Hi Jon,Classical "critical coupling" for coupled RF circuits is MUCH DIFFERENT than the "best" coupling for Tesla Coils. Critical coupling is defined as:
k(critical) = 1/Sqrt(Qp*Qs)An excellent discussion about coupled tuned circuits (from an RF design standpoint) can be found in Terman's "Radio Engineers Handbook", McGraw-Hill, 1943, Section 3: Circuit Theory. Although out of print, it is readily available from Amazon and other book sources, and may even be available as a large PDF file.
Using the above "critical coupling" criteria, a typical Tesla Coil with a primary Q of about 10-15 (including spark gap) and a secondary Q of about 250-350 will have a critical coupling of only 0.014 - 0.020. However, it has been experimentally determined that most spark gap Tesla Coils operate most efficiently with coupling coefficients in the range of 0.10 - 0.18. The higher the coupling, the more quickly energy will be transferred from the primary to the secondary (and vice-versa). When the coupling is too low, more energy ends up being dissipated in the spark gap before it can be transferred to the secondary. If coupling is too high, racing sparks and/or poor gap quenching occur. Setting up a coil for classical "critical coupling" will simply result in excessive primary losses and poor coil performance.
Once you have tuned your systems for best performance (under all power levels), the last adjustment tweak should be for "best" coupling (i.e., best performance with no racing sparks). Unfortunately, this adjustment is different for every coil. It is mainly dependent on the quenching capability of the gap, bang size, resonator size, and robustness of the secondary insulation system (including field control between the toroid and primary). At present there are no magic formulas or tools (other than some rules of thumb, such as keeping "k" below 0.15 or so). Optimal coupling for YOUR system must be experimentally determined.
Bert -- ******************************************************************** We specialize in UNIQUE items: coins shrunk by ultra-strong magnetic fields, Captured Lightning Lichtenberg figure sculptures, and scarce technical Books. Please visit us at http://www.capturedlightning.com ******************************************************************** Jon Danniken wrote:
Hello, I read the article on coupling written by D.C. Cox in the TCBA Volume 3, Number 2 (full article available for viewing on teslauniverse). In that article, DC provides a graph to show the effects of coupling on the secondary current, as shown here: http://teslacoils.home.comcast.net/CriticalCoupling.jpg What I am wondering is if it is possible, using a signal generator and an oscilloscope, to actually determine the point at which critical coupling occurs. Note: there is a page showing how to use a signal generator and an oscilloscope for other teslacoil measurements in the following link; would either of these methods work for determining the point of critical coupling? If so, what would I want to look for to investigate the effects of coupling, in an effort of achieving critical coupling? http://www.hvtesla.com/tuning.html Thanks for any suggestions on this, Jon _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla
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