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Re: [TCML] Crock Croft Walton Voltage Multiplier



On 5/4/12 6:53 AM, Bert Hickman wrote:
Although it may be possible to make an aqueous resistor using a long
length of small flexible vinyl tubing, I wouldn't recommend it since the
resistance may vary significantly over time or with temperature.

A better choice would be a string of commercial high voltage resistors
connected in series to achieve the desired resistance and hold-off
voltage (with a significant - say at least 50%+ - safety margin). New HV
resistors can be pricey, but you can sometimes find them on eBay at
relatively low cost.

For example, following are some possibilities where the vendor is also
offering sufficient quantities:

http://www.ebay.com/itm/260719938778 (200 Meg, 11 kV)
http://www.ebay.com/itm/251009542782 (100 Meg, 15 kV)

Bert



I've built a fair number of salt water resistors...

They DO have large temperature coefficients, but if you keep the concentration of salt down to where it doesn't start to precipitate out when the temperature gets cold, they're fairly repeatable.
If you're looking for something with many megohms and long distance (so 
you don't get surface flashover), a weak salt solution (or just tap 
water) would probably work. The resistance will gradually decrease as 
the end electrodes dissolve.
long pieces of some black rubber tubing (auto vacuum hose) also works 
(depends a LOT on the specific brand and lot).
So does spraypainting a spiral of some flat black paint on a long piece 
of PVC pipe.  You have to test your particular paint.. some is 
conductive, some isn't.  And, much to my dismay, a given brand changes 
from time to time.
Static dissipative or shielding paint are other possibilities, and they 
actually have well controlled resistive properties.
Ultimately, you need to be VERY aware of the physical size.  Flashover 
along a surface can be as low as 1/4 or 1/3 of the free air distance, 
and that's in a fairly uniform field.
IMagine your 300-400kV TC can throw meter long sparks, and you have a 1 
meter long resistor...
For extremes, take a look at the picture of the Siemens 1.2MV gas 
insulated switch in the centerfold picture in this month's IEEE 
Spectrum.  the actual switch that holds off the 1.2MV is fairly small in 
the middle, but the insulators are enormous.  (I can't find a link to 
the image, and googling siemens, etc. doesn't seem to turn up anything 
useful)
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