( 2 * π * f * C)

ESR can be calculated from the tangent of loss angle given as TANδ in the data sheets. ESR is frequency dependant. Capacitance is given in Farad, frequency in Hertz. ESR = (1 / (2 * π * f * C)) * TANδ = TANδ * Xc.

Thermal resistance (Rth) when given in data sheets are either Watt needed to raise the temperature by one Kelvin or degree Celsius the temperature raises by one Watt dissipation. Conversion from W/K to °C/W is to divide one by W/K dissipation factor.  °C/W = 1 / (W/K).

Ipeak is calculated from the dV/dT rating times the capacitance of the capacitor. Capacitance given in micro Farad times pulse rise time given in micro seconds will give a result in Ampere. Ipeak or Ipulse = C * dV/dT.

As a rule of thumb ESL is about 1.6 nH per millimeter of lead distance between the capacitor itself and the rest of the circuit. This also includes the leads of the capacitor itself. This only applies to well designed capacitors.

MMC calculator

MMC tank design calculator for SGTC, VTTC, DRSSTC and QCWDRSSTC Tesla coils. Results are guidelines to designing a MMC and should always be double checked in your final design! Most importantly is that voltage rating is the DC voltage rating, from experience this can used for good quality capacitors, AC voltage rating with frequency derating would be much lower.

Capacitor specifications are taken from data sheets at 100 kHz and some values for peak current, rms current, ESR and dv/dt are estimates from similar capacitors and graph read outs.

Inputs are in green. Outputs are in red. Formulas used can be seen below the calculator.

Basic MMC configuration – List of good MMC capacitors
Capacitance uF
Voltage rating VDC  
Capacitors in series
Strings in parallel
Price per capacitor  
MMC voltage rating VDC  
MMC capacitance uF  
Total capacitors  
Total MMC price  
Advanced options
MMC capacitor parameters
Peak current rating A  
RMS current rating A  
dV/dt rating V/uS  
ESR rating Find correct ESR rating
for your resonant frequency
dissipation factor
Tesla coil parameters – Examples are small, medium and large
Frequency kHz
Primary inductance uH  
Primary peak current A
On time uS
Advanced results
Primary impedance
(single cap)
Power dissipation
(single cap)
Temperature rise
(single cap)
ºC 0-5 very good, 5-10 good
10-15 not good, 15+ bad
  Actual values MMC rating
Peak voltage MMC VDC VDC
RMS current MMC A A
dV/dt for MMC V/uS V/uS
Peak current for MMC A A

Theory used

MMC voltage rating: MMC voltage rating = DC voltage rating * capacitors in series.

MMC capacitance: MMC capacitance = (single capacitor capacitance * amount of capacitors in parallel) / amount of capacitors in a string.

Primary impedance: Zprimary = SQRT(Lp / Cp).

MMC Xc, reactance: Xc = 1 / (2 * PI * F * C). F is frequency in Hertz. C is capacitance in Farad.

MMC Zc, impedance: Zc = SQRT(ESR^2 + Xc^2). ESR is the combined ESR for the MMC. Xc is the MMC reactanse from above.

Peal voltage MMC: DC peak voltage over MMC = Zc * primary peak current

RMS current MMC: Irms = 0.5 * primary peak current * SQRT(on time * bangs per second). Steve McConner.

dV/dt MMC sees: Actual dV/dt in V/uS the MMC sees = (2 * Pi * V) / F. V is peak DC voltage over MMC and F is frequency in Hertz.

dV/dt rating MMC: dV/dt rating in V/uS = Primary peak current / MMC capacitance.

Peak current for MMC: Peak rating = capacitor peak rating * amount of capacitor strings in parallel.

Kaizer SGTC I


This Tesla coil is my first and was build without any expenses worth mentioning, its the prototype from which I learned a lot about Spark Gap Tesla Coils, high voltage and where to find components in household items and trash.

In the development the first version was more of a proof-of-concept model build only from old microwave ovens, televisions and cable.

Mathematics and theory was not the leading part of this project in the start, but as optimizing went on I learned about tuning the Tesla coil for a better output, resulting in longer sparks.



WARNING!: Working with electricity is dangerous, all information found on my site is for educational purpose and I accept no responsibility for others actions using the information found on this site.

Read this document about safety!



There is no protection against RF spikes going back into the High voltage supply and destroying it. 

The secondary coil is limited by the amount of copper wire I had available from the cooling fan motor from a microwave oven. The diameter / height ratio of the secondary is far from optimal.

I used my flyback transformer as the power supply, it have a very limited current at about 1mA at 20kV, roughly estimated.

In the start I used home made salt water capacitors which are far from optimal.



High voltage supply20 kV from a flyback transformer
Primary capacitor
Primary coilinner 45 mm, outer 90 mm diameter, 1.78 mm diameter isolated copper wire, 6.6 turns.
Secondary coil50 mm diameter, 113 mm long, 800 windings, 0.127 mm enamelled copper wire.
Resonant frequencyTuned at around 655 kHz.
Topload60 mm diameter sphere, tennis ball wrapped in aluminium foil.
Input powerAround 20 – 30 Watt
Spark lengthup to 105 mm long sparks.





10th May 2008

The secondary coil was wound on a plastic tube, 50mm in diameter and 160mm in height. Its bottom was conical so practically there could only be wound wire on 110mm of the height.

The copper wire had a diameter of 0.127mm, varnished its diameter is 0.14mm, this very thin wire made it hard to wound nicely without overlaps.

It took me 2 days to wound the secondary coil, it was hard on the eye, arms and hand to do in one stretch. The plastic tube was mounted on a gear motor controlled by a frequency inverter so I could control the speed by a potentiometer.

11th May 2008

To keep the windings in place and insulate the coil it was varnished with common ship varnish, it was given a layer in the morning and one more in the evening.

12th May 2008

As the power supply I used my 20kV flyback transformer driver at 12Vdc input, the capacitor is a home made salt water capacitor with a capacitance of 2.7nF. The primary coil was wound of ordinary 2.5mm2 wire around a cut up soda bottle.

The topload is a sphere made of bobble wrap, tape and aluminium foil.

Spark gap is just 2 copper wires.

In the first test I could get 4-5mm sparks to a grounded object, the biggest problem was the too tight coupling between primary and secondary. Too tight coupling resulted in alot of racing sparks on the secondary coil, these are dangerous as they can destroy the thin wire on the secondary coil.

Here is a picture of some racing sparks I provoked to get a picture of it.

The primary coil was wound in a bundle and by adjusting the height of it, the coupling could be changed. It was now possible for 40mm sparks to jump to a fluorescent light hold in my hand.

15th May 2008

varnish, varnish, varnish, varnish, glue, glue and more capacitance…

The flyback transformer driver runs at 12Vdc input.

I made another salt water capacitor and installed it in parallel with the other, the total capacitance was now 5.9nF.

As it can be seen in the pictures there is sparks or just “violet light” coming from other parts than the topload of the Tesla coil, its corona loss and decreases the spark length.

To isolate the secondary coil further it was given 4 more layers of varnish and the top of it was glued all over with hot glue. The metal cap was the new topload and 54mm sparks could be achieved.

28th May 2008

The flyback transformer driver runs at 17Vdc input.

85mm sparks can now jump to my fluorescent light.

The bottom from a beer can is the new topload, it results in some spectacular pictures, the higher voltage on the flyback transformer driver is the best improvement towards longer sparks, but also racing sparks on the secondary starts reappearing.

As it can be seen in the picture taken in the dark, there is still corona losses, and with this particular coil it will be impossible to avoid it at these driver input voltages. It is not easy to insulate 100kV. The following picture is taken with long exposure to show the corona around the coil itself, unfortunately its not as clear in the picture as seeing it live. The violet field around the coil is faint, but can be seen in the picture.

This picture is taken directly above the Tesla coils topload.

21st June 2008

I bought 10 old high voltage capacitors at 150 dkr (25$) for the lot, a real bargain in Denmark compared to the joy it has brought me. I only use one of them instead of the 2 salt water capacitors. Its a Fribourg Condensateurs from 1961, 8nF rated for 20kV pulse driving at maximum 2MHz.

A new topload was made from a tennis ball wrapped in aluminium foil, its not as smooth a surface as it should be, but it works.

The spark gap now consists of 2x WT20 tungsten welding electrodes, these are able of withstanding high temperatures without vaporizing as the copper wires were likely to do over time. There is still room for improvement on the spark gap as its still just a single jump static spark gap.

By calculating the Tesla coil in JAVATC I tuned the circuits to theoretically be in resonance, but its only approximating as the construction is far from precise.

With the new improvements 105mm sparks will jump to a grounded wire.

I took a series of pictures with different exposure times. Sparks are about 90mm long.

In the next picture racing sparks can be seen at the top of the secondary windings, without a breakout point or something to jump to, the energy build up is too large for the coil and its under a huge stress.



This small project started as a proof-of-concept model, to see if the theory I had learned would work in practice. It has come a long way since I started on it almost 2 months ago.

I learned a lot more about the theory of the Spark Gap Tesla Coil, the maths behind tuning the circuits and the importance of planning the design before building. This is no surprise. So there have been spend a lot of time trying to optimize a Tesla coil build around a badly designed secondary coil, so the result will never be near optimal.

Despite all this, I am very satisfied with the results of 105mm sparks. There are several things to optimize in a final version of the Kaizer SGTC I, it would be a better spark gap, shorter and more suitable wires, shape of the primary coil and its coupling to the secondary and a topload with a smoother surface.