Tesla Coils: My notes and observations

I built my first coil

around 1986 in Phoenix, AZ. It consisted of a 12KV 50mA NST, a hand made capacitor made of glass and aluminum foil, sealed in parafin giving a capacitance of approximately 10pF, the spark gap was just AWG 10 wire, stripped on each end and threaded through 2 pieces of dowel with an aligator clip on the primary end enabling adjustment, a primary of AWG 10 wire wound vertically on a form of 8 dowels with the insulation stripped in places along the top few turns allowing tuning and a secondary coil of ~860 turns of 24 gauge enameled magnet wire wound on a 3" PVC form with an output "terminal" of a 2" trailer ball. This coil performed quite satisfactorily. It was quite capable of 18" arcs for long periods of time (several minutes at a stretch) with no detectable breakdown in components or performance.

The current design is to incorporate some more modern advances in materials and techniques. I resurrected the NST and the secondary, beginning on around 2/6/00. I began by manufacturing a new spark gap design lifted from Bert Hickman and consisting of a 4.5" ID PVC tube, 5" long, ringed with 12 pieces of copper pipe, 1" OD and 1" in length, 6 on either end. Each piece is bolted to the PVC with the end of the bolt sticking out to make a connection. This allows the total gap to be adjusted to nearly any width from 0.1" to 1.2", as the gap between each copper piece is ~ 0.1". There is a piece of 1" PVC with a cap on either end stuffed up the middle, acting as a damper for a forced air arrangement. I have on hand, but have not yet tried, a 4" fan to connect to the gap to force air through it for both cooling of the gaps and disrupting the sparks as recommended earlier by others.

Next, I built a new capacitor, again from a design by Richard Quick, consisting of 4 mil polyethylene plastic and aluminum flashing, 6" wide. I cut lengths 12" and formed a tab in each, with a hole for a mounting screw. The plates are wrapped in 8 layers of plastic, giving a dielectric of 64 mils between plates and 32 mils on top. There are 22 such plates in the cap. I measured the capacitance with a test bridge at work at 1 pF. Strange it should be so low, but whatever.

The primary is fashioned from 1/4" OD copper tubing, 40' long and wound on a 30 conical form placed 2" from the secondary and beginning about the same height as the bottom turn. This was made from 2-20' lengths joined end to end with a pressure fitting.

The whole shebang is assembled on a new framework of two levels. The top level contains the primary and secondary and the lower level holds the rest. Two holes are presently drilled near the base of the secondary, allowing the connection of the secondary and one end of the primary to the bottom tier. These are connected using a bus bar with screw attachment points. There will be another hole added later to accomodate the adjusting tap of the secondary.

I think it's interesting that all of the coils I've seen illustrated so far on the web are configured differently than mine. This includes the old one and the new one. While others connect the cap in series with the primary and the gaps parallel to the NST, I always saw it the other way around. It is part of my intent to explore this configuration and others upon completion of a working coil. I also intend to build a larger example, but more on that as the need arises.

Anyway, I completed the new configuration on 2/17/00. Upon powerup and a few adjustments to the gap, I found no noticeable output from the secondary. I concluded that the capacitor was not of a sufficient value to induce oscillations in either the primary or secondary, despite repeated tuning attempts. I decided to refurbish the heavy glass capacitor, and did so on 2/19/00. Unfortunately, the same results were obtained. It is now may plan to eliminate as many of the variables as possible, and start off fresh, as before. I'll use another 10 gauge helical primary, the old secondary, the newly refurbished cap and just try out the new gaps in an attempt to get the coil to fire first, then I'll introduce the new primary and finally the new cap.

That's pretty much where we stand at present, 2/21/00. The new primary wire will be purchased today, along with some hardware enabling a more positive connection to the capacitor and a means to squeeze the polyethylene cap a little tighter, (part of its problem, I believe).

2/23 The wind died down long enough to try the old configurations. No Joy. I still think it's the cap. Installed a new 10 gauge primary, 20 turns tapped at 10 to start. I'll be reducing that this weekend. I also have a new plan on a secondary winder. Very simple, very cheap. I'll also try the old spark gap method-just bare wire-since it worked immediately the first time. Grrrr......

7/12/00 I haven't added anything recently, but I did manage to get some fire from the old configuration somewhere around April, I think. I redid the glass cap, used the new gaps, retuned ad nauseum and managed to get some 2 inch purple sparks. Nothing like it used to be, or will be, but it was a start. My new assistant Sam was a little skeptical, shall we say of my methods, so for him I added a remote on/off switch.

I ran across a simple MMC configuration on the web. This seems to be a common item in the newer designs, so I thought I'd give it a try. The one I saw used 3 parallel strings of 9 0.047uF, 1.5KV caps in series, each paralleled with a 10 Megohm resistor for voltage compensation, giving a total capacity of 0.01 uF and a voltage rating of 13,500v. He was using a 10KV NST @50mA. My NST is 12KV at 50mA. I found some caps around I thought I could spare. I wasn't hopeful because they were only rated at 250V and 0.68uF. What the hell, though, I tried them anyway. I strung together 64 of the beasts, all in series, giving a capacity of about 0.015uF and a voltage raing of 16Kv. I first tried them without any c ompensation resistors, just for science, you understand. They lasted about 5 seconds. I could see some sparks around the ends at first, cycled power a few times and decided they needed comp resistors. I went looking for 10M resistors at a local bone yard, but only found 22M. Rewired the MMC, but still no luck. I got a nice fire going, though....Fortunately, it did no damage, since I was quite careful.

So, I got my buddy Bob checking into some different high voltage caps for me now. Hopefully, he'll remember today or tomorrow. (I looked and looked at the bone yard, but they didn't have any. :-((()

Oh, I also tried the new configuration of the gaps across the NST output and the cap in series with the TC primary. While this enables me to adjust a nice spark in the gaps without being hooked up to the TC, my cap blowing up made my results as per efficiency inconclusive. More on this later, as successes build.

8/21/00 It's alive!

But let's start where we left off. Bob brought in some seemingly beautiful caps. Glass bodies, oil filled and each rated for 5KV. Values were .01uF, .02 uF and .05uF. Arranged properly, they could yield a value of .01uF at 15KV. I paralleled 11m ohm resistors for bleeders. I also got the variac Bob gave me working. I set the variac down to 60%, wired up the caps, turned on the switch and poof! Up went 3 caps in flames! I could not believe it! After installing a few new caps and reducing the bleeder resistance across each cap to about 8megs, I fired it up again and POOF! 2 more caps went kerflooey. I left it alone for 3 weeks in frustration. I asked Bert Hickman about it. He said the glass caps are not well suited for this application because they can't withstand the inrush current or the frequencies applied by the oscillator circuit. Seems pretty crazy, but it makes sense, I guess.

In the interum, an article appeared in the Maricopa Communicator-the local weekly newspaper -about a man named Steve Elswick. Steve was the former president of the International Tesla Society, now bankrupt, and was living in Stanfield. Turns out he was invloved in the 13M coil project of Bill Wysock in Colorado Springs. This was a huge magnifier setup designed to simulate Tesla's coil experiments of 1899. I got a little excited, emailed him and we met on Sunday, August 5th, 2000. Let's just say it rejuvenated me. I decided to go the MMC route. I looked up Antique Electronic Supply in Tempe, AZ. They had a bunch of stuff that has Tesla written all over it! From old coil forms to new, polypropylene self-healing caps. These are the type I selected-"Orange Drop" brand, .005uF at 1600V each. I wired 10 in series, with 6.8M bleeders across each, three strings in parallel, giving 0.015uF at 16KV. I mounted it on a Lexan sheet about 8" x 10" and used 10 gauge wire at each end for connection.

We tried it first on Friday night at 50% power. The new gaps were used, along with the revamped 10 gauge, 20 turn primary, as well as the old NST and secondary, (12Kv 50mA, 17.75" x 3.5"=813.5', respectively). There were 10 turns in the primary, which was wound around an 8" sonotube form. (Ironic, no? (-:) Anyway, we got some small discharge at the secondary terminal, and the gaps sounded strong. I retuned down to a previous primary setting at 8 turns and the disharge got smaller, so I started back up. 12 turns improved the output to about 6" sparks. Jumping all the way to the top of the primary reduced output, so we were in the ball park. Optimum turned out to be 14 turns, yielding about 12" streamers.

Next came changing the configuration of the primary to the conical form. The form works great, but the 10 gauge wire wasn't long enough to get optimum tuning. Adding about 3 more feet improved this, but didn't get us back to before. I'm hoping to switch over to the 1/4" copper tubing primary tonight and give that a shot.

Rosie and I wound a new secondary this morning. On a 4.4" diameter PVC form we wound 24.5" of 24 gauge enameled magnet wire for a total of 1237 turns and 1425'. It's now drying after the 4th coat of polyurethane. As with the old secondary, there was a section of wire right off the spool that had some bad insulation. I did a double thickness coating of urethane, hopefully it will be enough to keep these turns from shorting or causing problems, as worked on the old one. I want to try it out maybe tomorrow.

That's it for now. Update after I change out the primary.



We've been on vacation and out of town since last Thursday, 8/24/00, but made some more progress before we left.

We took the coil and new secondary to my daughter's house for some demo's and trials. We played for a couple hours. After demonstrating the old coil, I installed the new secondary. I had also anticipated the need for a longer primary, as well as fashioning some new primary forms.

The forms were made from 3/4" particle board, (PB), cut 1' long and 4" wide, with notches cut along the length to accommodate individual turns of 10 gauge wire on one side and 1/4" copper tubing on the other. These forms are a test of concept in their present form and work reasonably well, although the PB is too crumbly to hold up to repeated winding. I plan to make permanent ones out of a harder wood, perhaps oak or plywood. These will be adjustable as to number of turns, proximity to secondary and configuration-either vertical or flat. Another form will allow two angles of conical primary, both inverted and normal, 45 and 60. The new primary was 100' of flashy red 10 gauge wire, of which I used 90'.

Anyway, the new secondary with the old primary performed amazingly well, giving nice 10" streamers, but obviously untuned with the old primary. After rewinding the primary and firing it up, we got HUGE streamers, well in excess of 16", a new personal record! These are the sparks that fly by themselves, not being drawn to a point or using a breakout point. This with the primary 6.5" from the secondary. I thought we could get better output with better coupling-moving the primary closer, so, I devised a plan to enable me to adjust the distance, and therefore the coupling, between prmary and secondary. This was a terriffic session!

I won't soon forget Robin's excitement at the resurrection of the coil. On the way home however, we ran into one of Arizona's notorious "monsoon" thunderstorms. It hit us with everything, including flooded streets for the last couple miles. I'd stopped along the way to put the smaller stuff in the truck-the cap, my tools etc, but the secondary, primary, table, NST and everything else was exposed. It sounds worse than it was, though. I was really only concerned about the primary forms, although I knew I was gonna redo them anyway. I tried it briefly the next day and got some sparks, but nothing huge. At least it worked.

One thing that concerned me during this run was the coil seemed to be shutting itself off after some time. I couldn't run it for more than a minute or so. I attribute this to current limiting in the NST, and perhaps a lack of tuning. The coil wasn't tuned well, runs inefficiently, draws too much current from the NST and off it goes.

Another thing I noticed, and forgot to mention, was I added a fan to the spark gap to squelch the spark faster. Current theory suggests this causes the frequency to be higher. It makes sense, and I'll be researching this more. Be that as it may, it seemed to me that the output rose slowly as the fan spun up. This also makes sense if current theory holds. As the spark squelches faster, the frequency rises in both the windings and up goes the voltage as a result.

After the demo at Robin's house, I left the coil for a week while we vacationed. Upon our return, I removed the primary and forms and redid them. I cut down the forms to be 2.5" wide and used only the ones set for 1/4" tubing. (The 1/8" cuts used before for the 10 gauge were too narrow and broke off easily.) Placing the notches 3.5" from the secondary, I rewound the primary placing 2 turns in each notch for a total of 26 turns. I had to install blocks between the top of the forms and the secondary because the forms aren't that rigid and bent towards the secondary as I wound. Oops.

Once completed and tuned all the way up, nice 12" sparks were evident. Tuning down half way showed less activity. Back up to about 18 turns proved good for now and that's where it was when I took these pictures.

Things are looking very good for this project. I just got some more caps and plan to make another MMC. the idea being to install it in the other leg of the primary circuit for testing and also for flexibility in various other configuration testing purposes.

Stay "tuned"! ;-)

9/14/00 Did a bunch of tweaking on 9/9/00. The main idea being to check some different configurations-cap before gap, 1 cap in each leg, and various stuff like that. Results were mixed, but one glaring point was, if 2 caps are installed, either in series or one in each leg, the primary has to be much longer for smooth operation. Thus, all the multiple cap configurations are suspended until I get a longer secondary. This may have to wait a while, but we had lots of fun with the coil, placing basically 2 different top loads on it, along with other objects and watching the sparks fly. I also installed the alligator clips to all connections to ease changes. Just clip, clip, clip and it's a new configuration. Works great! Gotta make several different lengths of jumpers, though. It got a little awkward at times.

One thing became plain during this session, too. I really need to decrease the coupling of the primary to the secondary. Under certain load conditions, there is a large corona present around the top turn of the primary that curves upward and in toward the secondary. While a beautiful purple color, it's wasted energy and sometimes causes arcing. Dangerous. This became clear on the latest session last night, too.

Last night, 9/13/00, I tried a tuning scheme suggested by Bob, mostly because of a conversation we had on Monday. I told him of a plan I had to indirectly measure the output voltage and especially frequency of the coil by connecting an antenna across my oscilloscope. He said I should use a tunable tank circuit instead. I pondered this and decided to try the old secondary coil as an antenna. To my suprise and delight, it worked amazingly well, relaying over 500 volts across it and showing a frequency of 500KHz! This, from across the room, 10' away! It showed nicely the classic damped ring of the gaps firing, as well as the instability of the gaps. This led directly to an adjustment of the gaps-a "calibration" actually. Adjustment notches in the plastic were enlarged to enable better, more uniform adjustment, and all gaps were set to [the thickness of a drywall saw I have], about 1/16". The coil was then retuned with the new total gap being ~9/16". I plan to widen each gap to enable both sets to operate independently. Anyway, Bob's idea is to tune the old secondary to the frequency of the new coil. This would allow the maximum energy transfer between the two, and create a transmitter/receiver pair. A simple circuit for detecting the output would enable me to investigate power output and range of the "transmitter". Maybe a signal strength meter or headphones to actually listen to the "broadcast". Leave it to a radio guy to put this bug in my head! :-) It could be a whole 'nother area of research.

Where was I? Oh yeah, Gaps. After reinstalling the newly "calibrated" gaps, retuning and monitoring showed the waveform to be MUCH more stable. It also increased the voltage present to just under 1000 volts! Before, the firings were irregular in both frequency and amplitude. The coil sounded scratchy and intermittent. Now, the waveforms are still intermittent in frequency, but largely the same amplitude and the coil sounds fairly smooth, and a LOT stronger. After tuning the primary, the streamers are over 24" long and all very robust. You used to be able to detect the less virulent ones as being shorter, but now they are all pretty long.

I played around with gaps and primary tuning for a while, then decided to add a cap in parallel, just to see. I had to retune the primary down several turns, but WOW what a difference! The output jumped amazingly high, but caused severe corona and arcing from the primary to the secondary. so I had to shut it down for the evening before I could get a measurement, but I'd estimate somewhere in the neighborhood of 1200 V. That could put the output in the neighborhood of 250,000 V! Today, it's time to toy with the primary a bit. I'll try the flat spiral configuration, along with widening the vertical coupling. Maybe I'll take some more comprehensive notes on different configurations, too. I'll let ya know.

10/13/00 Friday the thirteenth seems as good a day as any to update things. :-)

Several developements have taken place. First, an output terminal made of 4" aluminum ductwork has been successfully incorporated by increasing the capacitance on the oscillator circuit and retuning the primary. The coil fairly sings with this load installed, giving streamers well over 2' long. Unfortunately, the load puts a very high strain on the secondary, and I experienced burn through between turns at 4 different points along the secondary. Bob says it's due to an inferior insulation of the wire itself-single coating enamel rather than double. Rats. I was afraid of that. Twenty-six gauge teflon has been suggested, or double coated enamel or that old type hookup wire we used to use here. There's a roll of it available at Apache Reclamation, but finances prohibit its purchase, so far. Teflon has been ordered covertly. Anti-corona compound has also been oredered. Four week lead time from last week.

Other developments include purchase of a large drill press, (Happy Birthday and Thanks, Rosie! You're the greatest!), and subsequent completion of the adjustability of the primary coupling. The above mentioned burn through occurred because of too close coupling, resulting in repeated arcing from the very end of the primary to the secondary. Since the primary ended behind the coil from our perspective, the arcing wasn't detected right away, and much damage was done before discovery. An attempt to seal the holes with several coats of epoxy paint was marginally succesful, but quite ugly in appearance. A new secondary will have to be wound. Coupling will be reduced this weekend in an attempt to reduce corona from the top of the primary. Hopefully the new "goop" will suppress this too, as well as insulating the inevitable splice on the secondary.

The banked MMC's have performed absolutely flawlessly and continue to delight. Changing capacitance has a profound effect on the output and fine tweaks are now possible. "Patch panel" type jumpers have been incorporated between rows of caps, enabling very fine tuning of total capacitance. Each leg representing approximately 600 pF. This, along with the improved gaps and the adjustable coupling should provide hours of fun fine tuning. Where's that new wire at?! ;-)

I'm a little reluctant to fire it up at full power with the secondary burnt the way it is. It works great with no load, but it's a lot more fun to watch with that nearly single, heavy streamer.

Another experiment that needs to be done is comparing different grounding schemes. I currently ;-) have the bottom of the primary connected to the bottom of the secondary and one side of the NST. I want to drive a long piece of rebar into the ground and tie the secondary to that. Wire braid is also on the list of needs. This stuff isn't free, so it'll have to wait, too, but AR has tons in stock.

That's it for now. Hopefully, I can add some more pictures at full load before we wind the new secondary.

11/13/00 I managed to wind the new secondary last weekend. The form is the same size as the last secondary, but I could only get 891' of #26 wire because the Teflon insulation is thicker. I also "discovered" something that should have been obvious. After winding the coil, I attempted to seal it with a layer of polyurethane as always. I painted it on and went to do other things while it dried. Came back 15 minutes later and the urethane had run down the sides and was dripping off the form-Polyurethane does not stick to Teflon! Well, DUH! Anyway, the strain relief at the ends of the coil are improved, so hopefully it'll be ok anyway.

We fired it up and after retuning found several things. First, the output was quite substantial, but not the greatest. Since there was no evidence of corona around either the primary or secondary, I decided to increase the coupling. When moved all the way in, (vertical spiral 5" from seconday), the output set records, with few turns on the primary- perhaps 10 turns. While I like the output, I didn't like the sound very much. It seemed to be straining quite a bit.
One thing I should mention, procedure-wise; Anytime I make any radical changes, like a new secondary, gaps or cap, I start from ground zero as far as tuning goes. Gaps are reset to just barely fire, then back one, one MMC is connected, the primary is tapped all the way at the end and there is no output load, meaning just the wire sticking straight up. This configuration is tweaked, starting with the primary tap point until a strong discharge is achieved at the output. A load is attached and the coil is retuned until max ouput is achieved. The overall performance is analyzed and recommendations are entertained for improvements, such as adding more capacitance or increasing/decreasing coupling etc. This process is repeated until the maximum output for the configuration is seen.

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Revised: 10/13/00