600W Tesla Coil

Tesla_make_1 This is by far one of the more complex projects I have undertaken so far. This project requires a very good understanding of resonant LCR circuits and high voltage, high frequency currents to enable the safe construction and operation of a high performance Tesla coil. 

I have separated this project into two sections the first being a page on the theory of a Tesla coil and how they work and the second containing the step by step progress of the construction of my Tesla coil. 

The theory is to provide a novice or newcomer with sufficient information to understand and design their own Tesla coil. I found that the theory for designing and building a Tesla coil can be very complex and that the information required can be rather abstract and inconsistent. I have used many text books and hundreds of websites to try and gather enough information to enable me to be confident with designing and building my own Tesla coil.


This page contains the detailed construction and design of my Tesla coil and the second page contains the theory and design of a Tesla coil.



CLICK HERE TO SEE THE TESLA COIL THEORY PAGE





Making of the Tesla Coil

This page documents the construction process of my Tesla coil with descriptions and photographs on how it was built.

If during reading this page you are curious about specific theoretical details of the Tesla coil circuit and/or its construction, please see my Tesla coil theory page (link above).




Contents: Making of the Tesla Coil

Secondary Coil
Top Load
Primary Coil & Base
Neon Sign Transformer (NST)
Static Spark Gap
NST Protection Circuit/Filter
Variac Power Supply
Tank Capacitor Bank
The Completed Tesla Coil




Secondary Coil

The first part of the Tesla coil that I built was the secondary coil as I only had a single type of enamelled copper wire at hand and a section of clear PVC pipe. I decided that I could construct the rest of the Tesla coil based on the properties of this secondary.

The section of pipe used was an 800mm long transparent PVC pipe with a diameter of 60mm. It also had a handy black PVC cap with a hole in which was useful for mounting the toroid top load later. The wire I used is SWG-24 (0.5mm dia.) enamelled copper wire. It is a little on the thick side than what is preferred but it was all I had at the time and I decided that a new real of wire was too expensive (that was before I had known how much the rest of the Tesla coil would cost).

Winding the secondary coil is a very long and difficult task. It is important that the turns are wound neatly with no gaps and such that they don’t overlap. I wound mine by hand but many people rig the pipe to a lathe and wind it slowly on that. I started winding from the top (end with the black end cap) and worked my way down. I drilled a small hole in the top end of the pipe and threaded about 200mm of the wire through it such that the end of the wire was on the inside of the pipe. From this hole I wound the wire down the pipe until I got to the bottom.


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Once I had reached the bottom of the pipe I taped it in place securely and passed the wire up the inside of the pipe and secured temporarily. The coil consists of about 600 windings which is a little low for Tesla coil secondary coils which should have about 1000 windings, but this was due to using relatively thick wire. The secondary coil now needed to be insulated using polyurethane varnish. I hung the secondary coil up in the garage by passing thick garden wire through the pipe. This way allowed me to easily apply coats of varnish. In all it needed about six coats of varnish. To keep the bottom windings secure and to make it look neater I wrapped a strip of electrical insulating tape around the bottom edge of the pipe.




Top Load

I decided to construct a top load using corrugated aluminium air duct tubing. I cut two circles of rigid polystyrene and covered them with aluminium using disposable pie trays. Holes were drilled through the centres of the two discs to allow a bolt to go through them for mounting. A short length of 5cm diameter PVC pipe was glued to the two discs at their centres. This produced a former that allowed me to wrap the aluminium air duct tubing around to create a toroid. The tubing was cut to an appropriate length, wrapped firmly around the former and the ends glued together using epoxy resin. This produced a sturdy toroid top load that could be mounted onto the top of the secondary winding. The toroid has an outer diameter of 39.5cm and a cross sectional diameter (duct diameter) of 10.0cm. A long bolt was secured to the top of the secondary winding using large washers and the top load was then fitted onto this bolt.


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Primary Coil & Base

The primary winding needed to be mounted securely and by a good insulating material. I decided to use a couple of cheap PP chopping boards. The primary winding was going to be made using a 10m real of 8mm diameter copper tubing. I cut six strips of PP and drilled ten periodic 10mm diameter holes giving a winding spacing of 1.625cm. Also short lengths were also cut to support the strike rail. These were glued using impact adhesive and fastened using nylon bolts – PP is very difficult to adhere and fix securely due to its frictional surface properties!


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A 1cm thick, 50x50cm plywood base board was cut and varnished with polyurethane varnish. The winding supports were fixed to this base using cable ties. Three small holes were drilled on the bottom of each of the supports which matched with three pairs of holes on the base board for each support. Three cable ties were passed through the board, then through the support and back through the board and tightened.


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The next part was to wind the primary winding which proved to be very tiring and time consuming. The copper tubing had to be threaded through the holes in the supports starting from the outside. The problem was that after a few windings the friction between the tubing and the PP holes made it very difficult to continue threading further. I had to get help from a friend and had to use special (very) grippy rubber gloves (meant for people with arthritis such that they can open tight containers more easily) to complete the threading of the copper tubing. The winding has an inner diameter of 12cm and has a total of 9.5 windings which is equivalent to the full 10m length of the copper tubing.


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The secondary winding had to be located at the centre of the primary winding. A wooded plug was made using three circles of plywood, glued together and varnished. The central circles were made with a diameter such that the secondary winding fitted tightly over the top and met flush with the bottom larger circle. This plug was bolted to the primary winding base board with a long bolt. The end of the secondary winding that was left inside the bottom of the secondary tube after winding was soldered to the top of this bolt before the secondary was fitted onto the wooden plug. This allowed for a grounding connection on the underside of the board which would be connected later.


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A second identical base board was made and four wooden columns were doweled and glued at the corners. The top base board was then doweled and glued to the columns. This was to create a storage space underneath for the rest of the Tesla coil components.


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Connection to the primary coil from the rest of the circuit needed to be quick, easy and temporary to allow for tuning. Some brass clips were produced from some large crimping brass terminal connectors and were soldered to the ends of some high voltage insulated wire.


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Neon Sign Transformer (NST)

I managed to obtain a Neon Sign Transformer (NST) rated at 10,000V, 50mA running off 240V AC, 50Hz. It is an iron cored NST with overload and ground fault protection (which will probably have to be bypassed or disabled to allow the NST to function continuously in the Tesla coil circuit). I obtained this redundant but fully operational unit from a local neon sign company, whom I am very grateful to for letting me have this and many high voltage extras such as wire, mountings and a test neon sign all for £15!


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Static Spark Gap

My static spark gap was made using 7cm long lengths of 11mm diameter copper piping. I wanted the gap separation to be easily adjustable yet secure when in operation.

The first thing I did was to create an insulating enclosure to mount the spark gaps to and act as a vacuum chamber to aid in sucking air past the spark gaps using a hair dryer fan evacuating the box. I again decided to use PP chopping boards as the material for the enclosure. Once the box panels where cut to the appropriate shapes, I cut (using a woodworking router) a 10mm slot along the top of the two long side panels, 15mm away from the top edge. These slots allow 10mm diameter bolts to pass through and be fixed in place by tightening nuts at each end of the bolt. The panels were fixed together using small wood screws.

The method of extracting air from the box was to use a hot air dryer fan and motor as this allows a very large volume of air to be sucked out of the box quickly. The mounting for the fan and motor was taken directly out of the hair dryer as a complete unit and used as is in the spark gap in the photos. The motor was operated by 24V DC (from its label). A circular hole was cut in one end of the box to allow the motor housing to be inserted tightly. A small DC input jack was also added to the opposite end of the box to the fan to allow easy implementation of external DC power to the fan motor.

Lengths of 10mm diameter bolts were cut from a long length of threaded bar. The copper pipes were also cut to the correct length to fit in the top of the box. The copper pipes were slid over the bolts and two locking nuts were tightened either end of the copper tube centralising and holding the tube tightly in place. Locking nuts were used as they had a recessed lip that fitted well into the end of the pipes. Ten of these were produced and inserted into the slots. Nuts were then tightened at each to hold each of the bolts/tubes in place.


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The spark gap was tested using the NST for short periods of time to see how it functioned and to find what the maximum gap distance whilst maintaining sparks was. It seemed to work great - wonderfully loud!


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NST Protection Circuit/Filter

The NST protection filter is based on the renowned Terry Filter developed by Terry Fritz. I decided that the MOV’s (Metal Oxide Varistors) were not essential to the functionality of the filter. They were also very expensive and rather difficult to get hold of. The RC low pass filter and safely spark gaps should function just fine with out the varistors which are simply extra precaution for removing high voltage peaks. The full Terry Filter circuit diagram is shown below.


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My filter uses a pair of two 10W 2.7kΩ aluminium heat sinked power resistors connected in series, mounted on two large aluminium heat sinks. The snubber capacitors are a pair of two 20kv 1nF ceramic capacitors with 1MΩ bleed resistors across each capacitor. The safety spark gaps are made from three spherical stainless steel knobs. All the components were arranged and mounted on a rectangular sheet of PP from yet another cheap cutting board. Long connections were made with multilayered folded strips of aluminium foil to reduce the impedance of the connections.


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Variac Power Supply

The Variac or more correctly Variable Autotransformer was obtained for free from an electrical department workshop as it was surplus to requirements due to the availability of newer switch mode power supplies.

The variac I received has the following specification: Regulac ® Transformer, Type: RK8, Input: 240V 60Hz (single phase), Output: 0-275V 8A.

A variac operates similar to a resistor voltage divider circuit but also has an inductive property - an R-L voltage divider circuit. The two AC inputs (live and neutral) can be placed as a pair on any combination of terminals 1,2,4 and 5. The variac tap terminal is allocated as terminal 3. You can wire the AC input to make the variac operate in over stepping and standard stepping configurations. Over stepping configuration allows you to step the voltage above the input voltage of 240V to usually to about 275V. This is done by connecting the AC input across terminals 1 and 2. As the variable terminal passes terminal 2 of the inductor over stepping is achieved. Standard stepping allows you to step the voltage up to the input voltage of 240V. This is done by connecting the AC input across terminals 1 and 4.

Another feature that can be implemented on a variac is whether rotating the variable terminal clockwise or anti-clockwise (moving terminal 3 along the inductor either from the left or right) increases or decreases the voltage. This is done simply by symmetrically swapping the AC input terminals – e.g. if you have an over stepping configuration across terminals 1 and 2, and rotating the variable terminal 3 clockwise increases the voltage; swapping the AC input terminals across terminals 1 and 2 will result with a now decreasing voltage as the variable terminal is rotated clockwise as before.


Variac circuit diag


The variac did not come with an enclosure so I had to make one. I made a rigid box using 5mm thick plywood. The variac was bolted to the base of the box as it had bolt holes for mounting. A double mains socket was added to one of the sides of the box. A female IEC (Euro connector) power socket with built in 6A EMI filter was added to another side of the box to allow mains input power. A better EMI filter was needed than the once built in to the IEC socket. I purchased a high quality 20A EMI suppression module from eBay for about £15. I then started wiring it all up. The EMI module has to be wired in reverse (i.e. swap supply and load) to stop RFI from the Tesla circuit leaking back into the mains supply side. The mains from the IEC socket was wired through the reversed EMI filter and then to the variac (input) terminals. The variac‘s variable tap connection terminal was then connected to the double mains socket. Earths were connected from the IEC socket to the EMI filter and then to the double mains socket.

The top/lid of the box had a 14mm diameter hole drilled through the centre to allow the variable terminal dial rod to pass through. It was then screwed in place. A voltage indicator dial was created on the computer and printed out. It was then glued in the correct place on the lid of the box such that the dial gave an accurate representation of the voltage output – this was done using a voltmeter to observe the actual output of the variac. The large hand dial was fixed in place by tightening its grub screw.


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Tank Capacitor Bank

The main capacitor bank had to have a total capacitance of 2.25nF. I decided to use 36 ceramic capacitors each rated at 20,000V and 1nF capacitance. The best capacitors to use are polypropylene or polystyrene capacitors as they perform well at high frequencies but these are very expensive in comparison to ceramic capacitors which are not designed to operate with AC. I created two banks of 18 capacitors in series, with each bank of 18 containing 9 series paired capacitors of which all 9 are in parallel. I again used some PP chopping board as the form to mount my capacitors as seen in the photos. I marked out and drilled the necessary holes in the top of the mounting. The capacitor legs on each capacitor are too close together for 10,000V as they will arc over in air so they either could be potted or have the legs insulated. I decided to insulate each of the legs with 1.5mm thick wire insulation with a layer of heat shrink tubing and finally a thick coating of polyurethane varnish. The capacitor legs were passed through the holes in the top mounting board and the small amount of leg wire was soldered accordingly to neighbouring capacitors on the underside. Copper mains wire was used to connect the two bolts for terminals to the capacitors.


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The Completed Tesla Coil

I set up outside and connected the main parts of the Tesla coil together according to my circuit diagram (click here to see theory page). The RF ground was obtained by hammering an 80cm long copper tube into the earth. All wiring for connections was official 10kV rated wire from a neon sign company.


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** Note: Current tuning and testing is currently underway, check back soon for updates on the operation and testing **