Re: Coil diameter choice criteria

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

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

Hi Jim,

On 23 Jul 2003, at 14:48, Tesla list wrote:

 > Original poster: "Jim Lux by way of Terry Fritz <teslalist-at-qwest-dot-net>" 
<jimlux-at-earthlink-dot-net>
 >
 > At some point, the spark length starts to become a very large multiple of
 > the coil height, and primary strikes, etc. start to be become a
 > problem.  Also, the winding dimensions may be 5:1, but the secondary form
 > might be a bit longer..
 >
 > If I had to formulate a guideline, perhaps the secondary should be no less
 > than 1/3 the expected spark length, which in turn would be
 > 1.7*sqrt(watts)..  Mind you, particularly at higher powers where rotary
 > gaps are more common, break rates enter into the equation..
 >
 > So, here's my wag, without actually looking over what folks currently use...
 >
 > Watts   Spark   Sec Height      Sec Diam
 > 500     38"     12-18"          2-4"
 > 1000    54"     20-30"          4-6"
 > 1500    66"     24-36"          5-7"
 > 2000    76"     30-40"          6-8"
 > 5000    120"    40-60"          8-12"
 > 10kW    170"    60-80"          12-16"
 >
 > One could probably also scale the size of wire for the secondary, by using
 > an assumption that somewhere around 750-1200 turns are wanted, so, for
 > instance, at the 2kW level, where your secondary is say, 36" long, you'd
 > have about 25-30 turns per inch, which corresponds to, say, AWG 18-20

<snip>

Perhaps I can inject a wiresize guideline here to add to take your
note further:

       Closewind: aim for a minimum copper diameter of 6x the
skindepth at the *lowest* projected operating frequency (i.e. with
the largest terminal you are ever likely to use).

       Spacewind (1:1 wire/space ratio): go for a minimum copper
diameter of 3x the skindepth at the lowest projected operating
frequency.

      I know from experience these will give nice high Q secondaries.
Dividing the winding length (i.e. height of the former) by the space
required per turn gives the number of turns (and by proxy the
inductance). Since the capacitance is highly dependent on coil
diameter and height and is pretty much invariant with respect to
siresize, you can pre-calculate the operating frequency of the bare
coil and using JavaTC or some other program, the lowest operating
frequency with a terminal added.

      These figures as guidelines only. You may wish to shoot for a
higher inductance and hence smaller wire for whatever reason (e.g. to
get primary inductance and Xp higher).

Malcolm