My first year of high school I tried to build a functioning, high frequency Tesla coil entirely from scrap parts. This project is almost a cliche nowadays; thousands of dedicated hardware hackers have successfully created ominous and occasionally dangerous coils, and so-called “singing” Tesla coils are the new trend among hobbyists. But the project was one of my first earnest attempts to learn about something on my own and apply that knowledge to a non-scholastic project, and so I wanted to link to a few resources here that I found invaluable when I was first starting out:
The Powerlabs Tesla coil page. This is the most “professional” Tesla coil I have found that was built by a hobbyist. The craftsmanship is impeccable, from the precision of the secondary coil winding to the care with which the capacitor bank was assembled. The care is reflected in the results; I am very confident that this is one of the most efficient Tesla coils I’ve come across, as it appears to regularly generate 18-inch streamers despite its compact size
The trashy Tesla coil. I like this project because the author defiantly avoids using any store-bought components or parts, using piping and wiring entirely scavenged from his local rubbish yard. This site is also home to one of my favorite anecdotes from a hobbyist:
For some funky reason every time I switched on the power, the sprinkler system in the yard turned on. I’m not kidding here. The yard gets watered every time I fire it up.
Primary and Secondary Coil
The red, long coil in image at the top of this post is the secondary coil from my own Tesla coil, which took me about a week of winding 28 gauge enamel-coated wire over oven-baked PVC pipe. That the toroid is a doorknob is a good tip-off that the payload isn’t resonantly coupled. The pancake-spiral in the foreground is a remnant of my original primary coil design, which I based on tutorial found on this page.
I first realized how attainable a homemade Tesla coil would be when I saw just how simple it can be to make high-voltage capacitors at home in the form of Leyden jars, which can be made from a film canister or bottle and some aluminum foil. Using a CD jewel case and some foil, I’ve even made capacitors that can be charged from a CRT screen but which will produce 3-inch sparks upon discharge—although predicting the discharge rate and stability of Leyden jars against dielectric breakdown is almost an art when one is using plastics and glass with aluminum foil. The best page for an intro to Leyden jars and their uses can be found here.
Most Tesla coils use a step-up transformer even before the current reaches the primary circuit. This allows shielding of the electrical mains from sparks and shorts in the primary circuit, and it also allows one to get by using capacitors made from beer bottles, air gap discharges, etc. because a higher voltage primary circuit requires less finicky specifications (it would also be very difficult to use a spark gap to modulate the frequency if one was only using mains voltage). I originally ran my coil off of car batteries by using an electromagnetic buzzer and a pair of ignition coils in my primary circuit; however, if I were rebuilding it today I would instead use a neon sign transformer, which I believe offers much more reliable and safe performance despite running on mains power. Here’s a buying guide for NSTs for Tesla coils.
When I was in high school, I always found the spark gap to be the most mysterious component in the Tesla coil primary circuit. After all, the primary circuit is already an AC circuit, and it seems like forcing the current to regularly jump an air gap would induce significant power losses that would reduce the efficiency of the transformer. The latter point is correct, but it turns out that the spark gap is still worthwhile because the timescale of the AC cycles coming out of the HV transformer being used to drive the primary circuit is way too fast to effectively switch most Tesla coil designs, given the dimensions and couplings of the primary and secondary coils. The spark gap allows the capacitors to fully charge and discharge at a rate set by their time constants and the properties of the spark gap itself (since things like pointed electrodes can create corona discharge, reducing the effective dielectric constant of the air in the gap). As a result, the spark gap acts like a high-power switch at a low enough frequency to allow effective transfer of energy between the primary and secondary coils. A good description of the idea behind using a spark gap (instead of a high-power relay and integrated circuit or other solid-state switch) can be found here and here.