Tesla Coil Construction

The following information will describe the constuction of Tesla Coils. You will find information about details how to physically build the single components. Information about calculating dimensions is found in the Design Page. Photos are contained in the My Coil Page.
Building the primary capacitor
Building the primary Coil
Building the secondary Coil
Building the toroid
Building the main sparkgap
Building the power supply unit
Building the filter unit
Building the RF-ground
Powering up the system

Building the primary capacitor

There are different types of capacitors you can build. See the design page for more information. This chapter deals only with building the plate stack type capacitor, as this is the type of capacitor I use in my coil. I have built 2 capacitors, each 5,85nF, which I use in parallel. When constructing plate stack capacitors, there are some details to keep in mind. And I had some bad experiences.
First of all, carefully select your dielectricum. I bought a large roll of 0,1mm LDPE sheet. I would prefer a thicker type now, as it is a lot of work to cut the sheets. But: It is better to use several thin layers than only one thick layer. This is because a defect in a single layer wil for sure kill your capacitor. But if you have 10 layers, 9 layers will be still available. This practice is also applied in commercial manufacturing. My capacitors consist of 10 layers, which makes up a dielectricum of 1mm. I carefully examined the sheets for defects and only used the obviously good ones.
I preferred to use the stack standing upright. This is, because I think that air bubbles can easier leave the layers this way.
The layers of aluminium are made from 0,1mm aluminium foil. I did not want to use thinner material because of high pulse currents. All 45 plates have been cleaned and the surface has been made smooth. Special care was taken, that no small aluminium particles from cutting could get between the layers. I cut the edges round to reduce corona.
For building the stack, I made an assembly support which helped to keep the layers in place. I made packets of 10 sheets LDPE and punched holes into the 4 edges, which were used to fix them on the assembly support. Punching the holes also had the effect that all 10 sheets sticked together. Each LDPE sheet was cleaned from dust before use.
The finished stack must then be pressed together at both ends. For this purpose, I used 5mm Pertinax, which is a very hard material. 4 screws on the edges keep the things together.

The aluminium sheets are connected by using washers, inserted between the layers of each pole. A screw holds the whole thing together. This makes a very good contact for each plate.
The top is made of 5mm plexiglass. The contacts are made of aluminium moulding, 2mm thick, with a 90 degree angle. These contacts are screwed to the top plate. The screws are used as capacitor contacts. The hole in the middle is used for filling in the oil and can be closed with a screw. This top plate is then connected to the stack. The aluminium moulding then replaces 2 washers on each pole. After having mounted everything, I found, that my capacitor had no capacity. The reason for this was found quickly: The surface of my aluminium moulding was absolutely isolating. I had to disassemble and work with a file on it.
The complete unit then was inserted into the PP box. I first did the sealing with hot glue, although PP cannot be glued. But I found, that the connection was quite good, and as there is no mechanical stress on the connection, I used that stuff. Well, the problem turned out at a later time, when I found, that the hot glue was not resistant to my oil. After some time, it turned to some kind of dark yellow jelly. Unfortunately I also had used hot glue inside the unit. So I had to disassemble both capacitors and remove that stuff, mount again and refill with new oil. I now decided to seal the unit with PP by welding with hot air. Always check materials you use, if they are resistant to your oil. I did this with all components, but unfortunately for some reason did not test the hot glue I used!
Before filling with oil, I tested, if the sealing was ok by putting vacuum into the unit with a self made vacuum pump. After that, oil was filled in. Using vacuum, I tried to get air out of the unit. There was quite a lot of air in the stack, coming out in lots of bubbles. The capacitors after that had long time to mature before power was applied the first time. See the pictures in the My Coil Page.

Building the primary coil

The primary coil needs to be built from thick copper wire. I used 8mm copper tubing which is well suited for this purpose, as because of the skin effect currents will flow only near the surface. The tubing was bent into the form of the primary coil. You should not bend the tubing too much. Also avoid to straighten it first before bringing into form.
The form for the primary coil is made from 21mm chipboard, 50x50cm. The supports for the copper are made from 10mm chipboard. I use 6 supports for the complete coil, cut for an angle of 20 degrees. I built the supports in 2 pieces, one fixed on the base, the other used to fix the tubing in place.

The above picture shows the innermost support. The holes in the other supports are shifted a little bit upwards to achieve the spiral form. The coil has a total of almost 10 windings.
The red circles indicate the silicone isolation, and you can see the screws used to fix that thing. I needed that many screws because the mechanical forces of the copper tubing pressed into the form were very strong, more than I expected. I even mounted aluminium moulding to strenghten the construction. This and the screws turned out to be a problem. Although the tubing is isolated from the support with silicone and tests with my 8kV neon transformer proved, that everything was fine, my first real run with higher power showed, that the isolation was not good enough. I had to remove more and more screws and even the aluminium moulding on one support as there occurred a direct arcing. Fortunately, the mechanical construction did not suffer from that. I assume, that the copper adapted to the form over time and lost stress. ??
For the next coil I will use HDPE or LDPE instead of chipboard and I will build the supports in a single piece for stability and to avoid any metal pieces like screws near the tubing.
You also should mount a strike ring. This is a ring, that is made of copper wire, mounted some centimeters above the primary coil. Connect it to the RF ground. The purpose is to avoid long arcs striking the primary itself. Those arcs will instead hit the strike ring and cannot cause damage this way. See the pictures in the My Coil Page.

Building the secondary coil

Building the secondary coil needs a lot of care. I bought a 75mm PVC tube. The ends are closed with disks of PVC. In the upper end, there is a screw for toroid attachment. The metal of this screw is not blank in the inside. It is isolated by hot glue although it does not come into contact with the wire. The windings are made from 0,39mm enameled copper wire. There are 972 close wound windings. I made all windings by hand, this took some time but was still faster than first constructing some device for that purpose. No wire is put inside the tube. This is a tip from Richard Quick. The windings have been sealed with polyurethan coating (2 components). It is important not to use water soluble versions. But have good ventilation, as it stinks terribly. I did this work outside on my balcony. This coating is done in many layers, until the wires are completely enclosed and the surface is smooth.
The upper and lower end are then connected to contacts. On the lower end, I made a copper contact which can be connected with a screw, the top end is made with a copper plate, where the toroid will just be put on. There is a cylinder of plastics to provide some distance to the toroid. See the pictures in the My Coil Page which show the details.

Building the toroid

Toroids may be bought, but are very expensive. But there are possibilities to build a toroid on your own. The simplest way is to buy flexible aluminium duct, which is available at different diameters. The disadvantage is, that you are not free in the choice of the diameter. And you need to be careful when buying that stuff, as the diameter given is the inner diameter. For example, I bought 80mm, which in fact had an outside diameter of 90mm. First check availability before doing all your calculations. The aluminium duct may be streched, but do not stretch too much, as it looses stability. I left as much material as possible to achieve my inner circle radius. I have built my toroid by bending the duct around a small wooden circle plate, with the inner diameter I needed (100mm). Then I fixed the duct with hot glue. This results in a nice and stable toroid. Although the surface is not smooth on a toroid like this, it turned out to work well and cost was much lower than for a commercial toroid of that size.

Building the main sparkgap

The sparkgap is a very important part of the complete system and has influence on the performance. As mentioned in the theory page, there exist several types of sparkgaps. This section will only deal with the sparkgap I built for my coil. This gap is inspired by the information available from Richard Quick. It is a series static gap built with pieces of copper tubing mounted in a circle.

The copper tube pieces are 5 cm long and mounted in the PVC tube as shown above. Brass pieces are used to fix the copper with screws. The inner brass pieces are formed to fit the inner radius of the copper tubes to provide better electical contact. The outer brass pieces are used to connect the tank circuit. A maximum of 6 gaps is possible with this sparkgap. On the back, there is a small fan mounted for quenching purpose. This is a fan normally used as CPU cooler. I think, that I should use a stronger fan than this, to provide a better airflow. See some pictures in the My Coil Page. Other models of sparkgaps may follow at a later time, when I build them myself.

Building the power supply unit

The power supply unit provides the main power to the tesla coil unit. I have built a power supply unit in a separate case made from chipboard. This unit contains the variac as well as all switches and instruments and fuses. The output is 0 to 220V.

See the pictures in the My Coil Page. I decided to use a relay for higher currents, which actually switches the power on and off. I had problems when switching on my variac, because the initial current peak was too high and the fuses were blown (no load connected, just the variac itself). My solution was to connect an NTC in series, which reduces this current peak. For my next coil with higher power I will seek for other solutions.

Building the filter unit

The filter unit consists of resistors, inductivities (chokes) and capacitors, as you can see in the complete ciruit diagram of the coil in the My Coil Page. The resistors are just ceramic high power resistors, 11W off the shelf. The chokes and the capacitor are self made. See the pictures in the My Coil Page.

Building the filter chokes

My chokes are wound on toroidal ferrite cores. I tested several ferrites, and found, that the permeability is very different from type to type. I started with measurements by building an LC circuit with my choke and a known capacitor. By measurement of the resonance frequency, I could determine the inductivity. It was very difficult to get big ferrites. So I ended up with the types you see in the photo section. These turned out to deliver the largest inductivity. The chokes are wound on these ferrite cores with 30 windings of isolated copper wire. These cores are mounted into small boxes used for photographic films. A piece of plastic tube is put in the axis. This leaves a hole in the middle after filling up with cast resin. This way the chokes can be mounted with screws on the base plate.
The power resistors are mounted using aluminium plates.
The sparkgap is made from plexiglass and screws with rounded nuts at their ends. All safety sparkgaps are constructed this way.

Building the capacitor

The capacitor is built the same way as the main capacitor as a plate stack type with LDPE dielectricum.
The above diagram shows the construction of the filter unit capacitor. The capacitor actually contains both capacitors in one case, using common plates (the red ones) which internally build the connection between both capacitors. The unit consists of 8 common plates and 2x7 plates of 1/10 mm aluminium sheet with dielectricum made of 1mm PE. Dielectricum is build with 10 layers of 0.1mm PE foil. The capacity is 2x560pF. I first have built a case of plexiglass for this capacitor. But this case after some time got cracks and the oil poured out. I found, that some types of plexiglass need to be tempered before glueing. I decided to build a new case from polypropylene (PP) which I welded from pieces which I cut out of PP boxes used in the kitchen.

Building the RF-ground

My RF ground consists of a copper tubing, about 70cm long, which is driven into earth. It works quite well. But I am almost sure, that this is not as good as is should be. So I'm looking for a better ground. I was told by a friend, that it should be possible to get grounding rods from building sites where cranes are used. They use grounding rods for the cranes, and sometimes these rods are not used any more when the crane is removed. If you ask, you may get them for free. I will check that and hope to get some rods for my next experiments.


Powering up the system

It is very important that you NEVER apply full power to your system until it is properly tuned. Otherwise you will destroy your components and have to start again.

Tuning the primary circuit is done by applying a sinewave of variable frequency to the primary tank circuit with shortened sparkgap. This is done via a resistor of for example 470 ohm. Use an oscilloscope to monitor the amplitude for changing frequencies. At the main resonance point, you will find a strong maximum in output voltage.
Tuning the secondary circuit is done by applying a sinewave of variable frequency to the secondary base. This is done via a resistor of for example 470 ohm. Use an oscilloscope to monitor the amplitude for changing frequencies. At the main resonance point, you will find a strong minimum in output voltage.
Using these measurements you will be able to tune your system properly. You should not apply the full power immediately. First close your sparkgaps down and start with lower voltage levels using the variac.
© 1999 by Herbert Mehlhose