Re: 5 Turn Primary?

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

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-uswest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Nate (and any beginners):

On 21 Mar 01, at 8:11, Tesla list wrote:

> Original poster: "Nathan Morris by way of Terry Fritz
> <twftesla-at-uswest-dot-net>" <themfam-at-home-dot-com>
> 
> Question,
> 
> If the cap value can be changed so that the number of primary turns is
> reduced, then my question is, how do you know when to stop?  With the
> right cap value the primary could be limited to just a few turns. 
> There has to be some loss of efficiency as some point.  What is a good
> rule of thumb for knowing where to plan ones primary tap?  I currently
> have over twenty turns and I am planning on cutting that back to about
> 16 turns.  I have 40 good HV (10-30 KV ) caps on the way so I am 
> flexible with my cap configuration.  I would like to reduce the
> physical size of my primary as much as possible without jeopardizing
> performance.  

It is not the turns that count - it is the inductance. There is an 
infinite number of coils that can be wound with 16 turns, 5 turns, 
whatever, all with different inductances. There is no ideal number of 
turns. Any reduction in primary inductance is accompanied by a 
progressive loss of efficiency if the primary capacitor is kept the 
same. Doing this implies a reduction in either secondary C or L or 
both to maintain tune. You can usefully boost the efficiency of your 
coil by decreasing primary inductance (Lp) and increasing primary 
capacitance (Cp) but you must also increase primary voltage (gap 
spacing - Vp) to maintain the same primary bang energy (Ep). 
      Note also that reducing primary inductance (turns if you really 
must) is _generally_ accompanied by a reduction in the coupling 
constant (k) so the primary must be brought closer to the secondary  
if k is to be maintained. 
      The upshot is: you cannot reduce Lp and maintain the same 
efficiency if nothing about your secondary coil changes. You may find 
that lopping a couple of turns off the primary and reducing Ctop 
doesn't affect performance to a huge degree but only you can know 
whether the degradation is acceptable to you or not.
      Finally, I think some of the questions many people ask are best 
answered by a good ol' bit of DIY experimenting. That is exactly how 
most people on the list who are capable of answering your questions 
learnt it for themselves. To the world in general: it doesn't take 
much imagination to figure out how to wind a coil.

Malcolm