Tesla Coil


30mm Secondary Winding
Completed 30mm Secondary
Primary Coil Build
Primary Tank Circuit
Small Terry Filter
30mm Coil
Secondary Resonant Frequency Measurement
Simulated Streamer Test
Primary Resonance Test
Primary Resonance Test
Main Spark Gap
High Voltage Supply and Primary
Topload Upgrade
Topload Upgrade

My affinity with Tesla Coils (and high-voltage) started as far back as 1991 when I was first shown a circuit diagram, however it would not be until 1998, when I was specifically looking to build capacitors for a Copper Vapour Laser where I would again cross paths with Tesla Coils though a fellow coil builder.

16 years would come to pass before I embarked on commencing a build of components for a medium 100mm (4-Inch) coil, however this remains a project in the works following some major personal challenges.

100mm (4") Coil Project

In late 2014 I commenced building components for a 4-inch coil, commencing with the MMC capacitor bank, using 40 individual CDE-942C20P15K-F capacitors wired in two parallel strings of 20 to create a 15nF 40kV pulse-rated capacitor.

One other major component is the 'Terry Filter', a protection circuit used with neon transformes.

Originally, the coil was going to be powered wth a 15kV/60mA Neon Sign Transformer, however following the successfull construction of the 30mm coil, the 100mm project may become the basis for a Dual-Resonant Solid-State Tesla Coil (DRSSTC).

4-Inch Secondary
15nF 40kV MMC

Terry Filter
15nF 40kV MMC

MMC Construction - Bleed Resistor
Copper Buss Terminals

Precursor to 30mm Micro Coil

Slayer Circuit

In Late August 2020, the motivation took the better of me when I decided to pull the pin on one of the small slayer circuits that are available on ebay. My main purpose was to experiment somewhat, however I would not consider this small type circuit as a 'real Tesla Coil' so to speak.

Shortly after building this circuit, I came across a YouTube video on Plasma Channel; DIY 50,000 Volt Arc Lighter Powered Tesla Coil

After watching the video, this gave me the kick to sit down and actually look at designing my own coil.

30mm Tesla Coil Project

30mm Coil 3D Rendering
JavaTC Secondary Data
Completed Primary Coil
Low Powered Run

In early September 2020, I commenced the design of my 30mm Tesla Coil project spending some considerable time in JavaTC modelling various secondary parameters.

Initially, I was designing a slightly larger 50mm secondary, however I wanted to first experiment with winding a secondary with readily available small spools of 0.25mm enamelled copper wire of around 58 meters, prior to committing to a larger order of wire.

Initially, the motivating concept was to employ an arc lighter power supply, however following the task of winding of the secondary, the coil project took on a more serious approach. With an available 6kV/30mA Neon Sign Transformer I had in my inventory, I decided to change paths, which would also require construction of a matching Terry Filter.

The next task was the design of the primary coil. Several days was spent researching primary coil designs, finally arriving at an "inverted conical" coil inclined at a 30° angle.

A conical primary coil presents a more uniform and efficient distribution of the primary coil's electromagnetic flux to the secondary form.

For the primary coil, a 2.14mm copper capillary tube was selected, and a 5mm inter-turn spacing used. A strike rail using 2.3mm copper capillary was installed prior to initial testing.

Overall, the design and build of the coil focuses on several engineering specifications including; modularity, high-quality construction and materials, functionality and overall aesthetic appeal for the purpose of demonstration and education.

Construction of the project commenced on 7th November 2020 and officially completed on 25th November 2020.

The journey of this project has been collated and published as a Technical Document.

Tesla Coil Specifications

High-Voltage Input:
6kV/30mA Neon Sign Transformer, 180W
PFC Capacitor: 10uF

Primary Circuit:
Primary Capacitor: 3nF / 20kVAC
Primary tap Frequency: 1527kHz
Primary tap turn: T4.25
Inductance @ tap: 2.23uH
Total primary Inductance: 13.5uH

Resonant Freq with toroid: 1708khz
Resonant Freq without toroid: 2489kHz
DC resistance: 19.2Ω
Inductance: 1.720mH
Calculated No of Turns: ~500

Major Diameter (d1): 107mm
Minor (Ring) diameter (d2): 27mm
Calculated Capacitance: 4.62pF
Calculated Breakout V: 80.62kV

30mm Tesla Coil - Topload Upgrade

Following a successful run of the coil and obtaining a benchmark for performance, in the later weeks in October 2020, I had placed an order for a larger toroid. Service from the supplier left little to be desired except for the fact that finally after two months, the toroid had arrived.

The specifications for the larger Toroid:

Major Diameter (d1):  152mm
Minor (Ring) diameter (d2):  38.64mm
Calculated Capacitance:  6.61pF
Calculated Breakout V:  114.74kV

On the 27th December, experimented with the larger toroid configurations.

The additional capacitance of the larger toroid also required the re-tapping of the primary coil to bring the primary back in tune with the new lower resonant frequency of the secondary.

A further resonant test using an oscilloscope and signal generator was completed on the secondary coil to determine the resonant frequency, with the new resonant frequency confirmed at 1,360 kHz.

The primary tap was moved by one turn, which also accounting for a loaded secondary, this placing the system back in resonance.

The performance of the coil now increasing significantly with longer streamers reaching the strike rail and are of equivalent length to that of the secondary coil.

An additional experiment conducted with simply placing the two toroids on the coil, re-tuning of the coil was not required, however found that it also aided in the lengths of the streamers.

30mm Coil with Large Toroid

30mm Coil with Large Toroid
30mm Coil with Large Toroid

One interesting observation with a Tesla coil operating in the MHz range, the arcs are more fluid in nature.

Typically, for larger coils, these operate in the hundreds of kHz. In the case of my 100mm (4-inch) coil with a 300mm x 76.2mm toroid, the calculated resonant frequency (unloaded) is 287. 65 kHz, which would result in streamers with more fractal appearance and with a distinctive "crackle" sound.

Tesla Coil Resources & Images

Model 9 Tesla Coil, Canberra, Australia

The Model 9 Tesla Coil was designed and built by the late William C. "Bill" Wysock, Tesla Technology Research. The toroid is 76.2cm (30") diameter, the coil is capable of producing 3.5 million volts with longest recorded arcs of around 5 meters with an input of 5kva.

However, it's understood that the coil is operated at a significantly reduced input as such not visualised at its full potential when the exhibit is automatically operated each 15 min.

The followings series of images I took some time ago of the coil in operation. Lighting of the exhibit is not great, low light levels and illuminated with very blue (almost UV) light and very reflective glass make photography rather challenging.

The coil is housed behind glass and within a Faraday cage.

I have applied some fine-art processing of the images.

Model 9 Tesla Coil, Canberra Australia

Model 9 Tesla Coil, Canberra Australia
Model 9 Tesla Coil, Canberra Australia
Model 9 Tesla Coil, Canberra Australia

Model 9 Tesla Coil, Canberra Australia

Model 9 Tesla Coil, Canberra Australia

By Flavio Spedalieri, 2 December, 2020