I was firing up my magnifier last night and ran in to a number of
problems with what appear to be spurious resonances in my primary circuit.
I was using a 9 kV 150 mA power factor corrected (PFC) transformer. I have
doorknobs to ground and a safety gap across the transformer, followed by
iron powder chokes and air wound chokes in series with each transformer lead
between the safety gap and the spark gap. I use about 20 feet of 50 kV
cable between the two spark gaps. I am using quite tight coupling (about
k=.5) and my tune point was near where my 'scope said it should be (a little
lower frequency due to corona buildup). I was getting 3-4 foot sparks out
of my extra coil with about 1 kVA input.
The transformer calls for a rather large power factor correction
capacitor (I think around 15 uF). Initially, I had removed it from the
circuit and everything worked quite nicely, although my primary current was
quite high. When I added the PFC cap, my driver coil started arcing, and my
safety gap started firing. It appears that the PFC cap has somehow set up a
spurious resonance which is manifested on the high voltage side of the
transformer. My safety gap should not fire unless the chokes are creating a
resonant circuit and somehow interacting with the PFC cap. Anyone else
notice this problem? Tonight, I hope to add a couple power resistors in
series with the chokes to see if it quiets things down a bit, or I may throw
out the doorknobs and chokes all together since the spark gap is across the
Mark S. Rzeszotarski, Ph.D.