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Re: Number of turns for a secondary?



Hello Jim, ALL

Jim wrote: 
" I always thought that you decreased the gauge of the wire (fewer
 turns per inch) to keep the H/D ratio compatible with the number of turns
 (ideally somewhere between 4:1 or 5:1 & under 1000 turns). I thought that was
 why (aside from the current being carried) the larger coils always used a
 heavier gauge wire. Like I said I'm just curious & I know a lot of other
 factors come into play in the design. Thanks,  Jim"
 
Yes, Jim, this is ONE reason. However, the current capacity of the smallest
(usually used) wire is enough even for the largest of coils. Remember, the top
of the coil puts out a high voltage, but a very low current, whilist at the
bottom you will find heavy RF-current (which is one of the reasons you want a
thick grounding wire, plus of course the skin effect) and low voltage. An
ideal coiler wire would be thick at the bottom end and get thinner at the top.
However, this wire would be almost impossible to make. Wire size is not
depended on VA rating, but is defined by "ampacity" (current) alone. 

Back to coil wire size vs form: If you would use say a AWG 32 wire for a 12"
diameter coil with a winding length of say 55", you would need a lot of wire
(feet wise). This, plus the fact that resistance gets higher per feet as you
go down in wire size (meaning the AWG # goes up) will give you a lower Q (see
below) value for your coil. During coil construction you want to try to build
a sharp responding (high Q value) secondary coil for better spark output. This
is why a lower gauge number (bigger wire) is desirable to high power coilers.
A 16 gauge wire on a 2" by 13" form, on the other hand, wonīt get you very
many turns (this time lowering the impedance and again resulting in a low Q
coil).

Q definition: 

Q=Omega* L /R(dc at FRes)

Omega = 2* Pi* FRes 

so 

        2*Pi*FRes*L
Q=  -------------------
       R(dc at FRes)

For a high Q you want a high inductive value and a low DC resistance.

Coiler greets from germany,
Reinhard