RE: PlasmaSonic II tweaking

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

Original poster: "David Trimmell" <humanb-at-chaoticuniverse-dot-com> 

Dan, I have everything working pretty well now, but volume is still not
as high as I would like it, and base response is only ok. I am using a
Bogan PA amp to boost the CD players signal with 9K in series with the
audio out. This amp has an equalizer and compression controls which
helps a lot with audio quality. On my tube setup I could get deafening
volume levels, but I can see audio quality and consistent operation are
much better with this design. I am somewhat concerned about the
reliability of the FETS, perhaps I should look into getting some 600V
100A ones? ;-) It still blows me a-way that I could blow those FETS when
only pulling <200 watts, yes I know the instantaneous power can be many
orders of magnitude higher, but...

I have a video of operation here:
http://www.chaoticuniverse-dot-com/webdoc2.htg/SSTC/solid_state_tesla_coil.h
tm

Thanks for your assistance in getting this to work. Still much tweaking
to be done but that what I love to do except when I blow up $$ FETS!

David Trimmell

-----Original Message-----
From: Mccauley, Daniel H [mailto:daniel.h.mccauley-at-lmco-dot-com]
Sent: Sunday, November 23, 2003 6:46 AM
To: humanb-at-chaoticuniverse-dot-com
Subject: RE: PlasmaSonic II tweaking



BIAS will not kill your coil.  It basically works like this . . .

The TL494 PWM IC has an adjustable duty cycle from about 0% to 25% i
believe when using push-pull mode.
This duty cycle is controlled by the signal going into pin4 (BIAS) on
the TL494.  Duty cycle can be controlled via
other ways with the IC (i.e. error amplifiers etc...), but we aren't
using them for this particular circuit.
Now, the audio input to pin4 is an AC waveform which is capacitively
coupled to pin4 (BIAS)  So, the AC waveform (audio)
will have peaks above and below the BIAS voltage at pin4.  Ideally, you
want BIAS to be set at about 12.5% duty cycle or
half-way.  This way, with the proper audio level, you will get maximum
duty cycle range.  From 0% to 25%.  However, in
practice it may be a bit different depending on audio etc...  In my
coils, i usually have BIAS set at about 18-20% duty cycle.  It all
depends on the audio level, how your coil operates, etc...

Even if the BIAS was completely maxed out, it shouldn't really affect
the reliability of the coil that much.  I think i said that a high BIAS
would put out those thin long sparks in my last email, but i might take
that back.  I think there are a
few other causes.

One guess is the fact i think you said you tuned a bit offset of the
frequency.  I think if this was done, you might get
some serious hardswitching compared to having the coil tuned right on
frequency.

As far as testing FETs, just ohm them out with the ohmmeter and compare
G to S, S to D, D to S, S to G, and any combination
thereof among your four FETs.  If a FET is blown, it will be the one
with the different numbers.  I've blown quite a few
FETs in my day, although i got my two plasmasonics working now very
reliably.

Hope this helps.

Dan