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Re: Ground Plane/ Volumetrically Inefficient Secondary?

Original poster: "Alexander Rice by way of Terry Fritz <twftesla-at-qwest-dot-net>" <alex-at-rices.myip-dot-org>

26/05/2002 20:05:25, "Tesla list" 
<tesla-at-pupman-dot-com> wrote:

>Original poster: "Jolyon Vater Cox by way 
of Terry Fritz <twftesla-at-qwest-dot-net>" 
>My TC is presently grounded via a water-
pipe- there is no ground plane,

unless you are operating your coil 
floating freely in deep space then you DO 
have a ground plane - it is the thing you 
are standing on called 'earth', that is 
why it is important to have a good eath 
connection. The only time people sometimes 
make an artificial ground plane out of a 
whole load of AL foil on the ground is 
when an ordinary ground is not availiable 
eg. at teslathons

 only a
>bolt at the base of the secondary which 
serves as the connection to the earth
>lead and the primary return to the spark-
gap and the ignition coil.
>The primary is mounted on terminal blocks 
on 1/6" thick plywood board. The
>outermost primary turn -not surprisingly- 
has highest RF voltage- it draws
>fierce sparks to any grounded metalwork 
within a proximity of about 1/4 inch
>and develops a little corona in operation 

probaly doesnt matter

>What about the magnetic effect the ground 
plane- might  it not be better to use
>a star-shaped ground plane rather than a 
solid sheet of metal or foil to
>prevent induced current from flowing in 
the ground plane and causing energy

yes, if you were going to have a ground 
plane, or any other conductive opject in 
close proximity to the primary coil it 
needs something to stop it being a shorted 

>The primary a flat spiral consisting in 
its entirety of 11 3/4 turns with a max
>dia 12" (it is tapped at the fifth-and-a 
half turn) -given the power from the
>ignition coil is little over 40 Watts at 
best, might an alternative winding
>style eg. inverted-cone or helical be 
advantageous providing tighter
>primary/secondary coupling

possibly - i cannot say, my first OBIT 
(230w) coil had a solenoid primary and 
that had arc-over problems - i have never 
tried anything as small as 40w

>The web page 
>has a formula called "Available 
Transformer Power per Cubic Inch of 
>Coil Volume" or AVP, for short. basically 
this is the power density of the
>secondary for a given power input. The 
author suggests that for best results
>AVP should be in the region of 2.5 Watts/ 
cubic inch of secondary. The author
>also has formulae for predicting 
diameter, aspect ratio and (by simple
>calculation) height, for given input 
power, all potentially useful information.

whilst this presents an interesting 
approach to a normally guesstimated 
variable it is by no means law - other 
things will work

>My secondary ( built before I had 
knowledge of the above info) is a 3" dia 
>10 7/8" high, 440-turn winding of 24 SWG 
enamelled copper wire.

for such low power 10" is fine, 3" is a 
bit fat but not a problem in itself, the 
main problem is going to be that you have 
relatively few primary turns which is 
going going to push the operting frequency 
right up (my guess is about 1MHz) which 
reduces sparke length, that said it will 
still work, my first coil also was of 
unuasually high frequency - but i thought 
it was great! Now i need MORE power.... 
must have POWER uughh.... sorry, i forgot 
my medication this morning ;~) 

>Now calculating AVP for the coil 
parameters for my secondary on my computer
>spreadsheet, I have been unsuccesful so 
far in finding a transformer power
>where AVP was 2.5 Watts/ cubic inch.

well since your coil has a volume of 
(1.5"^2)*pi*10" = 70 CI then that suggests  
about 180w transformer power which seems 
reasonable enough, however if you were to 
make the coil even shorter you would have 
to use impractiaclly small wire to get a 
decent number of turns in.

 Does this mean that my secondary is
>volumetrically inefficient?

probably - who cares!

 Is there a near-universal formula for 
designing a
>good secondary?

No, but there are genaral guidelines, it 
should have about 1000 turns (anything 
between about 800 and 2000 is ok), its 
aspect ratio should be around 5:1 (very 
aproximately) and its diameter is based on 
the transformer power, 3" is probably good 
for about 700w, 4" --> 1500w, 6" --> 3500w 
8" -->6Kw+

my advice would be to start by winding a 
similar size former with smaller wire 
aiming for about 800 - 1000 turns which 
will allow you to make better use of the 
existing primary coil (it will tune 
further out) as well as lowering the 
operating frequency of the coil



Alexander JJ Rice
Rank : G-2 (Wahey!)
"Because the Geek Shall Inherit the Earth