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My FET T.C.

Ken,

You're doing very well with solid state coiling.

>2.  Input current (so far), about 4A -at- about 50 sparks/sec.  Spark rate
>variable from a hand-held "wand" from 1 at a time on up.

I take that this is some kind of interupter so that you can switch it on
and off at 50Hz

>3.  Driver-- 24, 85A, 500V power MOSFETs electrically in push-pull
>parallel, together driving the equivalent of 1 primary turn (in a
>patent-applied-for circuit).  Soon to be increased to 48, then--if my
>endurance holds out--to 72.  That will double, then treble the spark
>length (and the input current for a given spark rate).  The primary is
>untuned.

Do you mean 85A or 8.5A mosfets - why so many? they will handle
a peak current many times greater than the continuous current rating.
I thought that 8 was a lot. There seems to be some flexability about
the way that the term "push pull " is used by folks - I take it that you
mean that
both sides of the circuit are idendical low side switches driving a center
tapped primary.

>Soon to be increased to 48, then--if my
>endurance holds out--to 72.  That will double, then treble the spark
>length (and the input current for a given spark rate).  The primary is
>untuned.

you will increase it - you may not double it - there is a law of
diminishing
returns. Also FETs can be killed by over voltage. I tried this and managed
to fry quite a few FET's.  The problem is that when the FET's switch off
and
the magnetic field collapses a very high voltage is induced on the primary
- it could easily exceed 500V. Also if by resonance we achieve a very high
voltage on the secondary some of that is reflected back to the primary.

>4.  Maximum voltage, other than at the ~110 KHz output frequency, present
>in the primary apparatus at any time with any quantity of MOSFETs & at
>any rep. rate, 160V.  That's one hundred and sixty volts.  Safer for the
>kiddies.

And the FET's

>5.  Excitation frequency, always the instantaneous self-resonant
>frequency of the secondary.  

Which is drops when a spark is produced.

>6.  Secondary, about 1400T "pie"-wound -at- ~11T per pie.  Turn-turn
>spacing, n.l.t. 0.1".  Wire, mostly18 AWG tinned Cu with some 14 AWG
>phos.-bronze spring-wire for outer turns.

please expain pie wound? 

>7.  Top electrode, currently 4" c.s.d. x 14" mean diameter.  In the next
>few days to be increased to 5" c.s.d.  Material, Al flex-duct.

Bigger sparks as long as you have enough voltage to break out

>I hope before too long to peddle my plans for a pittance.  In the
>meantime, I'd value hearing from anyone making something similar.  But
>post it to the List!

You can see my stuff at 
http://ourworld-dot-compuserve-dot-com/homepages/alansharp/

Alan Sharp (UK)