Re: Help! My Coil Blew Up

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

Original poster: "S.Gaeta" <sgtporky-at-prodigy-dot-net> 

I didn't think it was possible for a Feedback design to oscillate at the
wrong frequency, unless the primary is phased wrong. Actually had it right
the first time, then switched it to the wrong phase because the oscillator
wouldn't start at all, but that turned out to be because my 555 was adjusted
wrong. The wrong phasing caused the circuit to oscillate at the coils second
harmonic once I tweaked the 555 frequency trim pot a bit. When I switched
back to the right phase, the circuit oscillated slightly higher(236KHz) than
what I measured Fr to be when I drove the secondary with the signal
generator(221KHz).

I didn't like the idea of an antenna either. It seems to leave the door wide
open for all kinds of noise gremlins to just walk right in. Especially since
there is no tuned circuit after it. I put a current transformer in the
secondary return when I was playing around with 12 volts, and just put a
scope across it. The voltage out of the CT was much too small to work as a
feedback signal, unless I added myself to the output terminal by touching it
with a hand held piece of metal. Then I could see the frequency drop
tremendously, and the voltage greatly increase. I am not sure if it was only
because I was sinking a lot more current out of the coil, or because the CT
was responding better to the lower frequency. That's what stinks about
blowing stuff up. You can't do any further testing until it's fixed!

Thanks for your input,
Sue
----- Original Message -----
From: "Tesla list" <tesla-at-pupman-dot-com>
To: <tesla-at-pupman-dot-com>
Sent: Thursday, February 05, 2004 10:16 AM
Subject: RE: Help! My Coil Blew Up


 > Original poster: "Steve Conner" <steve.conner-at-optosci-dot-com>
 >
 >  >I guess in spite of the age of the individuals involved, this SSTC crowd
is
 >  >just another "old boys" club.
 >
 > Touch?e
 >
 > The reason you probably got no replies is that there is just so many
things
 > it could be. SSTCs are complicated as heck and there are so many factors
 > interacting.
 >
 > Theoretically, if you are driving all four FETs off a single gate drive
 > transformer, it should be impossible for them to cross-conduct. I think
what
 > is more likely is that your FETs overheated due to poor gate drive. It
could
 > be too much ringing on the gate waveform causing them to fall in and out
of
 > conduction, or slow rise/fall times causing excessive switching losses.
When
 > they overheat, they fail short circuit. If one FET shorts, it will take
out
 > the other one on that side too.
 >
 > Another possibility is that your drive circuit was driving them at the
wrong
 > frequency to get zero current switching. If the frequency is slightly
below
 > resonance, the resonator load starts to look capacitive. (See Richie
 > Burnett's website, http://www.richieburnett.co.uk ) A capacitive load
forces
 > the FETs to switch high currents which again could overheat them.
 >
 > I tend towards the "wrong frequency" explanation myself. The antenna
 > feedback circuit isn't very "scientific", the phase shift (and hence the
 > oscillation frequency) is set by a bunch of things that aren't well
defined,
 > like the capacitance of the antenna clamp diodes and so on. As an EE it
 > gives me hives just looking at it.
 >
 > When reading the above, bear in mind I've never built an ordinary
"inverter"
 > type of SSTC, so it might be complete BS. If anyone out there is fond of
the
 > antenna feedback circuit, flame away, but you won't change my mind :P
 >
 > Steve C.
 >
 >