From: "Terrell W. Fritz"
Date: Fri, 20 Oct 2000 23:02:34 -0600
Subject: [TSSP] New Secondary Voltage Profiling Instrument
Hi All, After a four redesigns, testing, great thoughts, and many hours work... I think I have come up with an instrument that will basically read off the secondary coil's voltage profile. It does not seem to affect the secondary frequency or have any other significant affects to the coil's operation. Hopefully, it can even be scaled to a fully operating TC if the voltage is stable enough in such a case (my CW coil should be perfect of this!). Right now I have built this version for a coil driven by a sine generator. Both bare and top loaded coils can be studied. A detailed photo of the instrument is at: http://63.225.104.49/TeslaCoils/Misc/PaulNich/profiler.jpg Let me briefly explain the major parts. D1 - The glassy encased device at the top is a 1N34A Germanium diode that is well known for it's ability to rectify tiny signals. C1 - The purple square tied to the white rod is a 0.1uF polypropylene 50V surface mount capacitor. These capacitors hold an opposite role to the typical high-voltage high-current polypropylene MMC caps that work so well in that these aren't good for much of anything! Despite high hopes, they are being pulled off vendors shelves all over the world. Seems when you wave or IR solder them... They melt!!! Gosh!! Who would have thought... Fortunately, I have a bunch and I solder really carefully. Aside from the melting problem, they are remarkable devices! R1 - Although it is small, C1 can provide a very high current spike to D1 and blow it's guts out (a problem that plagued revision 2). R1 provides current limiting to protect the fragile D1. The white rod is a 1/8 Dia. 18 inch long butyrate rod I found in the valuable things bin. This is normally tied to a longer wood dowel so that the coil can be probed at a reasonable distance that does not affect the system. So here is how the instrument works. Tiny scratches are made in the secondary at every inch to allow direct electrical access to the windings. These scratches match up to the two probe tips on the profiler which are spaced at exactly one inch. When the profiler is placed against the secondary coil, direct electrical contact is made and the capacitor C1 is charged to the peak voltage across that 1 inch section of coil minus a little diode drop. The diode drop can be fairly easily mapped for different voltages and appears to be very independent of frequency (it is not very significant in many cases). The tiny instrument appears to be quite invisible to the large secondary especially once C1 is finished charging and it is no longer drawing any current. Once C1 is charged to the voltage of interest, the profiler can be easily drawn away and the voltage of the capacitor can be read off with a 10Meg scope probe and a digital storage scope that can easily "catch" the voltage of the capacitor before the scope probe has a chance to drain it significantly. The poly cap has very low leakage and can retain a voltage accurately for several minutes as long as the instrument is kept clean (washing with alcohol insures this). Voltage measurements can thus be taken along the length of the coil at every inch. A simple summation of these voltages along the coil will give the voltage profile. If the components are scaled to higher voltage devices, the instrument should be quite capable of working on a modest but fully operating Tesla coil. Although not yet tested in a actual situation, many obnoxious and unforeseen problems have now been solved (like super glue dissolving the capacitor...) so the pictured design is nearing perfection. Comments and suggestions are welcome. I hope to test it tomorrow and report on the initial results. As far as I know, this will be the first accurate direct reading of a resonating secondary's voltage profile. Cheers, Terry
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.