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Toroid construction hints



Original poster: "Ian McLean" <ianmm-at-optusnet-dot-com.au> 

Hello coilers,

I have a question regarding the use of aluminium corrugated ducting for
toroids.

I have two toroids on my system.  Bottom one is 2.5"x17" with a SS plate
(pizza plate) in the centre.  This joins via a 0.25" central hole to the PVC
bolt on the top of my secondary, and sits about 1 inch above the top
winding.  Top toroid is 6"x28", with no centre plate, it just sits on top of
the bottom toroid.  Looks great.

When run, the streamers come out nice and horizontal from the top toroid.
No breakout from the secondary winding is noticed, no racing sparks, and no
visible corona anywhere to speak of.  I have however noticed a small problem
...

My bottom toroid is foam noodle, carefully joined with epoxy, and very
carefully covered in aluminium foil strips, each glued down with sparing
amounts of PVA glue.  The SS plate has a 0.5" lip, so I cut a 0.5" slit
around the inner diameter of the foam noodle and the lip slots into this
slit the entire way around and is held firm.  It came out really smooth and
nice and (although I was worried about it with the glue) conducts around the
entire toroid from the central plate.

My top toroid however is 6" stiff aluminium corrugated ducting, held at the
join with aluminium tape ("FlashTac"), that stuff with the thick layer of
bitumen on the back.  That is all I could find in aluminium tape.  I cannot
get it to conduct to the toroid because of that thick bitumen adhesive, and
the coil favours this spot on the toroid for break out.  Why would it pick
this spot, when it is not conducting to the toroid?  Is it because it is
capacitively coupled to the toroid?, or is it just simply because it
provides a rough spot for breakout?  I thought I had smoothed the tape out
pretty well, but it is certainly not as uniform as the ducting itself.  Any
ideas anyone how I can improve this situation ?

Ian
Ian.