From: "Terrell W. Fritz"
Date: Fri, 10 Nov 2000 21:45:42 -0700
Subject: Re: [TSSP] Transients in primary waveforms
Hi Paul, At 11:53 PM 11/10/2000 +0000, you wrote: >Terry wrote: > >> My paper at: >> http://63.229.238.62/TeslaCoils/MyPapers/spark1/sparkgap.html >> has some waveforms in it that may be of use. > >Terry, > >Referring to sparkgap.html, last image, can you tell us where abouts >in the primary circuit the current probe was inserted? Probably between the gap and the secondary. A 0.01 ohm resistor to the fiber transmitter. > >I'm not surprised at the apparent amplitude of these current spikes, >although I disagree with the short wide strips suggestion - that >seems to me to be a recipe for greater parasitic capacitance and >lower series inductance, thus bigger and shorter current spikes. Short wide strips of copper for the connections cut down on impedance. That may make the spikes worse in retrospect but very lossy connections are known to cause all kinds of nasty hash. > >I'm assuming the current spikes are undesirable from the RFI point >of view, and from an efficiency point of view the high currents >are a source of un-necessary losses. The first spike is a terrible TV interference source that is very hard to stop. My big gap has ferrites on the rotor and stationary electrodes that help a lot, but still don't kill the spikes. > >Just a few rambling thoughts about these current spikes... > >If the primary tank, gap, and coil were all connected by negligible >lengths of wire, there would be no problem. Realistically there >must be some separation, so there is a 'line' capacitance to >charge up, and a line inductance which determines the rate at >which the charging occurs. The result is the transmission line >ringing seen in the last but one image of sparkgap.html. >The frequency and peak current could be dropped by introducing >some series inductance close to the gap, but the energy in the >ringing would be unaffected. Of course the ringing could be suppressed >by dampening the extra series resistance with some resistance, and >finding the right value would be equivalent to finding the right >terminating impedance for the primary transmission line, but >that's of no benefit to efficiency. Off hand I can't see a way to >prevent the initial ringing as the gap first fires. The gap is like a capacitor that is suddenly shorted. the currents and instantaneous hash is terrible... > >Let's try to estimate the energy lost due to the ringing. From >the last image in sparkgap.html, and taking the amplitudes of >the initial spike as, say, I = 100A and V = 2000V, and if L and C >are the parasitic reactances involved in the ringing, we have >roughly, > > V/I = 2000/100 = sqrt(L/C). > >and, if the resonance is at around 50Mhz, > > sqrt(LC) = 1/(2.pi.50e6) > >From these we get L = 60nH, C = 150pF. With a peak V of >say 1000V, thats 75 uJ injected into the ringing twice each cycle. >Terry, you don't say what the bang energy was in these measurements, >and the figures above are for spikes beyond the frequency response >of the probes, but on the face of it, it looks like the parasitic >ringing loss is probably quite small in comparison to the energy >carried at Fres. I think the "loss" is small but the burst of RFI is very big. Cheers, Terry > >Cheers, >-- >Paul Nicholson, >Manchester, UK. >--
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.