From: Paul
Date: Wed, 29 May 2002 20:52:28 +0100
Subject: Re: Top V Probe Design (was RE: [TSSP] Topload breakoutpotentials)
Bert wrote: > I think a larger amount of capacitance would be desirable in > practice to reduce AC ratio errors introduced by the stray > capacitance between voltage grading toroids/rings and to the > surrounding secondary winding. Indeed, in order to ensure that the output signal from the probe is largely a function of the toroid volts and not the coil volts, it might take quite a large 'column stage' capacitance to dominate the radial capacitance to the secondary, > at least 100X as large Perhaps. I just modelled a small corona-suppression toroid inside Bart's 8.5" secondary, at an arbitrary height, http://www.abelian.demon.co.uk/tmp/probe-ring.gif to get a total C to the secondary of 6.8pF. A 100X stage capacitance, with 10 stages in cascade puts an extra 68pF load onto the topload, compared with the existing C of 40pF. Dropping to 20X helps quite a bit, but then I think a circuit sim model of the whole network would show that the secondary voltage is influencing the output signal too much, say by more than 5%. > ROC sufficient to prevent corona and that's the other problem. On the whole the secondary maintains a roughly linear voltage rise, but we're expecting to see some quite large momentary departures from that and as soon as that happens, the grading rings will break down to the secondary. As soon as the behaviour gets 'interesting' the probe will fail. At the very least, when the voltage ramps up and the probe output reports a dip or clamping of the apparent topvolts, we'll never be really sure that it's not a bit of corona forming on one of the rings. I think only an oil filled column will contain the inevitable very high field strengths that will occur from time to time during the beat. The screened column, oil filled both inside and within an outer sleeve, does seem to have some advantages over more elaborate schemes. a) its easy to make with stock tubes of copper and plastic. b) isolation of the output signal from the secondary voltage is guaranteed. c) the division ratio can be accurately calculated and only involves two capacitances - that between the inner conductor and the shield tube, and that between the top of the inner conductor and the toroid. I'm assuming that the oil column continues right up to the topload, so that both the capacitances involved in the division are entirely oil dielectric. d) the capacitive loading on both the topload and the secondary is quite modest. I modelled a grounded sleeve of a reasonable diameter inside Bart's secondary and it dropped the Fres from 70kHz to 60kHz. That's only the tube effect, the capacitive contact of the probe inner conductor to the topload would have to be applied in addition to that, but it could be set to only a couple of pF. The highest gradients occuring on oil/conductor interfaces was around 150kV/cm. Mark wrote: > I would shoot for ... a 100,000:1 divider No need, I think to burden an already stiff set of requirements with the need for a high division ratio. Surely it is only necessary to drop the potential to a point where it can leave the probe base in an ordinary coax without worry of corona. A 1000:1 gives you a nice 500V signal which can be further divided in a more benign place. Such a level of intermediate signal is small enough to be manageable and big enough to be robust in the presence of an operating TC. A division ratio of 1000:1 could easily be arranged with the shielded column, say 1pF between inner and topload, 1000pF between inner and shield. The inner conductor is only carrying 500V, so it and the shield tube could be very narrow, limited only by the breakdown gradient under oil of the shield tube's outer surface. I think the response of the tube arrangement might be quite flat, but in any case, I shouldn't worry too much about producing a compensated divider. I'm assuming we'll characterise its response in software before we use it. The downside of this tube arrangement is that it may be necessary to completely fill the secondary with oil if there is no tube radius at which the gradient on the surface of the outer oil- containing sleeve is below the air breakdown threshold. -- Paul Nicholson, --
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.