On 1/26/12 6:47 AM, Carl Noggle wrote:
Me too. That's a really good idea. If we make the divider with parallel R and C sections, with good design we should have high enough bandwidth to make good measurements. The division ratio of the R divider should equal the ratio of the R section. If somebody has access to a bunch of old Allen-Bradley 2W carbon comp resistors, it would be a pretty straightforward build. Say, 100 10M resistors paralleled by 100 50pF 5kV ceramic caps, with a bottom stage of 1M parallel 500pF for a 1000/1 divider. Should even work as a probe for a high impedance scope input. It would be better to have the bottom resistor user-changeable to match different meters, etc.
This strategy works ok up to about 50kV, and then it's hard to make it work in practice. (40-50kV is about where ordinary HV construction techniques sort of start to stop working well, and you spend a LOT of time dealing with other effects. A worthwhile hobby, perhaps, or even a business, but I'd say that accurately measuring 100kV costs 10-100 times as much as accurately measuring 10kV...)
In your example, 100 50 pF caps in series would be 0.5 pF. The parasitic C of the divider chain (which is going to be pretty long, to stand off the hundreds of kV) is going to be MUCH larger than the C of the divider. Say you're designing for 500kV. Go look at a 500kV transmission line and see how long the insulator strings are.
As a rule of thumb, figure 10kV/inch (you want to be a LONG way away from breakdown), so that 500kV divider is going to be a minimum of 50 inches long. A 4 foot pole has a several 10s of pF capacitance to ground.
Ok, so you increase the C of the bottom compensation (which is what the commercial dividers do). Now, you're looking at hooking something with 10,20,30pF to your TC topload, which has a comparable capacitance. The commercial things hook up to a high power source (e.g. 500 MW power lines, so the "burden" of the measurement system is tiny.. the capacitance of the power line to earth dominates over that in the divider)
BTW, look at the power dissipation in the divider: 1Gohm.. 500E3^2/1E9.. 25E10/1E9 = 250W... That's a significant fraction of the power being put into the coil (even for a pig powered coil).
Take a look at what Ross Engineering does: http://www.rossengineeringcorp.com/hv_dividers.htm You're looking at something like the VD900 or VD450As you can see, it looks a lot like a tesla coil. And it will have an effect on your tesla coil just like putting another coil next to yours (i.e. it will detune it)
That's why a e Field probe some distance away is a good strategy _______________________________________________ Tesla mailing list Tesla@xxxxxxxxxx http://www.pupman.com/mailman/listinfo/tesla