# [TCML] Largest Secondary Coil "Drivable" by Primary Circuit

Futuret futuret at aol.com
Mon Sep 27 08:04:41 MDT 2010

```Scott,

I guess I should mention right off that the following is only a rough guide,
and is more than anything just a "way to think about the issue".  Certainly
a range of toroid sizes can work well in a particular coil, etc.......

Let's see... if a 660 watt coil gives 39" sparks from a 13" toroid
and gives a few simultaneous streamers for a "good looking" spark,
then we can use the formula:

Formula 1:

Toroid diameter (inches) = sqrt input power (watts)/2

example:  sqrt 660 W = 25.69       25.69/2 = 12.85" (which is close to 13")

example:  sqrt 11,000 W = 104.88    104.88/2 = 52.44"

If we assume that a 13" toroid has about 13pf, then the formula works for
toroid capacitance too at least in a rough manner.

The formula above probably gives a minimum toroid size for good results.
We can go somewhat larger.  In one test, I installed a 6" x 24" spun toroid
onto my TT-42 coil which uses a robust 12/30 NST and normally gives
42" sparks using a 13" spun toroid.  The coil draws about 800 watts.
(NST's can draw more than their rated power in some cases, especially
when driven by 140 volts).  When using the 6" x 24" spun toroid, the
sparks were only intermittently able to break out at all.  When they
did break out, there was only one streamer, and it was about 44" long
if I remember correctly.  Of course when I used a breakout point, then
the sparks were steady.  A formula for a toroid of such size might be:

Formula 2:

Toroid diameter (inches) = sqrt input power (watts)

But I don't recommend this formula which results in such a large toroid
that the sparks can barely or intermittently break out.

Going back to your 3kW (3000Watt) coil and using Formula 1 above,
we obtain ~ 27" as a minimum optimal toroid size.

Then again, a coil that runs at a high break rate of 300 to 450 bps,
might use a smaller toroid than a lower bps coil, due to the smaller
bang size.  Bottom line is you have to experiment to be sure I guess.

Again these ideas are rough, and are just a way to think about the issue.

John

-----Original Message-----
From: Scott Bogard <sdbogard at gmail.com>
To: Tesla Coil Mailing List <tesla at pupman.com>
Sent: Sun, Sep 26, 2010 11:19 pm
Subject: Re: [TCML] Largest Secondary Coil "Drivable" by Primary Circuit

John,
Good info, I was not aware of this relationship, but it does make sense.  It does bring to question however, is there a more concrete formula that relates input power to relative toroid size or capacitance?  So if one is designing a system that uses 3kW his toroid size should be X picofarads for longest spark before breakout can no longer occur under normal conditions...  It makes me wonder how much bigger I can go on my 4-inch coil that uses a 7*24; the thing makes sparks longer than my old system that happily ran a 12*40 and is consuming about the same power, just a smaller secondary...

Scott Bogard.

On 9/26/2010 8:59 PM, Futuret wrote:
> Brandon,
>
> The toroid size should follow the input power, not the secondary
> size.  If you keep the input power the same, but use a larger secondary,
> you should not use a larger toroid (unless the toroid was too small
> for the power in the first place).  If you increase both the input power and
> the secondary size, then the toroid size should also be increased.
> If you double the input power, then the toroid should be made
> 1.4 times larger approximately.  For example if the original input
> power used a 13" toroid, then when doubling the input power,
> you'd use an 18" toroid... just a rule of thumb.  The spark length
> should also increase about 1.4 times if you double the input power.
> If the original spark length was 40" then the new spark length
> using double the input power might be 56" or so.
>
> John

```