Introduction

This guide was written as an introduction to Tesla coil design and construction. It will mainly benefit people who want to build - or are working on their first Tesla coil. I've tried to cover most of the basic information needed to build a Tesla coil without getting too technical or detailed. I've listed several other sources of more in depth information at the end of the guide in the Helpful Links section. I'd love to hear your thoughts and suggestions. My contact information is in the Contact the Author section of the guide.

Move your mouse over the small pictures to see them enlarged.

Thanks,
Kevin Wilson

Safety

Tesla coils are potentially fatal. Please do not attempt to build a Tesla coil without knowledge of high voltage, high frequency electricity. Tesla coils produce ozone, ultraviolet light and loud noise. When running a Tesla coil be sure to have fresh air, hearing protection and do not look directly at the spark gaps. Try not to work alone and never work when tired. Have a fire extinguisher and safety glasses near.

Do you think you're ready? Take the Safety Quiz now and fine out!

Pre-build Checklist

Before you get started, let's make sure you have all the required materials, tools, etc.

Materials

Every Tesla coil is unique and the materials used will vary, but I'll list some of the materials you're likely to need.

• Power cord to plug into the wall outlet or mains wiring
• Power switch
• Line filter (optional but recommended)
• Power supply (NST, MOT, etc)
• PFC caps (optional)
• NST filter (optional but highly recommended)
• Spark gap materials
• Primary capacitors
• Primary coil materials (1/4 inch copper tubing or solid copper wire, plywood, plexiglass, etc)
• PVC pipe for the secondary coil
• Magnet wire for the secondary coil
• Polyurethane or varnish for secondary coil
• Top load materials (aluminum dryer duct, pie pan, metal bowls, etc)
• High voltage wire to connect the components
• Plywood, plexiglass, pref board, etc for mounting components
• Various nuts, bolts, washers, etc

Tools

You should have access to a good set of tools and equipment. A good workshop or garage with a nice workbench helps. The tools you'll use can vary depending on your choice of materials and construction technique. You should also be able to actually use the tools. I'll list a few things that you're very likely to need.

• Soldering iron and solder
• Drill
• Saw for cutting plywood
• Hacksaw for cutting PVC
• Wire cutters, pliers
• Measuring tape, calipers, ruler, etc
• Screwdrivers, sockets, wrenches, etc

Other Things

These are some of the most important things you'll need before you begin building.

• A design and construction plan - Download TeslaMap and create a design
• A suitable place to run the Tesla coil
• A willingness to invest a lot of time and effort

This is very far from a complete list! In fact, it's practically impossible to know exactly what materials you'll need, how you'll be constructing the Tesla coil and what will happen when you throw the switch. It's likely that parts will fail, your designs will improve, and different materials or construction techniques will be employed. Once you have a general plan, some tools, materials and some free time, go ahead and get started. Be careful and have fun.

Theory Of Operation

Nikola Tesla developed the Tesla coil around 1890. The original intent was wireless transmission of power. Imagine a world without power transmission wires, but a massive Tesla coil in the center of every town. To access the power, simply pound a metal rod into the ground. The earth would become the conductor that supplies unlimited power anywhere in the world.

A Tesla coil is a resonate transformer consisting of a primary and secondary LC circuit. Power is supplied to the primary circuit through a transformer. A capacitor in the primary circuit will begin to charge. Eventually the voltage across the capacitor will increase sufficiently and short across a spark gap. The spark gap will allow the capacitor to discharge into an inductor called the primary coil. The primary inductor is coupled to an inductor in the secondary circuit, called the secondary coil. Attached to the top of the secondary coil is a top load that acts as a capacitor. As the primary circuit oscillates, power is transferred to the secondary coil where the voltage is increased and arcs of lightning are discharged form the top load. The primary and secondary circuits must resonate at the same frequency to achieve maximum power transfer from the primary to the secondary. The circuits in the coil are usually "tuned" to the same frequency by adjusting the inductance of the primary coil. For a more detailed description I recommend the following sites:
http://www.tb3.com/tesla/theory.html
http://www.richieburnett.co.uk/tesla.shtml

Construction

Every Tesla coil is unique. In this guide I show the most common designs and materials. Your design and choice of materials will likely differ depending on availability of materials, design improvements and personal tastes. I encourage you to be creative and experiment with your own designs. You may find something that works better than anything yet developed. Building a Tesla coil is a good learning experience. Some of your components may not work the first time, or they may develop problems. Don't get discouraged. Keep redesigning and improving your Tesla coil. I've rebuilt almost every part of my Tesla coil at least 3 times over the years. The first time I fired up my first Tesla coil I got little 1 inch arcs. After some changes I've got 4 foot arcs :-)

Power Supply

The power supply is a step up transformer used to charge the primary capacitor.

Neon sign transformers (NSTs) are the preferred power supplies. Other transformers can be used such as oil burner igniter transformers (OBITs), microwave over transformers (MOTs) or the distribution transformers used in the power grid, often seen on telephone poles and sometimes referred to as "pole pigs". Pole pigs are sometimes given away by the power companies, but may contain hazardous chemicals and they are extremely heavy. Pole pigs have no current limiting and can easily kill you. I do not recommend using pole pigs to power a Tesla coil unless you really know what you're doing!

NSTs are usually fairly easy to obtain, safer than other transformers due to internal current limiting and are fairly robust when used with the proper protection circuit. Used NSTs can be much cheaper than new ones. They can be found at sign shops and salvage / recycling centers. Typically they either work or they don't. Testing can be done by wiring them up and checking for arcs between the output terminals, or each output terminal to the case (assuming the case is grounded).

If a NST dies, the cause of death is sometimes arcing through the internal potting material. Potting is an insulator, usually a hard, tar like substance. The NST can be resurrected by removing the top of the case and heating the NST over a grill. Baking in an oven is not recommended because of fumes and leaking potting material. Once the potting is melted it can be stirred to remove the short or poured out and replaced with transformer oil. This process is very messy and probably not worth the effort if another NST can be found.

NSTs have shunts or metal plates between the primary and secondary coils which limits the current even when the output is shorted. The current limiting makes NSTs safer and more robust than other transformers. The shunts can be removed to provide a bit more current, but the chances of winding damage increases.

The primary, low voltage side of a NST should be wired through a filter which is connected to the house or building mains. A PFC cap should be wired across the primary terminals, but the NST can be run without it. Common NST power outputs are 9kV, 12kV and 15kV @ 30mA to 60mA.

NSTs can be wired in parallel to supply additional current to the Tesla coil. Before two NSTs can be wired together they must be tested for compatibility. If the current output of the NSTs is significantly different, one NST will begin to overheat. Follow this procedure to test NST compatibility:

1. Determine the phase of the NST outputs by checking for arcs between the output terminals of the NSTs by connecting an output terminal of one NST to an output terminal of the second NST (leaving a small spark gap). If you see an arc then the terminals are out of phase.
2. Put a mark on the output terminals that are in phase. Also mark the low voltage input terminals since switching one of the input terminals will switch the phase of the output terminals.
3. Connect a 1 kOhm 1/4 watt resistor between the output terminals in phase.
4. Run the NSTs for a few minutes and see if the resistor gets hot.

Obviously, you need to disconnect power to the NSTs before toughing the resistors. If the resistor heats up then too much current is flowing through the NSTs and they should not be used in parallel.

PFC Capacitors

Power factor correction (PFC) capacitors are used to correct the power factor of the AC connected to the NSTs.

The power factor will be degraded due to the large inductance in the NSTs which causes a phase shift of the voltage and current. The capacitance in the PFC cap will realign the voltage and current phase. The amount of capacitance should be matched to the amount of inductance so the capacitance and inductance will cancel each other. The PFC capacitance does not have to be exactly matched to the transformer. Often the PFC cap is smaller than the recommended size. Go ahead and use it, every bit will help. The NSTs can be run without any PFC caps.

Be sure to use only "run" type capacitors, as opposed to "start" type capacitors. Start capacitors are designed to only be used for short periods of time, to start a motor for example. They will overheat and possibly explode if run continuously. Electrolytic caps should not be used as PFC caps, they'll also heat up and pop. The PFC caps should we wired across the low voltage inputs of the NST.

PFC caps can be found in salvage / recycling centers on AC motors, washing machine motors, refrigerator motors, etc. I believe it's against the law to bury PFC caps because they contain hazardous chemicals, and recycling centers will usually have a pile of them waiting for you. They can also be ordered on the Internet.

The optimum PFC capacitance is calculated as:
PFC Capacitance = (NST VA / (2 * pi * NST Input Frequency * (NST Input Voltage ^2))) * 1000000
The TeslaMap program will calculate the optimum sized PFC cap for your NST.

Line Filters

Line filters are used to prevent high voltage spikes from traveling back into the house or building wiring.

They usually consist of a capacitor to shunt the high frequencies to ground. Most will also use inductors to cut down the high frequency spikes. Some may have MOVs to shunt voltage spikes to ground.

The line filter should be wired in series with the mains power. When wiring the filter conventional wisdom recommends wiring the filter in reverse (the output leads to the house wiring). The logic being that the filters are normally used to protect a device from spikes in the house wiring, but we're using it to protect the house wiring from the device.

Filters can be salvaged from equipment, or bought on the Internet. It's possible to make your own, but it's usually much easier to but one. Be sure to use a filter that's rated for the power used by the Tesla coil.

NST Protection

The wire in the NST secondary coil is very, very thin and susceptible to high voltage spikes generated in the primary circuit. A filter will help protect the NSTs from premature death.

I've been using a filter designed by Terry Fritz for several years with great success (knocking on wood). Several other people have also had good success with the filter. The filter consists of several caps wired in series to shunt high frequency spikes to ground and high power resistors to help decouple the NSTs from the primary circuit. A spark gap allows high voltage spikes to pass to ground. The spark gap should be set just wide enough so it does not short when connected directly to the NST output. I omitted the MOVs, although I'm sure they would help. Each cap has a high resistance bleeder resistor across the leads to allow the caps to slowly discharge and prevent them from holding a dangerous charge. The bleeder resistors should not be in direct contact with the cap as arcing can occur. Several caps are wired in series to handle the high voltages from the NST output. The total voltage rating of the series caps should be 3 times the peak voltage of the NST output, although good quality caps can be run at their rated voltage.

The type of cap used is not quite as important as cap selection in the primary circuit. Film foil type are preferred, metalized caps should be avoided. Refer to the good / bad cap list compiled by Terry Fritz.

Primary Circuit

Primary Capacitors (MMC)

The primary capacitor is used with the primary coil to create the primary LC tank circuit.

The primary capacitor is usually made of several dozen caps wired in series / parallel called a Multi-Mini Capacitor (MMC). A single pulse type capacitor can be used, but they are harder to find, non adjustable and difficult to replace. Other types of capacitors can be made, including salt water beer bottle caps and rolled aluminum foil caps. Neither is a good option. Salt water beer bottle caps are inefficient and it's difficult to know how much capacitance you're working with. Rolling caps out of aluminum foil and plastic insulators have not shown much success. Often the plastic will have microscopic holes or weak spots that quickly short out. The caps are prone to exploding when small air pockets between the layers heat up. Despite the higher cost, I recommend sticking with factory caps. The primary capacitor is run under extremely demanding conditions. It's exposed to high voltages and very short charge / discharge cycle times. Factory caps can tolerate these conditions better than anything most of us can make ourselves at home.

Caps can be ordered on the Internet, although they may be difficult to find. Sometimes a fellow coiler will order several hundred and resell them to other coilers.

Normally 1.6kV to 2kV caps are used in the MMC array. Several caps are wired in series to provide adequate voltage rating. It's good practice to construct the MMC with 3 times the peak voltage rating from the NST. A typical MMC will have series stings of about a dozen caps. Normally a few series strings will be wired in parallel to provide adequate capacitance. The TeslaMap program has a MMC calculator that makes MMC design fast and easy.

The MMC can be constructed with tap points between the capacitors so the capacitance of the array can be easily adjusted. 1Mohm bleeder resistors should be wired across each capacitor to prevent the caps from holding a dangerous charge. The bleeder resistors should not be in direct contact with the capacitor case as arcing can occur. It's a good idea to solder the resistors to the underside of the pref board, or whatever you mount the caps on.

It's important to use the correct type of caps in the MMC. Some caps are not designed to handle the high frequency, high voltage charging and discharging in a Tesla coil. Terry Fritz has compiled a good / bad cap list.

The MMC should be placed so that damage from cap failure in minimized. I had a MMC cap pop sending plastic case shrapnel across the room. I've heard of MMC caps catching on fire. I recommend soldering the leads when connecting the caps. Twisted leads will eventually loosen and / or corrode resulting in a bad connection.

The optimum primary capacitance is calculated as:
Primary Resonate Capacitance = (1 / (2 * pi * NST Impedance * NST Input Frequency)) * 1000
Primary LTR Static Capacitance = Primary Resonate Capacitance * 1.414
Primary LTR Sync Capacitance = Primary Resonate Capacitance * 1.9
NSTs do not operate well with resonate capacitance. They should only be used with "Larger Than Resonate" (LTR) primary capacitors. The LTR static capacitance should be used with a static primary spark gap. The LTR sync capacitance should be used with a sync type gap.

Spark Gap

The spark gap is used as a switch to momentarily connect the primary capacitor to the primary coil. When the gap is shorted the cap is allowed to discharge into the coil.

Spark gaps must cope with extremely high currents. Most electrodes will quickly develop burning and pitting on the surfaces. Tungsten is a good choice of spark gap electrodes. It has the highest melting point of any metal. It can be found as welding rods, in drill bits, etc.

Many spark gap designs can be used. The most simple design is a static gap consisting of 2 bolts, wires, drawer knobs, etc. The gap is set to a specific width. The width determines the voltage required for the gap to short. The ideal gap will short just as the primary cap reaches it's peak voltage. These gaps should be designed to allow small and easy adjustments to the gap width. Knobs screwed onto bolts are a good choice. Adjustments are made by screwing the knob off or on the bolt.

Static gaps are the most simple gap design, but they have some shortcomings. Often the gap will continue to short after the cap voltage has fallen below it's peak. This happens because the air between the gap becomes ionized when the gap shorts. The ionized air is more conductive and allows the gap to remain shorted. The performance of a static gap can be improved by blowing air through the gap. This is called "quenching" the gap. The goal of quenching is the blow the ionized air out of the gap. I've used 12 volt computer case fans, other's have used vacuum cleaner motors. Generally the more air you can blow through the gap the better.

A Richard Quick (RQ) design uses several copper tubes to divide up the spark gap into multiple smaller gaps. The Richard Quick design usually performs better than a standard static gap. When the gap electrodes are stationary, the gap is referred to as a "static" gap.

A better gap design uses a motor to move the gap electrodes to control the gap shorting. This design is called a "sync" or "async" design, depending if a synchronous or asynchronous motor is used. NSTs should only be used with sync motors. Sync gaps come in 2 basic designs, disk and propeller. The disk design is more common and uses a disk mounted on the motor shaft. The disk has electrodes placed around the edge that rotate and line up with stationary electrodes to form the spark gap. A propeller design looks like an airplane propeller. The electrodes are mounted on the motor shaft and rotate to line up with stationary electrodes to form the spark gaps.

Care must be taken to avoid an electrode being thrown out of the gap at high speeds. Propeller type rotary gaps should always be mounted in a box or with some walls to contain a loose propeller. Terry Blake has a good bit of info on gap safety here: http://www.tb3.com/tesla/sparkgaps/safety.html

Typically the gap is designed to short or "break" 120 times a minutes (120 BPS) when run from a 60Hz supply. This will correspond to the 60 Hz primary cap charging. It sounds like the spark gap will be firing twice the required rate, but remember that the 60 Hz includes a positive and negative peak, so the gap fires on both peaks. The gap firing will have to be synchronous with the charging peaks. Typically the sync motor is rotated relative to the stationary electrodes until the best performance is achieved. Sync motors will always run at a multiple of the input frequency. Common speeds are 1200 RPM or 3600 RPM for 60 Hz input frequencies. The number of electrodes will need to be chosen to provide 120 BPS depending on the motor RPM. Small motors may not have enough torque to turn the disk and electrodes. In this case the motor may not start or it may lose sync. When the motor loses sync it will attempt to re-sync. During this time the RPMs will vary slightly as the motor "hunts" for the sync RPM. A propeller design can be used to reduce the load on the motor. I do not recommend a disk diameter smaller than about 5 inches. Smaller disks on higher RPM motors can have problems with gap quenching.

Old sync motors can be found on record players or old computer reel equipment. Some have been found at military surplus stores. New ones can be ordered online. Hurst makes good motors.

Primary Coil

The primary coil is used with the primary cap to create the primary LC tank circuit. The primary coil also couples to the secondary coil to transfer power from the primary to the secondary circuit.

Typically 1/4 inch copper tubing is used to make the primary coil. I've used 6 AWG solid copper successfully, although my hands were sore for 3 days from bending the wire. Some people have used flat copper ribbon to save space, but tapping the turns can be more difficult. Avoid using other metals like steel due to it's higher resistance at high frequencies. Leave about 1/4 inch spacing between turns. This will prevent arcing and allow space for a tap point. The primary coil can be constructed on just about any non conductive material. The material should be strong enough to support the weight of the copper. You'll need to design some means to hold the copper turns in place. Plastic wire ties or plastic bars with notches every 1/4 inch are common. I hung my coil of copper from the ceiling and let it hang down onto the form where I secured it with wire ties.

The primary coil can me made flat, called a "pancake" coil or in a cone shape. The angle should not be greater than 45 degrees. Generally larger Tesla coil use flat primaries and smaller coil can use cone shape primaries. Larger, higher power coils can experience over coupling between the primary and secondary coils if a cone shaped primary is used. I recommend starting with a flat coil.

The primary coil should have strike ring about 2 inches above the outer most turn. This ring will hopefully stop arcs from the top load from reaching the primary coil. If the primary coil is struck a voltage spike could kill the primary caps and / or NSTs. The ring should not be completely closed. One end should attach to the secondary earth ground. Smaller coils that do not produce arcs long enough to reach the primary coil do not require a strike ring.

Before making the primary coil you should know how many turns you'll need to tune the coil, the length of tubing of wire you'll need and the size of the primary base. The TeslaMap program can help you easily design your primary coil.

Secondary Circuit

Secondary Coil

The secondary coil and the topload create the secondary LC tank circuit. The secondary circuit also couples to the primary coil to transfer power from the primary circuit to the secondary.

The size of the secondary coil is generally governed by the power output of the power supply. For an average sized Tesla coil (about 1kW) you'll want a 4 inch to 6 inch diameter secondary coil. Smaller coils should have about 3 inch to 4 inch diameter, while larger coils should have at least a 6 inch diameter. The height to width ratio (also known as the aspect ratio) is important. If the coil is too short then you'll get a lot of strikes from the topload to the primary coil. The height of the coil should be about 4 or 5 times the diameter in an average sized Tesla coil. For example the secondary coil on a 1kW Tesla coil with a 4 inch diameter should be about 16 to 20 inches high. Remember to cut the PVC a couple inches longer than the winding height to leave some space on each end! Smaller coils should have a height to width ratio close to 6, while larger coils should be close to 3.

The secondary is typically thin (22 AWG to 28 AWG) magnet wire wound on a PVC core and covered with a few coats of polyurethane or varnish. Magnet wire is solid copper wire with a thin coating of varnish as an insulator. It's sold by the pound or the gram. You'll probably need about 2 pounds to wind a typical coil. Double build magnet wire is available with extra insulation, but it's not necessary. Aim for about 1000 turns on the secondary.

The secondary coil is usually wound on PVC pipe, although cardboard or most other nonconductive materials can be used. The PVC pipe should be clean and dry. Some PVC may come with a thin metal strip in it. This is used to help find the pipe after it's buried. Do not use this pipe as the metal strip will quickly short out the coil.

Winding the coil will take quite a while. Find a comfortable, well lit spot and plan to be there for quite a while. A lathe is ideal for holding the PVC pipe. Although I found that the lathe, even on it's slowest speed was too fast to wind the coil. So I just rotated the pipe by hand. The spool of magnet wire should be mounted so it will be easy to untangle during the winding. You may want to wear a thin glove to save the skin on your fingers. Start by taping the end of the magnet wire a few inches from the end of the PVC. Be sure to leave about a foot or two of magnet wire unwound on the end. Have some tape handy to easily hold the wire for rest breaks or untangling. Be careful not to leave any space between the windings. Keep some tension on the wire as you wind it. Tape the end of the magnet wire down when finished and leave a couple feet of extra wire. Hopefully if your calculations were correct you have just about a few inches of PVC pipe left. Start coating with polyurethane or varnish. Remember not to coat the foot of extra wire on each end. I usually coil this extra wire up and let it stick up and out of the way while I varnish around it. Follow the instructions on the polyurethane or varnish and apply several coats. Keep the pipe rotating as the polyurethane or varnish dries. I've used a lathe and rigged a hand drill on slow speed to rotate my PVC pipe. It's probably a good idea to coat the inside of the pipe to protect it from water absorption or whatever.

TeslaMap can design a secondary coil and tell you exactly how much magnet wire you'll need, the final number of turns and dimensions of the PVC pipe.

Top Load

The top load is acts as a capacitor in the secondary circuit.

The torus or donut, also called a toroid is the preferred shape. As the coil operates a charge will build up around the surface of the top load. A sphere will have an evenly distributed field strength over it's entire surface. By flattening the sphere into a toroid, the field strength will increase around the radius of the toroid. The arcs will break out where the field strength is greatest. The benefit of concentrating the field is to help direct the arcs and to allow a greater field to build up before breakout. Using a sphere will result in evenly distributed smaller arcs.

The most common method of toroid construction is to wrap aluminum dryer duct around an aluminum pie pan. You can also use a spun aluminum toroid. A topload can be made of practically anything with a smooth shape covered in aluminum foil. Avoid using "metal" paint. Usually there is not enough metal in the paint to create a conductive surface, and even if there is sufficient metal, it's usually quickly burned off.

The size of the toroid should be just big enough to allow a single, long arc to break out. If the toroid is too large the field strength will not be antiquate to allow breakout. Placing a sharp pointed object like a thumb tack (called a break out point) on the toroid will create a disruption on the field and allow the arc to break out from the break out point.

Generally the diameter of the toroid ring should be about the same as the secondary coil, meaning a secondary coil wound on 4 inch PVC pipe should use 4 inch diameter dryer duct. The overall diameter of the toroid should be about 4 times the ring diameter, so 4 inch diameter dryer duct should be wrapped around an 8 inch pie pan.

It's important to physically attach the toroid to the top of the secondary coil. You can get by with sitting the toroid on there, but eventually it's going to fall or get bumped off. At best you'll dent up the toroid or your primary coil, at worst there could be a short that blows out your primary caps or something else. A good way to connect the toroid to the secondary is to get a PVC end cap, drill a hole in the middle and insert a nylon bolt sticking up. Drill a hole in the center of the pie pan and slide it onto the nylon bolt. You'll have to use a nylon or some other non conductive bolt. A long, metal bolt will end up shooting an arc straight up.

Toroid capacitance can be difficult to calculate. I've found several different calculations that seem to be fairly accurate. Without knowing which calculation is best, I decided to use them all and take the average.

(For large or small toroids, Ring Diameter < 3" or Ring Diameter > 20")
Toroid Capacitance 1 = ((1 + (0.2781 - Ring Diameter / (Overall Diameter))) * 2.8 * sqrt((pi * (Overall Diameter * Ring Diameter)) / 4))
Toroid Capacitance 2 = (1.28 - Ring Diameter / Overall Diameter) * sqrt(2 * pi * Ring Diameter * (Overall Diameter - Ring Diameter))
Toroid Capacitance 3 = 4.43927641749 * ((0.5 * (Ring Diameter * (Overall Diameter - Ring Diameter))) ^0.5)
Toroid Capacitance = ((Toroid Capacitance 1 + Toroid Capacitance 2 + Toroid Capacitance 3) / 3)

(Ring Diameter is between 3" and 6")
Toroid Capacitance Lower = 1.6079 * Overall Diameter ^ 0.8419
Toroid Capacitance Upper = 2.0233 * Overall Diameter ^ 0.8085
Toroid Capacitance = (((Ring Diameter - 3) / 3) * (Toroid Capacitance Upper - Toroid Capacitance Lower)) + Toroid Capacitance Lower

(Ring Diameter is between 6" and 12")
Toroid Capacitance Lower = 2.0233 * Overall Diameter ^ 0.8085
Toroid Capacitance Upper = 2.0586 * Overall Diameter ^ 0.8365
Toroid Capacitance = (((Ring Diameter - 6) / 6) * (Toroid Capacitance Upper - Toroid Capacitance Lower)) + Toroid Capacitance Lower

(Ring Diameter is between 12" and 20")
Toroid Capacitance Lower = 2.0586 * Overall Diameter ^ 0.8365
Toroid Capacitance Upper = 2.2628 * Overall Diameter ^ 0.8339
Toroid Capacitance = (((Ring Diameter - 12) / 12) * (Toroid Capacitance Upper - Toroid Capacitance Lower)) + Toroid Capacitance Lower

TeslaMap can do all these calculations for you.

Misc Parts

Safety Gaps and Filters

Safety gaps should be placed across any component that can be damaged by high voltage. There should always be a safety gap across the NSTs and primary capacitors. The gap can be as simple as 2 bolts. No quenching is required. Set the gap width by connecting the gap directly across the NST output terminals. Close the gap so it shorts, then slowly open the gap until it no longer shorts when the NST it turned on. Verify by switching the NST off and on several times. You may be tempted to widen the safety gaps to get longer arcs. I don't recommend it. The primary capacitor can be stresses when it's safety gap fires. This basically creates a direct short across the caps. I recommend placing a low resistance (few ohms), high power resistor in series with the safety gap. When the gap fires the resistor will slow the discharge time and relieve the stress on the caps.

Wiring

All wiring should be as short an possible. All wires should be high voltage "GTO" wire. All connections should be clean. Soldering is the best way to connect wires and leads. When high current flows through a connection, it does not take much resistance to create enough heat to burn the connection. A bad connection will reduce the efficiency of the coil and can possible create a fire! There's always a chance that voltage spikes could find there way back into the house wiring. I recommend unplugging any sensitive or expensive electronic equipment before running the Tesla coil.

Grounding

Grounding is important for safety and operation if the coil. The power supply should be grounded to the house or building wiring. The case of the NSTs should be grounded. The NST protection gap should short the NST outputs to ground. The line filters should also be grounded. If you're using a variac or have a control panel placed in a metal cabinet, they should also be grounded to the house wiring. The bottom of the secondary coil and the primary strike rail should be earth grounded. This means a ground rod should be hammered into the ground close to the coil and connected to the magnet wire at the bottom of the secondary. Wetting the ground before running the coil helps conductivity to the earth. The ground rod should not be close to the house or building ground. Generally 6 or 8 foot minimum is recommended, but it really depends on soil conditions and other factors. Further is always better. If the Tesla coil ground rod is too close to the house ground rod then high voltage could find it's way into the house wiring.

Tuning

Tuning refers to the process of adjusting the resonate frequencies of the LC tank circuits to the same frequency. The coil must be tuned to produce the longest possible arcs. Usually the inductance of the primary coil is adjusted. The primary coil is the easiest component to adjust. The TeslaMap program can be used to get a good idea of the number of required turns on the primary. The typical tuning procedure is to tap the primary at the required number of turns and run the coil checking for the arc length. Adjust the tap point 1 turn and run the the coil again to check arc length. If the arcs are longer then you're moving in the right direction. Make smaller changes as you get close to the best tap point. Adding a pointed object like a thumb tack to the top load can help breakout and make arc measurements easier.

FAQ

Why build a Tesla coil?

Building Tesla coils is a great way to learn about electricity, electrical components, assembling and wiring components and safety. And creating lightning is totally cool.

How much does it cost to build a Tesla coil?

Cost will depend on many factors, mostly how much you can salvage. Used NSTs are often much cheaper, sometimes even free. MMC caps, magnet wire, good PVC and dryer duct are all difficult to salvage and will probably have to be bought new. A small coil can be built for under $100. A large Tesla coil with "nice" parts can cost several hundred dollars.

What parts will I need to build a Tesla coil?

Basically what's listed above and plenty of nuts, bolts, screws, wire connectors, maybe some plywood, etc. You'll need at least a basic array of tools.

Where can I get the parts for a Tesla coil?

Many of the parts can be found at the local home improvement store (Home Depot, Lowe's) or at salvage / recycling centers, or on the Internet. Check out Alan's store at Tesla Stuff. He has a nice selection of Tesla coil components including "hard to find" and "one of a kind" items.

Contact the Author

I can be contacted through email at thebiggiantkevin@hotmail.com I'm interested to hear your comments and suggestions.
I've searched the internet to find the best pictures. I've given credit to the source when possible. If I've used your picture and you'd like it removed, please email me at thebiggiantkevin@hotmail.com and I'll remove it. Thanks.

Helpful Links

These sites have helped me a great deal, and hopefully they can help you too.

Tesla Coil Mailing List
If you have a question about Tesla coils, it's probably been asked and answered in the searchable archive.

Tesla Stuff
Tesla coil components including "hard to find" and "one of a kind" items.

Wikipedia
A pretty good overview of Tesla coils.

Tesla Coil Web Ring
A list of Tesla coil orientated websites.

Hot Streamer
Website by the legendary Terry Fritz.

Steve's High Voltage
Interesting site with solid state and vacuum tube Tesla coils, Marx generators, induction heating and pulse power.

Classic Tesla
A good online JAVA design program from Bart Anderson.

DMOZ Tesla Coil Category
Open Directory Project with a list of Tesla coil websites.

www.richieburnett.co.uk
Lots of good Tesla coil information.

Video Of My Tesla Coil

Click here to download a 1 minute video clip of my Tesla coil producing 4 foot arcs. Thanks to my brother Russ for letting me use his garage.