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Re: [TCML] Safely Grounding a Tesla Coil



I don't think you are going to want to run a 15kV 30mA Tesla Coil on the window sill of your bedroom. Maybe you should consider running it out in the yard on top of the chicken wire.
Paul
Think Positive

----- Original Message ----- From: "Brandon Hendershot" <brandonhendershot@xxxxxxxxx>
To: "Tesla Coil Mailing List" <tesla@xxxxxxxxxx>
Sent: Saturday, February 06, 2010 7:58 PM
Subject: Re: [TCML] Safely Grounding a Tesla Coil


Hi jim,

Okay, so my setup is on my window ledge, which isn't very big, but big enough. It's not big enough to throw a 6x6 foot piece of chicken wire though. Does the counterpoise have to actually be under the coil, or was that just for demonstrational purposes? If I could put it in the front yard and run a wire through my window, that would be best.

Thanks a lot!
Brandon


On Feb 6, 2010, at 12:59 PM, jimlux <jimlux@xxxxxxxxxxxxx> wrote:

Brandon Hendershot wrote:
Hi Jim,
Could you explain the concept of "counterpoise" for me or provide a link to some documentation? I've never heard of anything like it...
Thanks btw,
Brandon


counter-poise, literally, "against" "weight" or "balance"

It's an artificial ground plane for your coil.

in this context.. think of your topload as one plate of a capacitor. what's the other plate? We talk about "free space", but really, it's the surface of the floor (ground) under the tesla coil, since it's closest.

The secondary is a series LC circuit.. the secondary coil being the L and the "top load and ground" being the C. Since it's a series circuit, all the current that flows in the L has to also flow in the C. So, imagine current flowing out of the top of the secondary winding into topload, through space, into the ground, flowing to wherever the wire from the bottom of the secondary is, and back to the coil. (in practice, there's other capacitances involved, e.g. from one segment of the winding to another segment of the secondary winding..aka "self C"; but that's by the by)


Our goal is to make sure that the RF current in that big capacitor (topload to ground) goes where we want it to (as easily back to the secondary L) and NOT to somewhere we don't want it to (consumer electronics in your house).

Now, with a BIG coil (or a transmitting antenna), the thing sits on the "ground" and you work on making the connection to "ground" as good as possible.. rods, radial wires, salt water, etc.

But for a smaller coil (say, up to 6 feet tall), you can make a "artificial ground"... say you made a big aluminum pizza pan 30 feet in diameter around your coil. Aluminum is a good conductor, so the RF current will flow in the pizza pan, rather than somewhere else. Turns out that chicken wire, or similar, works just about as well.

And, as for how big it should be: if you plot out the electric field distribution, you'll see that most all of the field is within a circle with radius equal to the height of the coil above the ground plane/counter poise.


Now.. for safety's sake, you don't want the voltage on that counter poise to be significantly different than "real ground" on which you might be standing, or to wiring in your primary side. So, you could connect the counterpoise to the "greenwire" ground (which is bonded to "earth ground" or to a "ground rod". Note well that this "safety ground" connection is NOT for RF. If the counter poise is laying on the real ground, the capacitance to the actual earth is huge, so in RF terms, it's already connected.. but you want a "DC" or "AC line frequency" connection for safety. You could put a big RF choke in series with the connection and it would exactly the same (and prevent any stray RF from getting into your electrical system.


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