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Re: coax (grounding shield & AC?)



Original poster: "Dave" <dgoodfellow@xxxxxxxxxxxxxx>

At one time I used rg-8 to feed my 6" Tesla coil. It was powered by a 19kv 4.2kva transformer, I could feed it with the 30 amps from the dryer outlet. I had two nice old axcel 40kvac .019uf capacitors in parallel in place. Rotary spark gap was right across the transformer output. My protection circuit, laugh if you will, was a couple of car stereo woofer chokes, mounted right on the hv terminals of the transformer. Hey, they were made to roll off 6db per octave, at starting at 100 hz, so I used them, and without a problem. So picture if you will this center tapped transformer, two woofer chokes, and two RG-8 coax cables with the center conductors on each hv output, and the shield attached to ground on the transformer end, I think that I left the far end unconnected to anything. There was 25 feet of distance between the transformer and the place where the Tesla coil components were set up. During this time with the coil arranged as I just described, I would get some terrible snapping arcs (up to six inches long) between components at the coil end of the coax. I had to pull the braided shield back at least a foot, and still be careful on how the wire and components were placed or these nasty popping arcs would happen. When I built a larger coil to use with the same transformer, I got rid of the coax and the woofer chokes, but this time I had the transformer within the base of the coil with the sg and caps. I wired all the components together with 8 gauge stranded thhn (household) wire. I noticed that I didn't have to be as careful with the proximity of cable and components, and I never did get those harsh snapping arcs between components as I did when I used the 25 feet of RG 8 coax between the transformer and the coil. When I read about Dr. R's experience with big zaps in unwanted places when he had coax in place, I remembered my similar experience during the time when I thought coax was the way to go. When I wire my next coil with the big transformer, I will use 10 gauge wire on the output of the transformer to go to the coil. I feel that there was something going on that I can't explain when I used the coax, and I wouldn't recommend my particular wiring arrangement.

Cheers, Dave
----- Original Message ----- From: "Tesla list" <tesla@xxxxxxxxxx>
To: <tesla@xxxxxxxxxx>
Sent: Sunday, May 06, 2007 10:07 PM
Subject: Re: coax (grounding shield & AC?)


Original poster: "Peter Terren" <pterren@xxxxxxxxxxxx>

I really have to question Dr Res' explanation of his observed 125kV sparks from a 14.4 kV RMS line as being due to an 8 times multiplier from a Blumlein effect. Far more likely I feel, is stray HV from the TC itself. If the ground was faulty or of higher inductance (perhaps even a few loops might do) or simply not connected (I've done it too), then an arc from base of TC to the primary occurs and has to go somewhere. It will then jump the easiest low inductance path to ground. Depending on the setup this may be across the bushing to ground.

I must admit to having used thick coax forever on all my MOT and PT powered coils for both of the hot lines with the coax ground grounded to the MOT ground. It does terminate in a Terry filter though with resistance, MOV's and resistors though. I do like the earthed surround as an additional safety feature from ground strikes and also from passing over other cables, conductive surfaces, grass, cars etc. Persoanlly I feel this outweighs the very unlikely potential for a Blumlein multiplier which is actually quite complicated and involves more than one spark gap for any significant multiplication if I recall.

I would advise that you DO use coax for safety unless and until there is some data to suggest otherwise. A suitable test setup might involve using a loaded primary circuit without a secondary which might generate spurious HV.

I was going to make a Blumlein multiplier as one of many little projects I want to do. I would be delighted to get 125kV but I am sure I wont.
Perhaps using water as the dielectric like these guys.
http://scitation.aip.org/getabs/servlet/GetabsServlet?prog=normal&id=IEESEM002001000156000P11000001&idtype=cvips&gifs=yes

Peter

Original poster: "M G" <gt4awd@xxxxxxxxx>

Can the Blumlein effect create problems on a smaller power level. For instance my 9kv 30ma NST? I would assume that quiet large coax cable would be used on a PIG setup, and of course a PIG is capable of moving a lot more amperage. Also, does the effect only apply to coax being used in Tesla Coil conditions or generally any current passing through a coax cable?
Thanks,
Matt G.


---------[ Received Mail Content ]----------

Subject : Re: coax (grounding shield & AC?)

Date : Fri, 04 May 2007 16:14:12 -0600

From : "Tesla list" <tesla@xxxxxxxxxx>

To : tesla@xxxxxxxxxx



Original poster: "Gerry Reynolds"!



Hi Matt,



What Dr R is getting at is the "Blumlein" effect. I have tried to

research this and believe it is a transmission line phenomenum when

improperly terminated. I dont know this definitively but believe its

possible to figure it out if someone would be willing to create the

effect. Presumably one would use a PIG and have a safety gap from

each HV bushing to ground to protect the PIG from damage. The

setting of the safety gap would be larger than usual so one would be

fairly certain that any firing was due to the effect and not to the

the voltages created by the TC. If we can find a volunteer, maybe Dr

R can help that person create the effect based on his

experience. Just an idea :-))



Gerry R.





>Original poster: "M G"

>

>"Don't run it with AC with ! the grounded shield on it."

>

>Hi, just wondering why it is not good to use the coax with grounded

>shield on for AC? I use coax for high voltage wire on my jacobs

>ladder. I did notice that if the grounding shield was left normal by

>the high voltage terminals of the NST it would pick up some of the

>electricity. A small, weak, arc on the other end of the shield would

>jump to anything conductive, such as your hand. Cutting the

>shield/coil of wire off about 1" from the terminals stopped the

>problem though. Is this the reason, or something else?

>Thanks,

>Matt G.
















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