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

mddeming at aol.com mddeming at aol.com
Mon Sep 27 07:09:01 MDT 2010

```Hi Scott, Brandon, All,

I don't think that there is a specific over-all formula, but here are some "constraining" parameters:

1) Total charge on the toroid Q=0.5CV^2
2) Total energy stored in  Toroid E=QV
3) V ~ fn(ROC of topload)
4) If D(toroid)>>>h(coil) or d( toroid)>>d(coil) ==> structural instability

I remember reading, but cannot now find the reference, that it is "desireable" to have ~ half the total sec C be in the topload.

Hope this helps,

Matt D

-----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
>
> SNIP!!

```