Re: tesla coil

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>

Hi Josh,

Neat!  Let's see what we have.


At 07:40 PM 1/2/2001 -0700, you wrote:
>Transformer voltage: Some where between 9-10kv. The transformer was given to
>me by a friend of mine at the local radio station and has no name plate or
>any type of label on it :-(.

Is this a NST type or current limited type transformer?  I will assume it is.

>
>Transformer current: 20-30mA, is there a way for me to measure the current
>of the transformer? possibly by setting a meter to read current and
>connecting it across the transformer before plugging it in?

You can measure the current with a meter but if anything is set wrong or
anything is not just right, it will fry the meter.  Best just to assume the
voltages and currents are ok for now.

>
>Primary coil inner diameter: 5 inches
>
>Primary coil turns: 3 made of 12 AGW wire.
>
>Primary coil distance between turns: .25 inches
>
>Primary cap size : 12nF
>
>type of spark gap: Multi point spark gap made with 8 1/2" copper tubes
>bolted to the inside of a 3 inch PVC pipe
>
>Secondary coil diameter: 1.75 inches
>
>Secondary coil turns: 680
>
>Secondary coil length: 8 inches
>
>Height of first turn of secondary coil above primary coil: 1/4" from the
>bottom of the primary (im using a Helical primary). I found that if i lowerd
>the secondary coil further into the primary the output increased dramaticaly
>( from 1 inch sparks to 3 inches).
>
>Terminal diameter (side to side): You guys are going to laugh but I am using
>a 1.5 inch Styrofoam ball wrapped with aluminum foil.
>Height of terminal (center) above primary coil: 10.75"
>
>I will be posting some pictures of the coil on my website as soon as I get
>it updated :-)
>
>Thanks for the help,
>Josh
>
snip...

Here are the numbers I get using a bunch of programs and formulas:

Your secondary coil's inductance is 4.03mH
The total secondary system capacitance is 4.56pF
Your secondary system frequency is 1.17MHz

For the primary to be tuned at 1.17 MHz you would need to tap the primary
coil at 2.38 turns with a 12nF cap.  That gives a primary inductance of
1.53uH and a primary system frequency of 1.17MHz like the secondary.

The primary to secondary coupling is around 0.08

So, I would try moving the primary tap point in to around 2.4 turns but try
a bunch of places around there to see where you get the best spark.

If that does not seem to help much I would try drawing an arc from each leg
of the transformer to ground to be sure it is working properly.  However, I
am assuming you have a current limited NST and can do such a test safely.
If not, we can help you out there.

The cap may be the next suspect.  If it gets hot or does anything odd, that
would be a bad sign.

Pictures would be great and help too.  If you send them as attachments to
(now with virus scanning ;-)):
terry-at-hot-streamer-dot-com

They will show up at: 
http://hot-streamer-dot-com/temp/

So that is the first shot at figuring it out.  See how it goes...

many of the numbers above are from formulas like those at:
http://home.earthlink-dot-net/~electronxlc/

Cheers,

	Terry