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Re: New Primary tested
Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>
On 16 May 2002, at 16:36, Tesla list wrote:
> Original poster: "Jolyon Vater Cox by way of Terry Fritz
> I have tested the new secondary (440t on 3" dia 10 7/8 high) with 4.7nf
> saltwater pop-bottle capacitor in primary tank.
> with full primary (11 3/4 turns) this resulted in poor spark length -less
> 1 inch.
> Best results were obtained with very small number of primary turns e.g..
> to 3 1/2 turns, when a circa 2" spark to ungrounded object was obtained.
> Calculations suggest the necessary primary inductance to resonate with the
> 4.7nF capacitor at a secondary self-resonant frequency (no toroid) of , say,
> 600kHz would be 14.97 uH -this figure looks somewhat dubious since the inner
> radius of connected spiral is 1.5" and the outer radius is 5" (the inductance
> of this was calculated to be only 1.8148148 uH )- although there are two 1.5
> metre lengths of neon cable between the capacitor/ spark gap and the primary.
> Might this not account for the missing inductance and if so, could I not in
> theory shorten the leads and use more turns on the primary?
Your calculation looks correct to me, and along with the actual
primary inductance suggests that the real problem with your coil is
that the primary has too little inductance, even taking into account
the lead length.
To check this, I just measured a 3m length of wire strung up
roughly as a 1 turn loop. I got about 5uH. Reducing the loop to a
parallel pair with a shorted end caused the inductance to drop to
around half that value.
The solution is to increase your primary inductance to 15uH and
get the leads coming from the cap and gap to the actual coil as short
as possible. You can always tap down a turn or two if you have to
compensate for the extra lead length. The gap and cap should be
considered an integral part of the primary circuit. It is probable
that you were exciting the resonator at a rather poorly responding
> Also -since a toroid was attached to the uppermost secondary turn- wouldn
> actual resonant frequency be lower than the self-resonant frequency and
> t the primary inductance have to be even larger than 14.97 uH to get a match?
Definitely. So the primary coil needs even more inductance. Not
having all inductance coupled to the secondary is not necessarily a
bad thing as long as it doesn't couple power into other energy sinks
which then robs the resonator of power.
> How is toroid capacitance estimated? Is there a reliable way of measuring
> toroid capacitance -I am wary of "non-referenced" capacitance (isn t
> capacitance always referenced to somewhere) and would like to know if/ how
> distance from the base of the secondary affects the capacitance of the
The distance from the bottom doesn't usually have as much effect as
the ratio of major toroid diameter to coil diameter does because
there is a degree of mutual e.s. shielding occurring between the two.
Bert Pool put a useful equation for calculating fee-space
capacitance of a toroid with a particular geometry to the list a week
or two back. However, that capacitance may be reduced once it is
placed on a resonator and may on the other hand be increased as it
will be in close proximity to other objects.
> The use of the pop-bottle capacitor in the circuit is precarious -the
> blew and the second is seen to be engulfed with violet corona under subdued
> lighting. I think 4 bottles in series-parallel would be less stressed and
> reliable -is this configuration likely to be OK?
I think it will be a lot more reliable once the coil is properly
tuned and the energy converted into output sparks. Right now, your
cap and gap are getting a thrashing and the transformer may not be
all that happy either.