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Ground Plane/ Volumetrically Inefficient Secondary?
Original poster: "Jolyon Vater Cox by way of Terry Fritz <twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
My TC is presently grounded via a water-pipe- there is no ground plane, 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 all of may not be a good idea to put
the ground plane on the reverse side of the primary substrate. Would it be OK
to move the ground plane to a small distance below the primary to prevent
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
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 and greater separation of the "hottest" parts of the
primary from the ground plane?
The web page (http://www.geocities-dot-com/lemagicien_2000/tcpage/techtc/tcd.htm)
has a formula called "Available Transformer Power per Cubic Inch of Secondary
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.
My secondary ( built before I had knowledge of the above info) is a 3" dia by
10 7/8" high, 440-turn winding of 24 SWG enamelled copper wire.
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. Does this mean that my secondary is
volumetrically inefficient? Is there a near-universal formula for designing a