The inventions of Nikola Tesla are all around us: radio, AC power, fluorescent lighting, and remote control devices are just a few. Tesla was ahead of his time, in many ways, and his work with high-frequency alternating currents has inspired engineers, scientists, geeks, inventors, artists, dreamers, and (frankly) quacks for more than a century. The Tesla coil is particularly fascinating because of the elemental, visceral nature of the electrical arcs it produces. It’s like watching lightning strike. Tesla himself used these spectacular effects to wow audiences with the wonders of AC electricity.
Since Tesla’s time, hobbyist “coilers” have made many discoveries and improvements to the basic design, achieving bigger sparks with less input current. With the advent of plastics, improved wire insulators, and a better understanding of theory, the modern Tesla coil looks very different from the original. The basic circuit and concepts are the same, but almost everything else is different.
One thing that is the same, in this project, is the capacitor design. Ours is made from glass beverage bottles, very similar to the champagne bottles that Tesla himself often used.
Along with the wonder and awe of a Tesla coil comes a significant level of danger. It is the responsibility of anyone who builds or operates a Tesla coil to ensure the safety of themselves and anyone who might come near, either during a demonstration or inadvertently. Whenever you approach the coil, unplug the power cord and hang on to the plug end as you work. If the location is not entirely secure, consider adding a safety key switch so you can pocket the key.
A Tesla coil’s high-frequency electrical field can damage or destroy cardiac pacemakers/defibrillators, hearing aids, and other biomedical devices. I’ve never seen this happen, but it’s imperative to warn audiences of the possibility before demonstrations.
Similarly, the Tesla coil can damage other sensitive electronics nearby. I have personally destroyed a stereo receiver, a garage door opener, a wireless phone system, and two PC network cards. It again falls to the maker to make sure that the coil is operated at a sufficient distance from any valuable electronics, flammable materials, pets, and of course small children.
There are many hazards to be aware of and in this single article we cannot cover them all. If in doubt, contact a nearby Tesla hobbyist or an engineer experienced in high-voltage devices and electrical safety. If you have any doubt about your abilities in this area, don’t attempt the build. Period!
Know the Danger
- Assume the capacitor is always charged. Capacitors can retain a charge for days. No matter what anyone else tells you, always safely discharge the six-pack capacitor yourself, and jumper it with a sturdy clip lead before touching any of the components. Keep the jumper in place when you’re not operating the coil.
- Do not operate the coil around small children or animals.
- Operate in clear spaces at least 20 feet from flammable materials. The electric field generated by a Tesla coil can create sparks within furniture and in the ceilings of structures. Sparks can ignite combustible solids, liquids, and especially vapors.
- Do not touch the NST terminals. Both sides of the neon sign transformer are “hot.” Some NSTs have exposed primary terminals carrying line voltages. Current at the NST secondary terminals is usually low, but the voltages are high enough to cause painful shocks and secondary injuries from loss of motor coordination.
- Do not stare at the sparks. Electrical arcs in air emit ultraviolet light that can damage eyes and skin on extended exposure. Clear polycarbonate sheet can be used to shield the spark gap and block most of the UV generated by the sparks.
- Do not operate the coil without proper ventilation. Electrical arcs in air produce ozone, nitrogen pentoxide, and several other nitrogen oxides that are hazardous to health. Note that nitrous oxide is not produced.
- Do not operate indoors without ear protection. This Tesla coil can produce hazardous levels of noise. It’s less of a problem outside, but indoors the sound is loud.
How It Works
Fundamentally, a Tesla coil is just a transformer, like the one that steps household electricity down to a voltage suitable for charging your cellphone. All transformers have two coils — a primary and a secondary — and most of those you encounter in daily life transform voltages based on the different numbers of turns in each coil. A Tesla coil works on a slightly different principle, creating the very high voltages needed to produce long arcs in open air mostly through the inductive difference between its primary and secondary coils.
More specifically, a Tesla coil is an air-core, dual-resonant transformer. Air-core means that the coils are hollow, rather than wrapped around metal or ferrite cores as in common transformers. Dual-resonant means that the circuits containing both primary and secondary coils are tuned to “ring” at the same frequency.
The combination of the primary coil (an inductor) and the capacitor (the bottles, in this design) create a resonant LC circuit that “rings” at a particular frequency. This is called the tank circuit.
Since both tank circuit and secondary coil are tuned to the same frequency, they pass energy back and forth when “struck” with an electric impulse. Imagine striking a bell near a drumhead tuned to the same note.
The electrode on the top of the coil is called the top-load. You can imagine the top-load as a capacitor with one side connected to the secondary coil, the other side connected to ground, and the air all around as the insulator between the two “plates.”
This Tesla coil is designed to be powered by plugging into a wall outlet, and uses a neon sign transformer (NST) to step 120V AC up to about 10kV at 25mA–30mA. Solid-state voltage converters are not appropriate for this application, nor are modern NSTs manufactured with ground fault protection circuitry. You’ll need a used or old-stock NST; fortunately these are not hard to find on eBay and, sometimes, Craigslist. Neon shops may have old units hanging around.
Design the Six-Pack Coil
JavaTC was instrumental in designing the six-pack Tesla coil. The output text file describing the six-pack coil is available here.
Spend some time playing with JavaTC, tweaking the specs for the six-pack coil, and you’ll quickly develop a feel for how the various design parameters affect one another. If you have to use a different transformer, make a different top-load, use a different wire gauge or any other major changes, you can use JavaTC’s auto-tuning feature to understand how to modify the design.
Build Your Six-Pack Tesla Coil
First-time “coilers” should follow this build as closely as possible. Use a neon sign transformer rated for 9kV at 25mA, strive for a main tank capacitance as close to 0.005µF as possible, and do not substitute parts if it can be avoided.
Plan your build carefully before you start. Do not just jump in and start building without reviewing every aspect of the design. High-frequency resonant circuits are very sensitive to small changes, and poor attention to planning can make the tuning process very frustrating.
Craftsmanship is also important. Take your time, particularly with the secondary coil, where a single crossed winding or a skimpy varnish job can easily result in a nonfunctional or very short-lived coil.
Good design, attention to detail, and patient craftsmanship will pay off with a long, noisy spark that draws oohs and aahs, applause, and admiration from everyone who sees it.
1. Wind the Secondary Coil
1a. Cut a 16″ length of 1½” PVC pipe. Clean up the ends by sanding.
1b. Mark and drill a ¼” hole in the exact center of both PVC caps.
1c. Build a simple winding jig as shown. Clamp or otherwise mount it right at the edge of your work surface.
1d. Chuck the secondary coil’s axle into your drill.
1e. Clean the surface of the PVC pipe with rubbing alcohol and dry it carefully.
1f. Make sure your drill has a full charge before winding the coil. If you have a spare battery, keep it handy. Using your finger (or a cable tie) to hold the trigger, start the tube spinning slowly.
1g. Apply an even coating of brush-on gloss urethane finish to the tube. To avoid drips and runs, leave the tube spinning until the urethane dries a bit. This first layer of varnish helps hold the wire in place during winding. After 20 minutes the urethane should still be a bit tacky.
NOTE: You’ll remove the PVC end caps later, so be careful not to varnish the cap/pipe seam.
1h. Stop the drill. Use electrical tape to fasten the end of the magnet wire to the cap at one end of the tube.
1i. Use a Sharpie to mark the winding start- and end-points at 2″ and 14½” from the starting end of the tube, including the cap.
1j. Set the direction of rotation so that the top of the tube rolls away from the wire spool. Use your non-dominant hand (or a helper) to run the drill slowly as you guide the wire onto the tube with your dominant hand.
NOTE: To guard against friction burns, wear a cotton glove or use a folded paper towel to guide the wire through your fingers.
1k. Advance the wire with wide turns until you reach the first witness mark, then start feeding so that the wire is close wound smoothly with no gaps. The trick is to angle the incoming wire such that it is slightly behind the advancing edge of the coil. Be patient and careful. Keep winding until you reach the second witness mark.
CAUTION: Do not allow the wire to cross a previous turn. If it does, stop, carefully unwind the coil, and correct the mistake. A few small gaps between turns will not matter, but a single crossover will create a short circuit that renders the coil useless.
TIP: The whole winding process should take less than 20 minutes. The varnish will hold the winding in place if you need to rest for a minute, but do not let it dry completely or it will not be sticky enough for you to finish.
1l. Close-wind the wire a few turns past the second witness mark, then open up the winding again until you reach the second end cap. Wind the wire about five turns on the end cap and tape it securely.
1m. Take a short break and change the battery on your drill, if necessary. As before, set the drill to slowly spin with a cable tie across the trigger. Apply three coats of varnish over the secondary windings, letting each coat dry according to the time listed on the can. Keep the coil spinning while each coat is drying to prevent drips and runs.
NOTE: No matter what it says on the varnish can, do not sand between coats since this will likely damage the wire insulation and ruin the coil. Also, when handling the finished coil, be careful not to ding or scratch the windings.
1n. Set the coil aside to dry thoroughly. Don’t even think about using it until the last coat of varnish has dried for 24 hours.
2. Install the Secondary Coil Terminals
The coil terminal connections need to be mechanically sound both at the bottom ground wire and at the top-load. You will connect the ground terminal first.
2a. Remove the tape from one end of the secondary coil and unwind the wire right up to the first witness mark. The wire should release easily from the varnish. Remove the pipe cap.
2b. Cut a ½”×2″ strip of shim brass or copper. Polish both sides with fine sandpaper or steel wool until they are bright.
NOTE: We use brass and copper as much as possible, throughout the build, to avoid magnetic materials that might interfere with the performance of the coil.
2c. Form the strip so that its curve matches the PVC pipe, using your fingers or a small-diameter mandrel.
2d. Apply a 4″ strip of electrical tape to the outside/convex side of the strip, and a sparse amount of CA super glue to the inside/concave side. Carefully glue the strip in place about 1/8″ below the winding using the electrical tape to hold it as the glue sets.
NOTE: Place the strip carefully. CA glue bonds quickly.
2e. After a few minutes, remove the electrical tape and tin the edge of the strip nearest the coil, where you will eventually connect the wire. Cut the wire so that it wraps smoothly around the pipe to contact the strip without kinks or bends. Clean the end of the wire using 600-grit sandpaper and then tin it and solder it to the strip. Finally, apply a dab of Goop adhesive to immobilize the wire near the connection.
2f. To make the top-load terminal, first unwind the other end of the coil to the witness mark and remove the remaining PVC pipe cap. Pass a ¼-20×1″ brass screw through the hole from the inside of the cap. Place a ring-tongue solder lug over the screw, apply a brass nut, and tighten well.
2g. To insulate the screw head and prevent arcing down the inside of the tube, epoxy a ½” CPVC pipe cap inside the 1½” cap.
2h. Use PVC cement or 5-minute epoxy to glue the PVC end cap back in place on top of the coil. Once the glue has set, wrap the loose end of the coil wire around the pipe and cap in a gentle spiral approaching the solder lug on top. Keep the coil tidy and avoid sharp bends. Tape the wire in place, temporarily, and cover it with Goop from the top witness mark just to the top edge of the cap.
After the glue sets, remove the tape, clean the end of the wire with 600-grit sandpaper, tin it, and solder it to the lug. Cover the final length of wire, from the edge of the pipe cap to the solder lug, with more Goop.
2i. Cut a 16″ circle of ½” or ¾” MDF or plywood. Drill a ¼” hole in the center, and counter-bore on one side so that the head of a ¼-20×1¼” brass machine screw sits flush.
2j. Mount the loose PVC pipe cap to the base using the machine screw and a matching brass nut inside the cap.
2k. Push the secondary coil into the PVC cap. Solder a 6′ ground wire (18–12 AWG) to the metal strip on the bottom end of the coil. Secure the wire to the base with a P-clip for strain relief.
CAUTION: Do not drill through the secondary coil form (the PVC) to attach the ground wire. Doing so may cause internal arcing.
3. Build the Top-Load
The top-load is based on a toroid-shaped floral wreath form I bought at a large hobby store. If you cannot find a foam toroid just like this one, there are other options for building top-loads, but these variations will affect your tuning. Use JavaTC to understand how a different top-load may affect your design.
3a. If your toroid has mold lines or other protrusions, smooth them out with a sanding block.
3b. Measure the inside diameter of your toroid. Mine was 8¼ inches. Use a large compass or trammel points to draw a matching circle on a sheet of ¼” or ½” plywood or MDF. Cut the circle slightly oversize with a jigsaw, then sand it to fit snugly. Drill a 5/16″ hole in the center of the disk.
3c. Cover both sides of the disk with strips of aluminum tape, overlapping them by about ¼ inches. Burnish it with the side of your Sharpie to smooth out wrinkles, then trim it flush with the disk’s edges and center hole using a utility knife.
3d. To make sure the disk is centered vertically inside the toroid, first set the toroid on your work surface, then arrange small blocks of wood inside to support the disk at the correct gluing height. With the specified toroid, ¾”-thick blocks work nicely.
3e. Apply super glue to the inside edge of the toroid, then quickly press the disk into place.
3f. Wait a few minutes, then run a bead of super glue along the joint for good measure.
3g. When the super glue sets, completely cover the toroid in 8″ strips of aluminum tape. Each strip should wrap all the way around the toroid and extend onto the top and bottom of the disk.
TIP: Fold each strip of tape in half lengthwise, sticky sides out, to find its center. Then align the fold with the vertical center of the toroid’s outer edge.
3h. Work each strip of tape down smoothly with your fingers to eliminate big wrinkles and voids. Then burnish it smooth before applying the next piece of tape. Overlap the strips about ¼” on the outside of the ring.
3i. Install a ¼-20 T-nut in the hole with a dab of 5-minute epoxy for good measure. Seat the nut firmly by tightening it against a matching bolt and washer. Then remove the bolt and washer and set the top-load aside while the epoxy sets.
4. Build the Six-Pack Capacitor
A capacitor is a device for storing energy, in the form of electric charge, between two conducting electrodes separated by an insulator, aka a dielectric. In this case, salt water inside the bottles is one electrode, the bottle glass is the dielectric, and the outer foil covering is the other electrode. There are a number of homebrew capacitor designs, but the bottle capacitor, aka Leyden jar, is by far the simplest and easiest to make.
4a. Wash the bottles thoroughly with soap and water. Dry them.
4b. Apply two pieces of aluminum tape, crossed at 90°, to the bottom of each bottle. Use scissors to trim the tape in a circle about ½” larger than the bottle’s base, then snip inward to create small triangular “petals.” One by one, fold and smooth the petals up along the bottle’s sides, working carefully to reduce wrinkles. When all the “petals” are in place, burnish the tape smooth.
4c. Cover the sides of each bottle with overlapping strips of aluminum tape, each of which wraps all the way around the bottle’s circumference, plus 1″ or so. Start flush with the bottom edge, covering over the “petals” from the previous step, and work your way up, overlapping the strips by ¼” to ½” and burnishing each strip before applying the next. Apply the last strip so that it ends just where the bottle curves inward at the neck. Wrap the top edge of the foil with two turns of electrical tape to reduce corona discharge.
4d. Thoroughly mix two cups table salt into 72oz warm water in a resealable container.
4e. Use a funnel to fill each bottle with saltwater to just below the top edge of the foil tape, then use your multimeter to measure its capacitance as shown here. When you are done testing, empty the saltwater solution back into your resealable container.
4f. This coil has been designed for a primary tank capacitance of 0.005µF. The individual bottles will be connected in parallel, so in an ideal 6-bottle capacitor, each bottle will have a capacitance of 0.005µF / 6 = 0.00083µF, or 0.83nF. Add, remove, or swap out individual bottles as needed to bring the total capacitance as close to 0.005µF as is practical. You can also adjust the capacitance of an individual bottle, by peeling a bit of foil tape from the top edge, or by removing some saltwater.
4g. Arrange the bottles on your worktable in groups of three and tape them together tightly with three or four turns of vinyl electrical tape at both top and bottom.
4h. Take two sets of three bottles and tape them together.
4i. Wind a length of bare stranded wire twice around the group of bottles, crimp ring-tongue lugs on each end, and stretch a small coil spring between them to keep the wire tight against the foil.
4j. Cut an 18″ lead from 12 AWG insulated stranded copper wire, strip the ends, and solder one of them to the bare winding. For strain relief, secure the lead to the winding a short distance away using a small cable tie. Cover the remaining length of the lead with vinyl drip hose for extra insulation, and then crimp a ring-tongue lug to the loose end. This lead will connect to the spark gap.
4k. Fill each bottle with saltwater as before, then top with a 1/8″ layer of mineral oil to prevent corona discharge.
4l. Connect the saltwater electrodes inside the bottles using a wire dip electrode made from two 19″ lengths of 12 AWG solid copper wire, each wrapped around a third 23″ length. Bend the six leads down, where necessary, to reach the bottoms of the bottles. Make good mechanical connections and solder the joints well.
4m. Cut a 30″ lead of insulated stranded 12 AWG wire, strip the ends, and solder one of them to the dip electrode. Run the lead through a length of ¼” vinyl drip hose to add extra insulation, then solder an alligator clip to the loose end. The alligator clip will be used to adjust the primary coil tap point during the tuning stage.
5. Build the Primary Coil
JavaTC is useful for calculating the number of turns in the primary coil given a particular NST, secondary coil configuration, top-load geometry, and main tank capacitance. If you use an NST rated at 9,000kV/25mA, build the secondary coil and top-load as described above, and achieve a main tank capacitance of 0.005µF (+/– 0.0002µF), this primary should work nicely. If any of those factors differs significantly in your build, use JavaTC to check whether the primary coil design needs to be adjusted before proceeding.
5a. Use a large compass or trammel points to mark a 16″ circle on ¼” plywood or Masonite. Preserve the center mark; you will need it later. Use a jigsaw to cut out the 16″ disk.
5b. Cut 8 strips of ½” MDF measuring 1½ inches ×6-1/8 inches. These will be the combs that hold the flat spiral of wire that forms the primary coil.
5c. You will cut the wire slots on a table saw using a simple spacing jig. Build the jig from a piece of 1×4 lumber, and a suitable 1/8″ steel pin, super-glued in place. I used a decapitated roofing nail.
5d. To cut the first slot, clamp the jig onto your saw’s miter gauge or panel sled. Mount a saw blade of about 1/8″ kerf and set it to cut 3/8″ deep. Set the MDF comb blank against the jig, covering the slot, with its end against the pin. Activate the saw and cut the first slot in the comb.
5e. To cut subsequent slots, simply advance the comb along the jig, index the pin in the preceding slot, and make the cut. Proceed in this manner, cutting 15 slots in each comb.
5f. Cut 8 strips of 1/8″ Masonite or birch plywood 3½”×½” to make the strike ring supports. Drill a 1/8″ hole near the end of each one to pass a zip tie.
5g. Affix a strike ring support to one end of each comb with wood glue.
5h. Give the combs a coat of urethane varnish.
TIP: If you live in a humid area you may want to dry the combs in an oven for ½ hour at 250°F first. Apply the urethane while they are still warm.
5i. Use a pencil and ruler to divide your disk into 8 equal “pie slices.” The angles don’t have to be exact; eyeball-accurate is fine. Mark two screw holes on each line, at 3¾” and 6″ out from the center.
5j. Center-punch and drill 7/64″ holes at your 16 marks, and countersink them from one side so that a #4 wood screw sits flush.
5k. Cut a 2-3/8″ diameter hole in the center of the disk using a hole saw or jigsaw.
5l. After they have thoroughly dried, mount each comb on the disk with two #4 brass wood screws. To prevent splitting, drill a 1/16″ pilot hole for each screw before tightening it into the MDF. Your coil form is complete.
NOTE: I arranged my combs in a very shallow spiral, with each comb 1/16″ farther out from center than the one before. This is not necessary for the coil to function, but it makes the coil spacing a little neater.
CAUTION: The comb screws should be solid brass, not plated. In large, high-power coils, steel screws can get hot enough to burn loose from the primary coil form. This is called inductive heating.
5m. To wind the primary coil, you need about 60′ of 12 AWG insulated solid copper wire. First smooth out any kinks by fastening one end to a secure object (like a trailer hitch or signpost), wrapping the wire once around a wooden dowel at least 1″ in diameter, and pulling through the whole length, letting the wire slip around the dowel as you go.
5n. Drill a ¼” hole in the plywood disk near the first comb slot (at the center of the spiral).
5o. Pass about 12″ of wire through to the underside of the coil form, then start laying wire into the slots in the combs, forming a smooth spiral. Continue until there are four unwound turns remaining.
5p. Go back and seat the wire you have already laid firmly at the bottom of each slot, and then apply a drop of super glue to keep it in place.
5q. Wind the last four turns and mark them between each pair of combs as shown here. You will strip the insulation from short sections of the wire, and it is important that they be staggered to prevent accidental shorts between adjacent turns.
5r. Remove the outer four turns of wire from the form and use wire strippers to cut the insulation where marked. Put the wire back in the slots, and use a utility knife to slit and remove the small sections of insulation between the cuts. Seat and secure with super glue as before.
5s. Arcing between the top-load and the primary coil is potentially dangerous, and bad for the coil. Our design includes a strike ring mounted above the primary to shunt these wayward arcs directly to ground, like a lightning rod. Cut a single loop of six AWG bare solid copper wire and attach it to the strike ring supports with small zip ties through the holes. You can also use copper refrigeration tubing.
NOTE: Make sure to leave a ~1″ gap in the strike ring. If the two ends touch, the coil will not function.
5t. Slip a short section of vinyl drip hose over the strike ring, positioning it to cover the gap and support the free ends of the ring. Use a heavy-duty soldering iron or propane torch to solder a 6′ length of insulated stranded wire, 18 AWG or heavier, to an arbitrary point on the ring. Run the wire down one of the supports and away from the coil for a later connection to RF ground.
6. Build the Spark Gap
“Coilers” have produced many designs for spark gaps over the decades. This is a simple, static “multi-gap” spark built from ½” copper pipe couplings and an acrylic or polycarbonate baseplate. It is designed to be adjustable between one and six gaps, depending on where you attach the leads. With the specified 9,000-volt NST, use four or five gaps. JavaTC tells us we want a gap of about 0.042″ between our couplings. CD-ROM plastic is about 0.043″ thick, so pieces cut from an old disc will make handy spacers.
6a. Cut a 3″×8″ plate from ¼” acrylic or polycarbonate sheet. Drill holes in the corners for later mounting to a wooden base. Use a brad-point bit to avoid cracking the plastic.
6b. Remove any sticky labels from the pipe couplings. Make holes in five couplings by clamping each one in a vice, marking a dimple with a center punch about 3/8″ from one end, and drilling a 3/16″ hole through one side.
6c. Use tinsnips to cut six spacers roughly 1″×1″ from an old CD-R.
6d. Run a piece of blue painter’s masking tape along one long edge of the plate to serve as a guide when gluing the pipe couplings. Clamp a block of scrap wood to one end of the plate, with its inside edge 1-3/8″ from the end.
6e. Apply a bead of Goop to one of the drilled couplings, opposite the hole, and affix it to the plate, against the wood block, with the hole facing up.
6f. Insert a CD-R spacer and glue one of the undrilled couplings to the plate beside the first. Be careful not to get any glue on the spacer. Repeat, using the second undrilled coupling.
6g. Glue the four remaining drilled couplings to the plate, holes pointing up, with spacers between. Clamp a second wood block against the last coupling to hold it all in place while the Goop sets overnight.
6h. The next day, unclamp the blocks and remove the spacers. Install a #6-32×3/8″ brass machine screw in each drilled coupling, from the inside. On the outside, add a split washer and a nut, and tighten firmly. You will add an additional nut when you connect a lug.
7. Build the Terry Filter
Tesla coils are hard on neon sign transformers. I myself have killed several over the years. Coil hobbyist Terry Fritz understood that the high frequencies and high voltages developed in a Tesla coil could cause arcing inside an NST and lead to the formation of “carbon tracks” which eventually ruin it. In the late 1990s, he published designs for filter network circuits to go between the NST and the rest of the Tesla coil circuit to prevent this harmful feedback.
The resistors and capacitors form an RC filter that attenuates high frequencies from the Tesla coil. The resistors bleed high voltage from the filter caps, and will also bleed the charge from the main tank capacitor if the NST should fail. The chained metal oxide varistors (MOVs) form a “surge suppressor” that shunts big voltage spikes (such as ones from the top-load arcing to the primary coil) straight to ground.
7a. Cut an 8″×10″ plate from ¼” acrylic sheet.
7b. Use a center punch to transfer the hole locations from the print to the acrylic plate.
7c. Drill the holes where indicated, using a 1/16″ bit for the caps, resistors, and varistors, and a 9/64″ bit for the screws.
TIP: Use brad-point bits to avoid cracking the plastic, and drill into a sacrificial piece of MDF or plywood to keep the bit from binding as it breaks through.
7d. Cut two 10″ rails of 1×2 lumber and attach the plate to them using four #6×1″ brass flat-head wood screws.
7e. Attach a 1″ corner brace at each end of the 2 large wire-wound resistors using a #6-32×½” brass machine screw, 2 brass #6-32 hex nuts, a #6 split washer, and a #6 flat washer to attach each brace.
7f. Assemble four safety gap terminals. Each terminal consists of a corner brace, a ¼-20×1¼” brass round-head machine screw, a ¼-20 brass cap nut, and two ¼-20 brass hex nuts.
7g. Assemble the two sides of the RC filter circuit from 14 pairs of resistors and capacitors. First, insert the leads of a capacitor through the holes in the baseplate. Then, bend the resistor’s leads, as needed, and insert it beside the capacitor. Now bend each resistor lead over, wrap it once around the adjacent capacitor lead and clip off the excess to connect the two components in parallel. Proceed to the next resistor-capacitor pair until you have finished a row of seven pairs. Repeat this step for the other side of the RC filter circuit.
7h. Now bend one lead of each capacitor over, wrap it once around the near lead of the adjacent capacitor, and trim the excess, connecting all seven RC pairs in series. Repeat this step for the other side of the RC filter circuit.
7i. Insert a chain of seven varistors beside the seven RC pairs. Bend one lead of each varistor over, wrap it once around the near lead of the adjacent varistor, and trim the excess, connecting all seven varistors in series. Bend the free leads of the first and last varistors over, wrap them once around the free leads of the first and last capacitors in the adjacent filter circuit, and trim the excess, connecting one varistor chain and one side of the RC filter circuit in parallel. Repeat this step for the other side of the circuit.
7j. Solder all connections securely. Leave the first and last capacitor leads on each side of the circuit intact, but cut away any remaining excess leads.
7k. Mount the two large, wire-wound resistors and the four safety gap terminals to the baseplate using #6-32×1″ brass machine screws and matching hex nuts.
7l. Referring to the schematic, photos, and assembly template, complete the Terry filter circuit using short jumpers made of 16 AWG insulated stranded wire and terminated with matching #6 ring-tongue lugs. Reinforce the insulation on the long NST ground jumper by running it through a length of vinyl drip hose.
NOTE: When you wire the screw terminals, add a #6 split washer between the lug rings and the hex nuts to keep the threads from vibrating loose.
7m. To connect the RC filter circuits to the wire-wound resistors, simply crimp lugs to the capacitor legs and bend them over to the resistor screw terminals. The filter-ground connections will require short jumpers soldered to the capacitor legs.
TIP: The Terry filter has six terminals for connecting other parts of the Tesla coil circuit: two NST power leads, the NST ground lead, the RF ground lead, and the two sides of the main spark gap. Assemble these terminals with the hex nuts on top (not underneath) of the baseplate.
7n. After assembling the Terry filter, set each safety gap to 1/8″ by adjusting the bolts and nuts, using a 1/8″ drill bit as a gauge. You will tune this distance later so that the gaps fire only if a high-voltage spike occurs.
8. Build a Momentary Power Switch
You do not want to be plugging and unplugging a cord to turn the coil on and off, so it is best to make a simple momentary switch. This design should be able to resist significant abuse.
8a. Cut a 25′ 3-wire extension cord about 5′ from the male end.
8b. Remove the knockouts at each end of a 1-gang household electrical box.
8c. Pass the cut ends of the cord into the electrical box through the knockouts. Install a Romex clamp on each end for strain relief.
8d. Strip 6″ of outer insulation from the cut ends of the extension cord, cut away any internal reinforcing cords, and strip the ends of the wires.
8e. Crimp ring-tongue lugs to the green ground wires, and attach both of them to the box’s grounding screw.
8f. Reconnect the white wires by crimping them together with an insulated butt-splice connector.
8g. Connect the black wires to the normally open lugs on an SPST pushbutton switch rated for 2A, 120V AC service. I recommend you use a safety key switch; that way you can pocket the key and no one can accidentally energize the coil.
8h. Drill a hole to fit the switch barrel in the center of a blank 1-gang electrical box cover. Mount the switch in the cover with its bundled hardware.
8i. Install the cover on the box with its bundled hardware.
9. Assemble the Coil
9a. Set up a non-metallic table in an area safe for operating the coil.
NOTE: Do not try to operate the coil on the floor, as reinforcing steel in concrete foundations may interfere with its operation.
9b. Terminate the NST’s factory leads with #6 ring-tongue lugs to fit the Terry filter’s terminal screws. Connect the NST “hot” outputs to the large wire-wound resistors. Connect the NST output ground to the Terry filter grounding terminal. Do not forget to add lock washers between the nuts and the lug rings.
9c. Super-glue 3 small spacer blocks cut from ¾” wood or MDF to the base of the secondary coil.
9d. Slip the primary coil down over the secondary coil and set it on the spacer blocks. Be careful not to scratch the secondary windings. Run the primary coil lead out through the space between the primary and secondary coil baseplates.
9e. Align the brass screw on top of the secondary coil with the T-nut in the top-load, and rotate the top-load to thread them together. Do not overtighten.
9f. Connect the remaining coil components using the shortest possible stranded leads. The wire used in the charging circuit and the secondary circuit can be narrower (no lighter than 18 AWG), but the wire in the tank circuit must be thicker (no lighter than 12 AWG) to handle the large impulse currents.
Use the shortest possible leads to minimize losses and reinforce wire insulation wherever possible by running leads through vinyl drip tubing, or, in the case of heavier gauges, a clear Tygon hose. Avoid contact even between insulated wires. Make good connections and terminate all leads with crimp-on fork or ring-tongue lugs.
10. Ground the Coil Safely
WARNING: Do not operate a Tesla coil without a proper ground. Do not use the ground connection on an electrical outlet. Instead, connect an independent “RF ground.”
10a. Run a length of insulated 10 AWG (or heavier) stranded copper wire from the Tesla coil to a metal water pipe that you know is buried in the ground, or to a dedicated ground electrode near your electrical service, and connect it securely.
10b. Gather the ground leads from the secondary coil, the strike ring, and the Terry filter. Add the ground lead to these three from your top-load strike point, and connect all four together using a large wire nut with the RF ground lead running to your water pipe or other ground electrode.
11. Tune the Coil
Once the coil is assembled and properly wired in a safe place, you can begin to tune. Theoretically speaking, the goal is to bring the resonance of the tank circuit to the same frequency as that of the secondary coil circuit. Practically speaking, the goal is to find the point on the primary coil to attach the alligator clip from the six-pack cap that gives the longest spark from the top-load. Tuning is potentially a very complex subject, but this methodical approach should yield good results.
11a. Make a capacitor discharge tool so you can safely short out the six-pack capacitor between experiments. It is just a 10″ length of 12 AWG or heavier solid copper wire mounted on a handle of PVC pipe. To discharge the cap, touch the wire to the dip electrode and the outer foil simultaneously. Make sure to keep your fingers clear!
11b. Tape a short bit of bare small-gauge wire to your top-load to act as a breakout point. Set up a tripod or other device to hold a grounded wire near the breakout point. This is your strike point.
11c. Hook up the spark gap so that only two gaps are in use.
11d. Connect the alligator clip lead from the six-pack capacitor to the outermost point on the primary coil.
11e. Set the ground wire on the tripod so that it’s about 10″ away from the breakout point.
11f. Double-check all the wiring and prepare to activate the coil. Plug the NST into your momentary switch and your momentary switch into mains power.
11g. Push the momentary switch to activate the coil for a few seconds. You should see arcs in the spark gap and hopefully see an arc from the top-load.
11h. Shut off the coil and disconnect the power cord.
11i. Discharge the six-pack capacitor with the discharge tool.
11j. If you did not see an arc from the top-load, move the ground wire on the tripod a bit closer to the breakout point. Repeat from Step 6.
11k. Move the alligator clip tap one turn inward on the primary coil. Move the ground wire on the tripod a bit further from the breakout point. Repeat from Step 6 until you have found the optimal turn of the primary coil for attaching the clip lead.
11l. Repeat Steps 6–11 to determine the optimal tap point on that turn of the coil for attaching the clip lead, moving the clip from one bare tap point to the next.
11m. Adjust the connections on the multi-spark to increase the number of active gaps by 1. Repeat Steps 6-9. When the gap stops firing, back up one position.
This coil has been designed for easy construction from common materials. There are a number of improvements that might be implemented to enhance performance.
» A more robust dip electrode can be made using glass bottles with screw-on plastic caps. The individual dip electrodes consist of a carriage bolt inserted head-down into each bottle with the threads protruding through a hole drilled in the plastic bottle cap. This is secured with nuts and washers. Individual dip electrodes can then be interconnected using jumpers terminated with ring-tongue lugs.
» An air-quenched multi-spark featuring a fan, blower, or suction system would help keep the spark gap clear of ionized gases that, otherwise, tend to accumulate and impede performance.
» A larger-diameter secondary coil would couple more efficiently with the magnetic field of the primary coil. Similarly, a conical primary coil would improve coupling as well as reduce flash-over to the secondary.
» A more powerful high-voltage supply transformer, whether an NST or otherwise, could of course be made into a more powerful coil.
» Commercial capacitors suitable for off-the-shelf use in a spark gap Tesla coil are rare and expensive. A multiple mini capacitor (MMC) is an excellent DIY alternative. An MMC consists of many smaller, high-quality, high-voltage, off-the-shelf capacitors wired in a series/parallel arrangement to reach the necessary tank capacitance. MMCs are more durable and perform better than bottle capacitors.
- JavaTC calculator — classictesla.com/java/javatc/javatc.html
- JavaTC output for the Six-Pack Tesla Coil — 6pack_javaTC.txt
- Terry filter layout template
- Primary coil CAD files: Primary_assm.dxf or primary_assm.stp, comb2d.dxf or comb.stp