Re: Perfecting my final design...

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

Hi Adam,
         Having built and used several stacked primaries, here are
some comments FWIW:

On 13 Jun 2005, at 16:21, Tesla list wrote:

> Original poster: "Adam R." <arabraxas@xxxxxxxxxxx>
>
> Hello, this is my first time to this "forum"...well, I'm working on a
> "second version" of my coil (ie, I want to make it as professional as
> possible) and would like to make sure I am doing everything correctly.
>
> Here are some specs: The secondary is 4"x18" (24.2mH), a .02uF cap
> bank, 15-60 NST, and a Terry Filter for protection.
>
> I have a question on the primary--I'm trying to make it high
> quality/durable and compact. What I'm trying is something that I've
> seen before and have been wanting to give it a go: a simple dual layer
> primary. It will be 1/4" OD Cu tubing ten turns, each wound in
> opposite directions (so 20 turns total, top and bottom layers).

All turns must be in the same direction. Typically, the coil spirals
inward on one layer and outward again on the other. Good insulation
between layers is required with particular attention given to
creepage distances between the upper and lower outer turns where the
voltage gradient between the coil ends is a maximum.

 So
> each layer will be ABOUT 0.0306 mH. Does this all sound right?

30uH/layer sounds about right but it depends heavily on the spacing
between turns, the mean diameter and the number of turns. Be aware
that when you stack two spirals the coupling is heading towards 1
(probably about 0.9 or greater) so the total inductance will be
around 70 - 80% higher than the sum of the inductances of the two
individual layers.

 I'm
> worried I might build it and find I need more inductance! Or am I
> making it overkill with two layers of TEN turns? Because the total
> width of the base is 17"x17", I need to get the size right (I have
> acrylic pieces and they are expensive... cannot mess up!) My coupling
> to the secondary is about 1".

There is not enough data on your coil to make any further specific
comment on the suitability of your proposed primary.
     Another way of boosting the inductance is to greatly reduce
spacing between the turns. One of my primaries has the same length of
tubing as a double-layer primary and scores only 10 - 20% less than
the stacked arrangement because of the close spacing (another way of
boosting the overall coupling within the coil). You can get away with
this trick since the volts/turn is reduced for a given primary
voltage and many more turns. Also note that the voltage gradient
between turns is a maximum between the outer turns.

Malcolm

> Second question is with the rotary spark gap. I'm making a simple
> propeller gap using a cheap (ie low power) Oriental Brand motor. I'm
> pretty much sticking to what is commonly done, but what length should
> the rod be? 6" or 7"? It's my intuition to go with the latter...
>
> Any other recommedations? I'd like to get this coil working very well
> and not miss anything or screw something up.
>
> Thanks for the help!
>
>
>