Re: 24" secondary

["Barton B. Anderson" <tesla123-at-pacbell-dot-net> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Hi David, 

Tesla list wrote: 

>
> Original poster: "David W" <Dav_W926-at-hotmail-dot-com> 
>
> Hi everyone, 
> I recently discovered a piece of 24" white PVC pipe about 10+ feet long at a 
> local farm near me that I might be able to someday use.  however, I have
> never 
> wound a secondary that large in diameter before and I was wondering how I
> would 
> go about doing something like that. 
>     Do I need to space wind it? what does spacewinding achieve, voltage 
> insulation between turns?


It's up to you. Space winding will cut down on losses in the secondary due to
proximity effects where the the windings or in close proximity to eachother.
Space winding increases turn to turn distance and reduces this loss. But also,
space winding means a higher frequency which affects skin depth losses. The
higher the frequency, the higher the losses. One may be more significant than
the other. However, there is another effect I'll discuss below. 

>
>   
>     What guage wire should I use (is 14g too lossy?)


Sounds ok to me. 

>
> And, should I wind it to around 1600 turns (John Freau? -- BTW, I have 
> noticed that John frequently mentions that more turns seem to be more 
> efficient, but no one else seems to acknowlege his claims or discuss them...)
>
> with say a 5:1 aspect ratio or go for the traditional 900 or so turns?


I will then acknowledge John's claims. More turns on the secondary does a
couple important things. It reduces the resonant frequency and lowers skin
depth losses. Because of the lower frequency (cap size being a static value)
the primary turns is increased effecting the surge impedance reducing gap
losses (which you may read on the list is very significant. 

>
> also, if I do spacewind it, what if tightwound insulated wire is used instead
>
> of enameled wire to achieve the spaces?


Insulated wire will probably work ok and will add quite a bit of spacing
between turns. Possibly more than you want to add. 

With 14 awg magnet wire, you can expect about 1700 turns close wound. A self
resonant frequency of about 36kHz. More turns on the primary and lower gap
losses. 

With 14 awg and a spacing equal to the total wire diameter (0.0674" - double
insul.) you can expect about 900 turns with a self resonant frequency around
70kHz. Less turns on the primary and higher gap losses. 

If you use a smaller wire (say 18 awg), you will add more turns. The spacing
diameter is less. If you used the same method for this wire size, you'll end up
at 1400 turns and Fo of 46kHz. Again, more turns on the primary, etc.. etc..
etc.. 

Obviously, the methods can be adjusted, such as space winding only 1/2 the
diameter of the wire used, increasing turns, lowering Fo, increasing Pri-turns,
reducing gap losses, etc.. etc.. 

You should first decide what you will drive the coil with (PT, PIG, etc..) and
the cap size you wish to use. Figure the approximate spark length you would
like to achieve, then pick a good number of turns on the primary to keep gap
losses minimized, and work out the secondary to match. 

BTW, with the 14 awg close wound at 1700 turns and a cap size of 128nF
(resonant for a 10kva, 14.4kv pig xfmr) you would end up with about 20 turns on
the primary (not too bad). I used a 72" x 30" toroid size here. The 128nF cap
size would be perfect for a 24" x 120" coil. 

Sure to give amazing spark lengths. A well built RSG is important here. All in
all, this could add up to a lot of cash to build as well as a lot of room to
run it (and store it). 

Take care, 
Bart 
  
  
>
> thank you all for your advice 
> Dave