Ramsey Electronics PG13 Instruction Manual page 13

We have included two 0.25mm spacers for you to do experiments with.
Since the transformer is specific about its "sweet spot", we couldn't run the
drive circuit directly from a pulse width modulator circuit (PWM). We may have
been able to tune it up really close while there was no load applied, but as
soon as we would draw a spark, the frequency would change, and our output
would drop considerably. For example if the "sweet spot" was 20kHz, and we
were driving the circuit with 20kHz, we may have 20kV on the output. Then, if
we add a new load on the output that changes the "sweet spot" to 19kHz, but
we are still driving it with 20kHz, the output may drop to only a few kilovolts.
Because of this we decided to make the transformer self-resonating. This
means as the load changes, so will the frequency, so that the transformer is
always running in the sweet spot.
The way this oscillator works is by alternately saturating the core, first in one
direction, then the other. R1 and R5 are used as a "kick start" for the oscillator.
These resistors provide some current to turn the transistors on. Because no
two transistors are perfectly alike, one will turn on before the other, providing
an imbalance.
In Figure 1, The transistor that is turned on stays on, forcing the other
transistor to turn off by directing current through the feedback winding of the
transformer in the direction required to turn the other transistor off, and turn
itself on even harder. For this illustration we'll say that Q3 is turned on while
Q4 is turned off. As this is occurring, magnetic flux in the core is building along
with the current being drawn through Q3, because of the side of the winding
Q3 is attached to. This current is drawn through the center tap of the primary
winding through the winding, and finally down through Q3 to ground. This
rising magnetic flux in turn is inducing voltage and current in the high voltage
secondary, as well as the feedback winding. This current in the feedback
winding pushes the transistor on even harder up to the point that the core of
the transformer saturates.
Fig 1.
+12V
Direction of Flux
+V High Voltage
-V High Voltage
Fig 2.
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PG13 13
Direction of Flux
+12V
-V High Voltage
+V High Voltage