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Re: First coil- need help

Original poster: "Malcolm Watts by way of Terry Fritz <twftesla-at-qwest-dot-net>" <m.j.watts-at-massey.ac.nz>

Hi Jolyon,

On 9 May 2002, at 17:14, Tesla list wrote:

> Original poster: "Jolyon Vater Cox by way of Terry Fritz
<twftesla-at-qwest-dot-net>" <jolyon-at-vatercox.freeserve.co.uk>
> I have rechecked- the primary OD is 9" and the ID is 3"
> The height of the secondary is 8.125" (almost as big as the primary
> diameter) so I
> presume they are comparable.
> Ammeter says 3 A on the primary of the ignition coil -I am using a Radio
> Shack
> 13.8VDC 3A regulated supply so max power would be 41.4 Watts.
> I am using a 7 tube zigzag gap with 1mm between the copper tubes -which are
> 15mm
> in diameter, so if breakdown voltage is 3.3MV/ metre between rounded
> electrodes the
> max voltage (perhaps) is 19.8kV.
> The ignition coil-based HV supply has been improved with the addition of a
> diode
> string (60 x 1N4007 so total PIV = 60kV) to prevent the capacitor
> discharging back
> into the HV winding - this has resulted in more reliable firing of the gap
> especially
> with larger capacitors; sadly these do not seem to give good performance -is
> because they lower the primary resonant frequency so far it becomes
> impossible to get a good "match" with the secondary?

Those diodes have very slow recovery characteristics and aren't 
terribly good for rectifying high frequencies. In fact at high enough 
frequencies, they'll behave more like resistors.
> Why is spark length to grounded object less than spark length to ungrounded?
> Is this anything to do with the secondary being more in tune with the
> toroid-to-ungrounded-to-ground capacitance than it is with the ordinary
> toroid-to-ground capacitance perhaps?

That is a characteristic I have noted with both small coils running 
from low power, and larger coils with very poor primary and/or 
secondary Q's.