Re: TC resonance estimation?

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

Original poster: "Mike Panetta by way of Terry Fritz <twftesla-at-qwest-dot-net>" <ahuitzot-at-mindspring-dot-com>

On Sun, 2002-07-21 at 14:18, Tesla list wrote:
> Original poster: "Malcolm Watts by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
> 
> Hi Mike,
> 
> On 19 Jul 2002, at 8:14, Tesla list wrote:
> 
> > Original poster: "Mike Panetta by way of Terry Fritz
> <twftesla-at-qwest-dot-net>" <ahuitzot-at-mindspring-dot-com>
> > 

> > Would a coil of other then a 6:1 aspect ratio fix the problem with
> > losses?  I was thinking about making a 12" x 2" coil until I saw that
> > the resonance frequency (with a particular top load) would be somewhere
> > around 1MHz...  This was someone elses project that I found on a web
> > page out there on the web (it was a google search for ' small tesla
> > coils '), so MMMV (My Milage May Vary ;) I guess...  Do you know of any
> > web sites off hand that would describe how I would go about choosing my
> > resonance frequency, and what the tradeoffs are of the various size
> > coils?
> 
> There is a simple rule I use to make a design converge with 
> acceptably low losses. If the coil is spacewound, aim for the wire 
> diameter to be a minimum of 3 skin depths thick at the lowest 
> frequency of operation you will run the coil (i.e. with the largest 
> total - terminal+coil - capacitance). If the coil is to be 
> closewound, I'd go for a minimum wire diameter of five skin depths at 
> the lowest operating frequency. I don't have a ready made program to 
> do it for you sorry. Perhaps I should write one. Using this rule and 
> iteratively designing a resonator will give you a definite final 
> result. 
> 
> Here is the basic algorithm:
>     Start with knowing the desired frequency, and a projected maximum 
> output voltage so you can choose the height of your coil. Then choose 
> a desired aspect ratio. Next, choose your desired top terminal and 
> find the capacitance of the structure (there is definite answer which 
> doesn't require you having to know the inductance in advance). A 
> simple calculation now yields the required inductance to do the job 
> and from there, you can calculate the number of turns using Wheeler's 
> equation rearranged (you know L, height and radius). Calculate the 
> wire size required, then compare the diameter with a calculated skin 
> depth figure (which needs only the frequency and the multiplier a la 
> close/space wound). If the required wire diameter is significantly 
> smaller than the skin depth calc tells you it should be, you know 
> that an increase in coil size is required. The converse is true if 
> you wish to make the coil smaller.

Ok, so I need to do a bit more reserch on what the upper limits are on
the FET switching rates in what I will be using on my SSTC I guess. 
That way I will be able to figure out what frequency range I want to
target.  What simulation tools do you guys use?  Are they free?  I think
I should definately simulate this stuff before trying it out because I
do not have the money to blow on fets and such :).

> 
>     The guide to wire diameter is based on experience and I am hoping 
> that something more concrete might be refined by the TSSP project 
> (Paul ;). It may be that the guides are too conservative or it may 
> not. I do know that cramming heaps of inductance into a small coil is 
> a total loser.

What is the TSSP project?


> > Do you think a google search on the names you gave above would return
> > any useful results?  I think I may do the search anyways and find out
> > myself ;).  As for the top load, I was thinking of using a steel float
> > that I have as the top load, but I do not know how well it would do. 
> > Its not spherical, but its kinda more like a short cylinder (maybe 3
> > inches) thats has a hemisphere at each end to cap it.  As you can see, I
> > am trying to keep this as cheap as possible to start with by using as
> > much of things I already have on hand as possible ;)
> 
> Look out for Wheeler's formula for inductance and Medhurst's formula 
> for self-capacitance in the list archives. No need to repeat what is 
> already there. If the above sounds like work, it is. However, it 
> increases the amount of "design" in the design procedure.

Ok, will do.  I do not mind the extra design work, as currently I am in
no real hurry, and I seem to have all the time in the world since I do
not currently have a job...

> 
> Regards,
> Malcolm
> 
> 

Thanks,
Mike
>