This project was inspired by Stefan’s mini-TC project (named Vitamini. Check under the Links section for his web page). It was to be my first attempt at a bipolar half wave Tesla Coil system but I later changed it into a simple quarter wave Tesla Coil. The objective remained the same: To produce a small, good looking, portable Tesla Coil that can be taken and set up anywhere in as little time as possible, and still provide an impressive display (provided by it’s relative high power).
Given that it will be a demonstration model, it also needs to be safe, hence the primary transformer was specifically chosen to be of very low amperage output, so as to make it less likely to cause heart fibrillation, and the major H.V. parts (tank capacitor, spark gap) were to be mounted inside a box, so as to stop people from coming into contact with them. Other safety-specific design details are the insulated primary, and the secondary which is not connected to the primary. Now onto it’s components:
The coil form chosen was a polypropylene (PP) tube. PP was chosen because of it’s low RF dissipation factor (so low, in fact, that most HV capacitors use polypropylene as their dielectric. PP is only beaten by PE as a plastic dielectric, and it remains a good dielectric over a greater temperature range), good mechanical properties (although PE performs even better in RF circuits, it is not rigid enough for a coil to be wound around it), and low water absorption per weight (unlike PVC, a PP form does not need to be dried before winding). Overall, PP is probably the best material choice for a Tesla Coil form. The specific tube chosen was a “Tangifor C” vitamin tube, because, as mentioned above, it was made of PP, and it was thin enough, and of a suitable size (note: this is a 16-pill tube, hence it is longer than the 10-pill tubes vitamin C normally comes in). To your right, you can see the secondary materials: Two Tangifor C tubes, and a 1.5-kilogram spool of 36AWG wire (0.127mm diameter!!!). Note: 1.5kG is enough to wind almost a thousand coils (70kM), however, it was the smallest spool they had… Before winding the secondary, it is vital to thoroughly clean the coil form. In this case, the paint residues had to be sanded off with fine (#250) grit sandpaper, as most paints contain conductive compounds (metal or carbon compounds being the most common ones).Than, each coil form is thoroughly varnished.. This doesn’t really matter much on a PP form, but I like to do it just the same and it makes it smoother to wind after the sanding.
Finally, the coil form is wound… I always wind my coils by hand, simply because I do not have access to a lathe. This particular coil took 6 and a half hours of exhaustive winding… Three interesting things to note on this picture: An ignition coil (blue), a 7-Cd changer stereo, and the tail of my F-15 model plane.
Fortunately, after much work and several retries (AWG 36 wire has a tensile strength of 250 grams, so it breaks all the time), the secondary was completed… Here you can see it prior to varnishing, being held between my thumb and first finger. It also gives you an idea of its minute dimensions.
And here it is… The recently wound and varnished secondary masterpiece dries outside the window on a sunny day… With an improved winding technique, I managed to wind this coil (by hand) in 2 and a half hours.
Specs: The secondary coil form has 2.8cm diameter, and is 13.5cm tall. Actual winding height is 12cm, giving it 970 turns and a aspect ratio of 1:4,3. This is actually low for such a small coil but I couldn’t get a better coil form.
A 12-turn 15 degrees inverse conical coil made from 10AWG (5mm diameter) multi stranded copper wire, with 6mm inter turn spacing (to prevent flashover) and 1cm pri-sec spacing (so as to keep the coupling within workable values). The coil form is acrylic (from a microwave oven door!), which is a real pain to work with. I used a 30,000RPM angle grinder attachment on a dremel moto tool to build it in under 4 hours 🙂 Everything is held together with cyanocrylate glue and epoxy. Looks good too…
Here you see a 110 – 220V in 7.5Kv, 30mA out Neon Sign Transformer (NST). I bought it new for $60 at a transformer shop. I plan on using it to power my twin coil, as well as perhaps other (future) coils… It is not a very powerful unit, but it is small and portable, and yet delivers 225watts, which should be enough to produce sparks in the 30cm (1′) range… Along with this transformer I also got 10 meters of thick, double insulated neon sign cable: A must for any serious HV tinkerer 🙂
A 10-segment vacuum quenched series static gap. Each section is a 2cm diameter 8cm long copper pipe. The total gap is 5mm, which, given the electrode geometry, is equivalent to about 8Kilovolts. This is done so I can make use or resonant charging in the transformer – capacitor system to boost the output voltage. The copper pipes are held by steel bolts with round heads and nuts going right through the pipes and pressing them against the sides of the box. Steel washers are used (4 per pipe) to distribute the heat more evenly. Cooled by its two high speed CPU fans, this gap has run for over 15 minutes and remained COLD. I believe that at the power level I am running now it will never overheat.
A capacitor bank made from 4 TDK UHVï¿½12A 173K strontium titanate doorknob-type pulse capacitors (removed from a nitrogen laser). Each capacitor is rated at 50kV and 1.7nF. Total capacitance is therefore 0.0068uF. This is a bit on the low side for my transformer (it resonates at 60Hz with 0.011uF capacitance), so I’ll be adding a second capacitor bank as soon as I manage to find them. I am also ran some tests with low density virgin-grade polyethylene flat plate capacitors and obtained promising results.
The capacitors here are all interconnected with 3mm thick, 15mm wide, 23mm long pure copper buss bars, and the whole bank measures 26X6X5cm.
A 3 1/4″ rubber model aero plane wheel covered with aluminum tape… Should be a bit larger, but until I find something more appropriate it’ll just have to do.
Well, not quite… But here is a test run snapshot: The bright white light comes from the spark gap. If you look carefully at the secondary (under the purple sparks) you can see that it is GLOWING. This is caused by corona breakdown. A small spark (arrow) can also be seen running down the secondary. This spark melted the insulation over the wires and ruined the coil… So I’ll be working on a larger system from now on. It now seems apparent that this very small system will not handle the high voltages it is developing, and therefore I will be forced to upgrade into a larger system. Here is a snapshot of what the NEW system will look like:
The winding height is now doubled (25cm) and the system stands 40cm tall, with its and 4.2 (1.68)cm diameter secondary. The primary remains the same size, but the primary conductor is increased in thickness. The primary coil form is now made of polypropylene, for it’s reduced brittleness (the last coil form cracked) and RF losses and the new secondary is acrylic (not ideal, but certainly better than my next option: PVC). Still tabletop, but it puts out half a meter long sparks! Click on the picture to be taken to its official homepage.