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Re: [TCML] Divide and conquer



Good ideas. Locating the divider inside the secondary should significantly reduce space charge effects.

However, a capacitive divider may provide more accuracy than an aqueous resistor or other resistive divider chain. In fact, HV resistors should not be necessary for this application since there is no DC component on the TC output. An AC divider would provides excellent high-frequency response, it shouldn't significantly reduce TC secondary Q, and the TC can easily be re-tuned for any changes in TC operating frequency.

Specifically, a series divider chain could be made from small PP film or COG/NPO Class I ceramic HV capacitors, housed inside a corona-resistant PMMA or polycarbonate tube, and centered within the TC secondary. The larger divider output cap is normally connected to the noisy TC RF ground, so it may be necessary to use a battery-powered buffer and analog fiber optic transmitter - receiver pair to get a clean output signal.

Bert


Carl Noggle wrote:
Has anybody tried putting a many-stage RC ladder voltage divider down the center of the secondary?  The E field is uniform (vertical) there, and there's probably not much space charge.   I keep thinking about it, but I haven't tried it yet.  Maybe a string of 20 M Allen-Bradley 2W resistors with some HV ceramic caps in parallel.  The time constant should be at least 10 times the time of one cycle, 10pF or so.

It could be that due to the well-behaved field inside the secondary, that we could dispense with the capacitors and use a long CuSO4 resistor.  Be sure to use copper end plugs and measure it often, since chemistry enters into the equation.

It's easier to measure the voltage of a Marx, since the output impedance is quite low.  Usually some sort of geometrically-calibratable capacitive divider is used.

--- Carl




On 2/27/2018 6:27 AM, jimlux wrote:
On 2/26/18 8:42 PM, Bert Hickman wrote:
Chris,

It's a very challenging design problem. And, the divider will be quite sensitive to changes in the surrounding E-field. Since TC sparks create shifting space charge regions with corresponding fluctuating E-fields in the surrounding air, accurate direct measurements can be done only if the Tesla coil is not actively creating streamers/leaders.




you might look at commercial products made from this:
Not a lot of design info, but

http://www.rossengineeringcorp.com/products/measurement/hv-voltage-dividers.html

is an example.

I'm sure there's stuff out there about designing a divider to work with the output of a Marx at megavolt levels - way above what a TC will put out.

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