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Re: flat strip primary
Sam, Bob, and all,
Well... not quite. Because the primary purposely has inductance, the
turn-turn voltages can be quite large during the time that the tank
circuit's energy is being fully stored in the tank capacitor. At the
very instant the main gap fires, the FULLY charged voltage of the tank
capacitor appears across the primary as the tank circuit begins to ring.
So, if your main gap's breakdown voltage is 20,000 volts, and you're
using a 10-turn coil, you'd expect to see at least 2000 volts between
turns.
But, it can actually be much worse than this! While a helical style
primary WILL share the voltage stress equally between turns, other
styles will not evenly distribute the inductance from turn-turn. With
other primary winding styles, such as a flat of inverse conical primary,
the outermost turns contribute significantly more inductance per turn
than the innermost turns, and the turn-turn voltage stress will tend to
be concentrated in the outer turns. For example, for a 10-turn flat
(Arcimedes spiral) primary, the last turn contributes about 18% of the
total inductance, so the worst-case inter-turn voltage between turns 9
and 10 in the example above will actually exceed 3600 volts!
Since this phenomena is purely driven by the inductance of the tank
circuit, it is virtually independent of the resistance (no matter how
low) of the primary inductor. Tesla used to demonstrate voltages
developed across low-resistance inductors by forcing large RF currents
through heavy copper busbars and then lighting incandescent bulbs
between various points along the bars.
You want to use at least 1/4" spacing between turns if you're only using
air as the insulating medium between turns. If you use LDPE between
turns you should be able to reduce this spacing considerably as long as
you have a relatively long "creep distance" along the poly insulation.
Because of dielectric interface effects between the air and the poly,
the RF between turns will "jump" a longer distance along the poly's
surface. Use at least a 1/2" margin of poly on each side of the primary
strap to prevent inter-turn flashovers along the surface of the poly and
you should be OK. This technique should result in a very nice, compact
primary.
Safe coilin' to you!
-- Bert --
Tesla List wrote:
>
> Original Poster: "Samuel Rosset" <samr-at-chez-dot-com>
>
> >Original Poster: bob golding <yubba-at-clara-dot-net>
> >
> >Hi all,
> > i just got a load of 20 mm x 1 mm(3/4" x .06") flat copper strip from the
> >local scrap yard. I think is used for lightning conductors. I am thinking
> >of making a flat strip primary with some of it. Does anyone have any
> >experience of doing this?. What i need to know is how much spacing I will
> >need between turns. I am thinking of using some 1 mm poly between the
> >turns, but how many layers do i need. I am using a 9 kv transformer rated
> >at 60 mA. Should I use the same rule of thumb as with caps and assume 300
> >volts per mil. How do I work out the equivalent dimensions to round section
> >to plug in to win Tesla?. This is not to much of a problem for me as I
> >have a bridge, but It would be helpful. So many questions not enough time.
> >
> >many thanks
> >
> >bob golding
>
> If I'm right, you want to use your strip your your PRIMARY right???
> Well, you don't need to worry about the spacing between turns. Electricity
> wont jump from one to another, because the difference of potential between
> turns is very low (copper is a good conductor). A primary is not a cap.
> between the plates of your cap, you have a difference of potential of 9KV
> (even more in fact), that's why you have to be concern about distance
> between the plates.
> So you don't have to worry about electricity jumping between turns.
> Nevertheless, spacing is important, for it will determine the inductance
> (and therefore frequency) of your primary. Use Ed Sonderman's spread sheet
> to design the "perfect" primary for your secondary...
>
> I hope It will help...
>
> Sam