From: "Terrell W. Fritz"
Date: Wed, 18 Oct 2000 20:05:18 -0600
Subject: Re: [TSSP] Surprising secondary voltage profiles
Hi Paul, I ran a voltage profile E-Tesla5 plot of the little 3x10 coil that Mark refers to: http://users.better.org/tfritz/site/papers/nonlinearcoils/NonLin.html http://users.better.org/tfritz/site/papers/nonlinearcoils/markphd.txt I then superimposed that onto the profile I got in the paper: http://63.225.104.49/TeslaCoils/Misc/PaulNich/contours.jpg The profiles look remarkably different. The hand plotted profile extends much further down the coil. This "may" indicate that the profile is not convex. However, it may also point to a flaw with the hand plotting method I used in the paper. I used a grounded rod and approached to coil until the tuning went down by a given amount. In retrospect, the coil was probably far less sensitive near the base than near the top for this method. Thus giving the elongated profile. If the E-Tesla5 secondary voltage contour was far off, it should give a significant calculation error. The program gives an Fo frequency of 1566.4 kHz. I measured the coil at Fo = 1597.6 kHz and Mark got 1.615MHz. Errors of -2.0% and -3.1%. The program assumes a center top terminal which tends to give a slightly low calculated Fo (all my coils are terminated to center posts...) but this small coil did not have this terminal. The BASIC and C versions with source code of E-tesla5 are at: http://users.better.org/tfritz/site/programs/E-TESLA5.ZIP One thing I worry about is how the capacitance is distributed on the coil. Near the base, the capacitance is the ground plane is high since it is so close. Near the top of the coil, the capacitance is also high since the top of the coil needs to charge the large top area above the coil. The top turn of the coil has substantial current in it since it is charging so much area. Duane Bylund and I have both noted that LEDs placed on the last turn will illuminate even though one would think the current would be zero. The top and bottom capacitance tend to be far higher than in the center of the coil due to these effects. This gives sort of a bathtub curve of capacitance distribution along the secondary: Robert Jones (wherever he is?) was struggling with how to determine this capacitance distribution in his secondary model last I heard. I think the high capacitance near the base of the coil suppresses voltage buildup and the high currents spraying into space near the top of the coil tend to make that nasty concave curve: http://users.better.org/tfritz/VoltDistBare.jpg It is interesting that this curve implies the voltage is greatest just before the end of the coil!! I still don't belive that but the measurements there are and E-tesla5 seem to live with this just fine... By setting the capacitance distribution in various ways, you can probably get just about any curve you want. Unfortunately the "right" curve is illusive. Perhaps some of my babbling here will be of use. This is how I think about the secondary voltage profiles and it all seems to work for me. I think it can be equated to the spring ruler analogy but I have never pinned that down. Maybe all this will spark some ideas... I am still working on ways to measure the profile. So far most things tend to see field stress rather that absolute voltage. Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.