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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