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RE: Terry's DRSSTC - First light ;-))

Original poster: Terry Fritz <teslalist@xxxxxxxxxxxxxxxxxxxxxxx>

Hi Steve,

At 05:36 PM 3/7/2005, Terry Fritz wrote: 
Hey Terry,

Congratulations on your first light! It sounds like it went very well with
no _unexplained_ failures.

Thanks :-)) Yes, all the failures have indeed been well explained! Hate to have to scope IGBT blowups one after another trying to track down some odd glitch somewhere.... But the HC logic and even the optical gate drivers seem to function perfectly well :-))) I have all those that have gone before me to thank for working out the rough parts ;-))


As to what happens when an arc hits the primary. The di/dt caused by a
streamer hit is colossal. You're basically connecting a big metal lump
charged to half a million volts to your circuit, through about a foot of
"wire". The streamer channel oscillates around its quarter wavelength
frequency (less because of toploading) which on your coil would be about
40MHz.


Yep!!


If you take the whole thing to be a resonant circuit ringing at 40MHz, with
15pF capacitance charged to 500kV then you can see that one quarter cycle
later the peak streamer current will be pretty big because the inductance is
so small. I got over 10 million amps in a quick back of the envelope
calculation.

Sounds a little "high"!!, but the current is certainly enough to mess everything up in a number of ways... The H-bridge and logic just cannot take that kind of hit and remain stable.


Obviously that's not true because the streamer channel is lossy, but it
still shows that there is a LOT of RF current trying to fight its way to
ground through your primary and H-bridge. Since it's such a high frequency
it will cause thousands of volts worth of di/dt voltages to appear across
"ground" wires and such like, screwing up your circuit completely.

I have thought long and hard about how to protect against this and the only
solution I can think of is "Don't let sparks hit the primary EVER"

The strike rail should fix it. But Steve (Ward) seems to have still had some problems.... Have to keep an eye on this one.



Also I noticed Steve Ward said-

>I put metal caging around the base
>of my coils, which seemed to work fine until i switched to primary
>feedback (now the fence sucks up a lot of power for some reason)

It sounds like you switched poles and are on the lower one now. I had a
third winding on my dummy resonators connected to a light bulb to help me
with tuning. I found that on the upper pole, the light bulb didn't light
even though lots of power was being transferred. On the lower pole it lit
brightly. The reason (as far as I know) is that on the upper pole, the
primary and secondary currents are 180' out of phase so their magnetic
fields cancel in the space around the coils. On the lower pole the fields
add. Strange but true.


YES!!  INDEED!!!!  A wonderful observation there!!

Cheers,

        Terry



Steve C.