Top voltages was Re: Safety gap issues

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> Hmmm... That is.. the secondary inductor is still putting 
> current/charge into the topload, and the charge isn't being lost 
> fast enough through corona and streamers.

That does seem to be what happens. It's mostly noticeable with 
DRSSTCs rather than classic coils, for two reasons-

1) These coils typically use a breakout point. It's harder to push a 
smooth toroid with no point over its breakout voltage, because it has 
a large surface area where multiple streamers can appear all at once.

2) The DRSSTC works "funny" compared to a classic coil. A classic 
coil takes a lump of energy in the primary capacitor and transfers it 
to the topload capacitance where the streamers then eat it. A DRSSTC 
uses its primary and secondary as a matching network, to transfer the 
energy straight from the inverter to the streamer load. You can't 
think of it as a two-stage process any more- "tank capacitor to 
topload, topload to streamers"- since the total energy transferred 
can be ~5 times greater than either the tank capacitor or topload can 
store. This is not just speculation, we have measured it experimentally.

Anyhow, what this means is that you get quite a long, continuous 
burst of energy instead of it all getting transferred in 3 cycles or 
whatever. It's easy to see what is going on during the burst 
especially if you use a plane wave antenna or similar.

What I noticed on my DRSSTC: If I fire it without a breakout point, 
the output voltage leaps to the breakout voltage of the smooth 
toroid. Then the streamer forms and the voltage is suddenly pulled 
down to about 0.5-0.7 of the breakout voltage, where it stays until 
the end of the burst. The exact number depends on DC bus voltage- the 
higher I turn this, the better the output voltage holds up.

With a breakout point, there is no initial leap, but the behaviour 
from then on is pretty much the same. The envelope just rises to 
about the same voltage as previously (ie ~0.6 of the toroid breakout 
voltage) and stays there until the end of the burst. The thing to 
note is: The voltage it stabilises at is far above what it takes to 
make corona appear on the breakout.

Steve Conner
http://www.scopeboy.com/