RE: Help! My Coil Blew Up

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com> 

 >Any ideas, EE?  :-)

I think secondary base current feedback is the way forward, using either an
RC circuit with adjustable R, or a sweepable bandpass filter, to produce
adjustable phase shift. But all feedback schemes suffer from the same
problem, in that heavy streamer loading tends to kill the "Q" of the
resonance. So the drive circuit needs to also have an oscillator to keep it
going if the feedback won't.

Also, the Tesla resonator has several resonant modes, and the driver circuit
needs to be "told" which one to excite. The antenna feedback circuit favours
the fundamental, but not by much. I have heard some people mention that
their feedback SSTC jumps to the 1/2 wave mode when pulling arcs to ground.
Sure it probably makes the arcs fatter, but the high switching frequency can
kill the FETs. That is another reason why I favour putting a bandpass filter
in the feedback circuit, it will make sure it stays on the fundamental (1/4
wave) mode all the time.

There is a guy over here who has done great work with the UC3872 resonant
lamp ballast controller chip in place of the TL494, driven off a base
current transformer. There should be details on his website

http://thedatastream.4hv-dot-org/

If I ever build a "real" SSTC (rather than an OLTC) I'll do it in a similar
way to what he did.

Finally (getting more off the wall) there is a nice old-fashioned oscillator
circuit (the bridge oscillator) that uses a series tuned circuit to ground
as the resonator. A high-powered bridge oscillator could base-drive a Tesla
resonator directly and would lock to the resonance with no other form of
feedback needed. Adding another tuned circuit with a variable capacitor
somewhere in the loop would keep the oscillation going under streamer
loading, and allow phase adjustment, as well as defining the mode.

This scheme would be best with tubes rather than solid state because of the
high voltages needed for direct base drive. I'm actually tempted by this
one, but my house is full of Tesla coils already :(

Steve C.