TCBA Volume 12 - Issue 1
Page 17 of 18
Input - Output
Q. A book I am reading states that self-capacitance in a Tesla coil (secondary) should be kept as low as possible. However, it does not provide any information on how this can be accomplished?
A. The first thought that comes to mind is the practice of spacing the windings of the secondary inductor. This can be done in two ways: (1) Select wire with an insulation thick enough to keep the turns of the conductor distanced by the diameter of the conductor wire. That is, if the diameter of the actual conductor is 1 mm then the thickness of the insulation should be at least 1/2 mm. (2) Wind the turns in parallel with a non-conducting material of the same outside diameter of the wire (fishline, etc.). The material used to keep the turns separated may be left on the coil form or removed once the turns have been anchored with an insulating liquid (shellac, etc.)
Another practice is to wind the turns closely but in sections. After so many turns have been applied, skip about 1/8" and continue to wind another section. Continue winding in this manner but widen the distance between sections by 1/8" or more. If the first and second sections are spaced, say, by 1/8", then the second and third section would be spaced by 2/8" (1/4"). The third and fourth sections would be spaced by 3/8" and so on.
Perhaps the best way to reduce, if not eliminate, the effects of self-capacitance in the secondary coil is to wind it in a flat spiral. This requires the construction to be a bit more complex than when winding a cylindrical form. The flat wound Tesla coil can be a very interesting project (see Tesla's patent #645,576). Incidentally, it is better if this type of construction is mounted in a vertical position (90 degrees from the floor) especially if the floor is of concrete. When parallel to the floor, there may exist a capacitive coupling between the coil and the floor surface (as though they become two plates of a capacitor).
I have not found self-capacitance to be a problem. My preference to use methods 1 & 2 (paragraph one) is due to a desire to increase the distance between the primary and the upper high potential portion of the coil. This may be necessary where projects of high power are concerned.
Q. Has any work been done in the area of eliminating the spark gap? Such devices might include thyratrons, ignitrons, magnetic blow-out coils, etc.?
A. You may find it hard to believe but there is much work being done on improving the spark gap. Certainly, there are products that can be used to replace the spark system. The trouble is that the items you mention don't come cheap. For example, the Hughes Research Laboratories will sell you a “Crossatron Modulator Switch” for $4000 (about one-tenth the original price). The CMS is a fast closing and opening switch combining the best features of the thyratron and hard-vacuum thermionic switches. It would be interesting if some Tesla coil enthusiast (with plenty of dough) would test one of these devices.
Q. What is the best material to use for a coil form: Bakelite, Lucite, cardboard, Sonotube?
A. Any insulating form free of moisture and properly sealed will work. Thin wall forms are less lossy than those of heavy thick construction. When selecting a coil form, I tend to follow my own guidelines: seal properly and think thin.
Q. This is in reference to “What's Your Frankenstein Quotient” questionnaire on page 15 of Volume 11, #2. A question asks “What contribution did a man named Charles Strickfaden make to both the original 'Frankenstein' in 1931 and 'Young Frankenstein'?” According to the article on the preceding page of that issue, his name was Kenneth Strickfaden, not Charles Strickfaden.
A. You have sharp eyes. I spotted the error, too, but thought it was nothing more than sloppy editing. Thanks to TCBA member John O. Foster, I learned that Charles was a brother to Ken. He was a musician and well known in movie circles. Evidently, the personnel in the promotional department were more familiar with Charles than with Ken. Did any other readers detect the error?
Q. My father and I are building our first Tesla coil and are confused about where the spark gap and capacitor are placed in the primary circuit?
A. Circuits “B” and “C” are the correct methods.