This post has been sitting as a draft until I got around taking pictures. The actual coil was completed in March of 2013.
In light of my participation in National History Day, I took the opportunity to have the topic of Nikola Tesla’s AC Power Grid for this year’s theme (turning points in history). Tesla is also the conjurer of lightning by way of his Tesla coil, something I’ve wanted to build for a few years now and had designs drawn up, the math figured out all in my head just waiting for the time (and funds) do start building. Early 2013 presented itself nicely.
- 12kv 60ma Franceformer neon sign transformers for the HV source. I bought two of them via Craigslist, matching, and put them in parallel to get extra current and spark length (120ma @ 12000v = 1440 watts = 60″ of theoretical spark length).
- 26kv 26.25nf MMC capacitor bank. Composed 6.5kv 0.015uf polypropylene caps I (bought from Alltronics). 28 caps arranged into series strings of 4, 7 strings in parallel. Surplus made this a relatively cheap solution! However, this is close to the minimum capacitor value I calculated. Additional parallel strings will be added in the future to increase power. Each capacitor has a 1M 1/2 watt resistor to bleed the current quickly after a run. THe whole assembly is loosely shoved into a schedule 20 PVC tube, hot glued at the ends, and then zip-tied to the base.
- 7mm vacuum-qenched spark gap. Copper pipes arranged across from each other, plumbed with PVC to a vacuum to suck air through them to reduce heating. The cables are bolted directly to the pipe fittings.
- Primary coil. 1/4″ copper tubing, 12 turns spaced 1/4″ apart. This is about 40 feet of copper refrigeration tubing on 1/2″ tall HDPE mounts. The mounts were constructed by drilling 1/4″ holes in the center of 1″ wide strips, then cutting down the center of the holes – yielding 1/2″ square bars with semicircle indentations for the tubing to rest into. 192 zip ties secure all the tubing into a tight spiral.
- Strike Rail. 2″ above the primary resides an earth-grounded loop of copper tubing to prevent arcs to the primary and blow the NSTs. Not a continuous loop to avoid efficiency loss via induction.
- Secondary. ~1800 feet of 23 gauge magnet wire. 900 turns around 6″ PVC drain pipe. This has an outside diameter of 6 7/16″ and a wind length of 28″. the ends are about 8 inches in lead, wrapped in 4″ Kapton tape to prevent unwinding.
- Top load. 6″ dryer duct around 12″ pie pans, 24″ in diameter and the whole thing wrapped over with a few layers of aluminum tape to make it super smooth and hold the most charge. A single breakout point made of a thumbtack allows for long arcs. Fastened to the secondary with a bolt in the center. This will likely be the first upgrade to increase the spark length.
- Secondary mount. A 6″ endcap is held below the primary MDF platform with wood blocks. This allows for adjustable K-factor, or how tightly coupled the primary and secondary are by using spacers in the bottom of the cap. A bolt in the center provides a tap point.
- Wiring and control box. All wiring is 10 gauge jumper cables with appropriately size crimp rings. The 10-gauge extension cord leads back to a metal box with two 20 amp breakers (only one used)
Performance: About 4-5 feet of spark! The coil is super loud to the disappointment of my neighbors and you can really smell the ozone. It’s able to light fluorescent tubes wirelessly from about 15 feet away.