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Re: Overcoupling??

At 04:45 PM 5/7/99 +1200, you wrote:
>Hello all,
>             Here is an excerpt of a discussion I've been having 
>lately with John Freau. It is just part of the conversation so gaps 
>will exist but you'll get the gist. Terry's mention of turns ratio 
>in regard to the topic spurred me to send this to the list. I hope 
>John doesn't mind me sending this.
>
>Regards,
>Malcolm
>
> ------- Forwarded Message Follows -------
>
>From:           Self <DIRECTORATE/MALCOLM>
>To:             FutureT-at-aol-dot-com
>Subject:        Re: Secondary Inductance (fwd)
>Date sent:      Mon, 26 Apr 1999 11:45:46 +1200
>
>Hi John,
>          I thought I'd have a crack at doing a proof of the turns 
>ratio idea.  To help decide the method of proof, I thought I'd have a 
>look at a practical example where different aspect ratios give the 
>same Cs which I failed to do last time.
>     
>     I'll take a Diameter of 10" and Height of 30" for both coils as 
>a starting point. According to Medhurst, Cself for this coil is 
>0.61*10*2.54 pF = 15.5pF close enough.  I'll choose a height for the 
>new secondary coil of 10" and by knowing Cself = 15.5pF, arrive at a 
>diameter of 13" for this coil (Cself comes out to 15.3pF which is 
>pretty close).
>
>Let Lp = 100uH which gives Np = 37 turns.
>Let Ls = 10mH which gives Ns = 371 turns for the first secondary and 
>the expected turns ratio.
>
>For the new secondary, both Ls and Cs have to be the same so I need 
>to find Ns to achieve this:  193 turns.
>
>Interesting! That says I can do better than the turns ratio if the 
>primary looks like a secondary and the secondary has the aspect ratio 
>of a typical primary (sort of). 
>
>Ns/Np = 193/37 = 5.21    and SQRT(Ls/Lp) = 10
>
>Wow!  I hadn't expected that.  I simply have to try this. I expect 
>that I'd get a reasonable if low coupling constant by overlapping the 
>secondary on one end of the primary to a small degree.
>
>The physical layout poses plenty of problems when it comes to 
>generating high voltages in a small size but the argument is for 
>academic interest. Let me try for a more moderate sized coil that I 
>can whip up on the workbench:
>
>Let Lp = 10uH, Dp = 4" and Hp = 5"   Then Np = 13 turns
>
>Let Ls in both cases = 5mH and let me choose Ds = 2" and Hs = 10" for 
>the first secondary. That gives me Cself = .71*2.54*2 pF = 3.6pF
>That gives me Ns = 707 turns
>
>SQRT(Ls/Lp) = 22.36   and Ns/Np = 54.4
>
>Knowing Vout prop to SQRT(Ls/Lp) says that voltage step up falls 
>short of the turns ratio which appears to be the usual scenario.
>
>Now let me choose a second secondary where Ds = 3".  If Ls remains 
>the same, then Hs is 3" to meet the Cs = 3.6pF requirement (I chose 
>those based on getting the same capacitance or at least one of 3.5pF).
>This forces the new Ns = 311 turns.
>
>Now Ns/Np = 311/13 = 23.9  which is still slightly worse than 
>SQRT(Ls/Lp).
>
>So I conclude that there exists a configuration where one can do 
>better than Ns/Np but also that I am unlikely to better Ns/Np for the 
>normal coil geometries we employ.  Still not a formal proof 
>unfortunately. In view of the finding, I'd now like to try the first 
>configuration I came up with. It would be amusing to get a longer 
>spark than the primary gap setting from the shorter coil than I could
>get from the taller one.
>
>Of course this all ignores that one would expect 4/PI times the 
>step-up ratio from a distributed circuit. 
>
>**NOTE TO LIST MEMBERS - that last statement is made on the 
>assumption that the Corum's theory has some validity which might not 
>be the case.**
>
>Better check me out on all this. I may have gone off the rails 
>somewhere.
>
>Cheers,
>Malcolm
>


Hi Malcolm,

	Very interesting indeed!!  My only comment, which I am sure you are aware
of, is to keep the coupling coefficient from primary to secondary the same
for both coils in your testing.  "Theoretically", both coils will behave
exactly the same despite their very different dimensions.  It may be
interesting since the coupling from primary to secondary is going to
"bathe" both coils differently.  It will be very interesting to see where
theory and reality differ in this case!  Of course, theories are adjustable
where reality is not :-))

	Are you going to have a top terminal for these tests?  I assume not, due
to how they would affect both coils differently.  If you are, I would be
interested in the dimensions so I could look at the field distributions
around each of these coils.  

Neat work you are doing here!

	Terry