Ramsey Electronics PG13, Instruction Manual

Page: 12 / 44

PG13
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12
CIRCUIT OPERATION
What is going on with this board may look simple at first, but it is actually
quite a difficult design to get working properly and reliably. A lot has to be
considered with magnetics when dealing with high voltage, high frequency
transformers. Unlike power transformers like the one powering the entire kit,
high voltage transformers have a “sweet spot”, or a resonant frequency where
they operate the most efficiently. The goal of the design is to get it working
above human hearing, otherwise the screech of high frequency from a plasma
discharge is deafening. When choosing the transformer for this design, I
wanted the best of everything: High Voltage output, High Current, High
Resonant Frequency, and the ability to generate this from a relatively low
voltage.
The transformer company we found delivered four different transformers to
us to experiment with, and a bunch of plastic spacers of varying widths. The
four transformers had increasing numbers of secondary windings, but all other
factors similar. The problem is that the more windings there are on the
secondary, the more inductance there is, meaning the resonant frequency
would be lower. The largest transformer which had 6500 turns in the
secondary would have been perfect to get 12 volts up to 25kV using a low
number of turns on the primary, but the resonance was around 13kHz. The
sound this emits is intolerable for any length of time. The coil also had the
problem of having very thin wire resulting in a low current output. They have to
use fine wire to make it fit in the transformer’s plastic case.
The smallest coil had 2000 turns on the secondary, which isn’t quite enough
to get 25kV from 12V, even in a push-pull configuration. The problem here is
we really need more than one turn of wire on the primary to make an effective
output. An advantage would be that the transformer oscillated around 35kHz,
well above hearing, but almost too high for some effects we would like to
make.
The transformer we wound up using was the third size, which has 4000
windings on the secondary, which gives us plenty of high voltage output. It
also has the larger sized wire, and with the proper spacers would oscillate
right around 18kHz. This frequency is above most people’s hearing, but your
dog won’t like this too much.
So what do those spacers do? Without getting into magnetics too much, they
lower the saturation point of the ferric core. This means the core saturates
faster with a larger gap, which also translates to a higher operating frequency.
This also means, however, that since the core saturates faster, less energy
will be transferred from the primary to the secondary, which reduces power
output.