The driver is composed of the primary and secondary coils. This is not a resonant system! This is not a Tesla coil. This is a low impedence, tightly coupled, oscillation transformer, fully obeying simple transformer action whose output frequency is determined by the primary tune and secondary frequency splitting. Nothing is resonating here as quarter of half wave line!
Since the output voltage of the secondary is determined by the simple turns ratio and coupling factor, we want the highest input voltage to the primary as possible. The critical requirement of this driver is that it has the lowest possible impedence with reference to ground. Having said this, it can be seen that two critical requirements must be met.
The first is the very finest ground connection must be secured! The casual coiler cannot begin to appreciate this statement. Tesla once said that the bulk of the expense for the Wardenclyffe tower was in the incredibly deep and labor intensive grounding system he required in order "to secure a firm grip upon the earth".
The second important part of the foregoing involves using wire with a large surface area for the socondary winding. This reduces the internal RF losses of the secondary and further lowers the output impedence.
Generalizations are overabundant in the Tesla coil community but I am forced by space to say that the inductance of the secondary should be low to moderate. We have found that 4-10 millihenries works well for the medium sized systems.
Needless to say from the above discussion, the resonant frequency is not critical, as nothing resonates. If one must characterize things, the quarter wave resonant frequency of the driver secondary should be at least two to four times higher than the resonant frequency of the third or extra coil which is the true output device of the system. The primary is built along more conventional lines. Heavy walled copper tubing is the best material here. We prefer moderate to high inductances between 30 and 70 microhenries. This allows the use of more economical capacitances.
One thing we have always found amazing is the tendancy of many builders to place large amounts of capacitance in the primary circuit! In some cases we have seen as much as 0.2 microfarads of capacitance was used on a 6 kilowatt Tesla coil which produced 6 to 7 fett of arc. It is amusing to remember that Tesla was able to get fantastic results from a huge 50 foot diameter system that used no more than 0.153 microfarads of capacitance. It just shows how far out of balance some systems can be.
When we hear of a builder that uses more than 0.1 uF of capacitance, we wonder about the builder. Our 10Kw Nemesis used only 0.09 uF of capacitance and produced straight line, point to point arcs of 14-15 feet. We believe this system could have produced longer arcs, but we were space limited. Large capacitances are best used only with low voltage input systems where increased energy is needed. The magnifier normally uses very high input voltages and can utilize a smaller capacitance.
The coupling between the driver primary and the secondary is crucial to the operation of the system. It is the main distinguishing feature between a magnifier and a normal, classic two coil system. This is where all of the power handling capability of the magnifier is derived. Most Tesla normally operate between K=0.05-0.15. The magnifier requires couplings over 4 times greater (K=0.4-0.6). For simplification, the closer the primary is placed to the secondary, the greater the coupling. Some of our work of late has been in investigating the use of various possible flux concentrations to assist in this area.
Finally, there is the question of insulation between the primary and secondary. With coupling factors of K=0.4-0.6, the proximity of the primary to secondary will break down air easily. We use large 90 mil thick sheets of polyethylene in multiple layers to suppress the arcs and corona. We are currently investigating oil immersion of the driver system to eliminate many or the driver insulation problems. One arc from primary to secondary will destroy the driver instantly. The current intensity of one of these arcs must be seen to be believed.
Tesla damaged the dynamos of the Colorado power plant, but not while running his magnifier and overloading the generators as so many of you have been led to believe. He overloaded nothing! He had disconnected the extra coil and was arcing the secondary driver output to ground when the short waves created by this terrifying process traveled back to the dynamos and faulted their insulation! Once the dynamos insulation broke down, they power-arced and destroyed themselves. Where did we hear this new stuff? From Tesla's own pen! It is clearly and fully delineated in the Colorado Springs Notes.
We have run extensive tests on materials and wood, paper, cardboard, phenolic tubing and other such materials must be avoided at all costs. The best materials are modern thermoplastics. The best of these being Teflon, polypropylene, polyethylene, styrene and PVC. Don't use granpa's pretty stuff. Use what the intervening 100 years has given you to better the system. Tesla would have!!!
Tesla gives us an important hint as to the inductance of the extra coil. He says that the extra coils momentum, (inductance), must preponderate over that of the secondary. This makes sense too. We are striking a bell, (the resonator) with a clapper, (the driver). A 12 ounce silver bell would be ill served by a one ton clapper. The inductance of the resonator should be at least three times that of the secondary. We don't want the driver overpowering and swamping the resonator.
In engineering terms we want to match the impedence of the driver to the base of the resonator. For the calculating crowd, we refer you to the Corum's excellent PC program, TC Tutor. My task here is to put you on the right path without tangling you up in your underwear. A bit of experiment will verify what Tesla, the Corums and I say. The pictures included here will serve as proof enough for most.
The resonator must be viewed as an isolated system! This includes its terminal capacitance which has the effect of storing energy, electrostaticaly shielding the top of the coil and lowering the base impedence of the resonator. This allows for increased base current which translates directly into more output voltage and energy. Odd, that is just what we are looking for! The purity of a free ringing, unfettered resonator cannot be appreciated by the casual coiler until viewed first hand.
We prefer to use a wire gauge equal to or just smaller than that contained in the driver's secondary coil. The resonator's final resonant frequency is more determined by the terminal capacitance than by the wire winding in our machines. We have settled on a ratio of less than 2:1 for the extra coil's L:D ratio. Our best systems are often 1.5:1. We have achieved sparks of 7 times the resonator winding length which Tesla never seemed to achieve, according to his notes. We attribute this to modern materials and more efficient gap systems than he had available.
Plus, we have the benefit of modern theoretical work of the Corums and well developed transmission line theory. In all of our work, we have noted that the largest toroid possible always tends to extend the results of the system. The terminal capacitance should dwarf the resonator if spark is desired.
The principal magnifier developers in our group get together often for brainstorming sessions. Such sessions are very fruitful and separate from our monthly meetings. We have decided on a number of modular approaches to magnifier construction. The key to our success is the construction of numerous magnifier systems of vastlt different characteristics. Rather than build just one system and refine it forever, we tend to build a system, refine it as much as possible and determine its limitations. We then tear it down and construct another system using knowledge based on the experiences acquired from the previous system. We number each system and currently are working on magnifier #9.
The power levels utilized range from 300 watts to 10,000 watts. We have never achieved less than 3 times the resonator winding length in spark and have recently exceeded 7 times that length as the system efficiencies increase. We share our information openly with others via our video report tapes and educational series tapes14. We have published information in The Tesla Coil Builders Association News, Electric Spacecraft Journal and R & D Innovator. We have met some of the most wonderful people at our yearly Teslathons in Virginia.
It is important that good information see the widest possible dissemination. There are just too many false starts based on untried information or pie-in-the-sky theoretical approaches. We have sought to document our work at every step of the process via video, still photography and the written word. We have over 450 still photographic images and offer over 100 hours of video tape reports as proof of our work based on real experimental efforts.
The limits of the system hold to Telsa's original verbiage, that the system limits are so remote that they are imposed only by the limits of the components and materials to tolerate the strain. This is where our efficiencies far exceed Tesla's. Modern materials and techniques allow almost a full order of magnitude reduction of the Colorado Springs machine. The input power handling capacity per unit volume has increased dramatically and there is a wide open field for the Tesla coil builder or experimentor.
We tend to use mathematics as a tool rather than a crutch. It was once said when equating mathematics to a lamp post, that a good scientist will use it for illumination. A bad scientist, like a drunkard, will tend to use it for support. We have made many mistakes and our failures outnumber our successes. We have worked from false assumptions and probably will continue to stumble now and then. We are, however, confident that ultimately we will understand as Tesla did, the intricacies of the magnifier system.
We are still aggressively pursuing magnifier research and, like Tesla, have completely abandoned the two coil system. We have added many new insights for construction and shared them with the Tesla coil building community. I hope we have also opened new areas for amateurs and professionals to assist in the discovery of new and innovative techniques.__ RH
1:Nikola Tesla, Colorado Springs Notes-1899-1900, Nolit, 1978
2:Tesla-Scherff correspondences, 1899-1939, Tesla Museum, Belgrade
3:Nikola Tesla On His Work With Alternating Currents, Anderson, Sun Publishing, 1992
4:The Tesla Coil Builders Guide to the Colorado Springs Notes of Nikola Tesla, Hull, Twenty-First Century Books, 1994
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