Re: [TCML] DRSSTC and targets to strike

[Steve Ward <steve.ward@xxxxxxxxx>]

So about that... in the case of that one system it seemed to protect the
IGBT, but i really dont hold high confidence that it makes the thing
spark-proof.  One thing that really is an issue is corona rectification, and
i was not aware of this when i posted that drawing.  Basically, the streamer
deposits a net charge onto the primary, which manifests itself as a large
common mode voltage vs ground.  Ive measured the voltage across the bus
bypass cap, and it was easily getting up to hundreds of volts after a few
wimpy sparks hit it.  So now ive added some bleeder resistors across this
capacitor (1M ohm), and also up its value to about 1uF for larger coils.
 Its always really hard to validate these protection schemes because the
failures can seem so random anyway.  The theory is to keep all of the
electronics assembly at the same RF potential so that no insulation is
stressed, and the best way to do this is with low inductance bypassing
capacitors.  But still, the discharge for a big toroid is *really* fast, so
its hard to be sure you are protecting things properly.  Another thought i
had was to put TVS diodes from each of the bridge outputs to ground, though
you'd want to be really sure that the TVS's couldnt possibly fail, otherwise
you'd create a fault condition on the bridge output.  I havent had much
opportunity to test these protection schemes.

So personally, i try to guard the primary with strike rails and sheets of
plastic (so the streamer can creep over the surface to the strike rail).
 And i put extra strike protection around the electronics as well.
 Basically just like lighting protection for a building... its best to keep
the spark current out of the circuit completely.

Steve

On Wed, Jul 20, 2011 at 8:26 PM, Justin B <boshjack@xxxxxxxxxxx> wrote:

>
> Bart, For this reason I used 0.1uF capacitors on the bridge and ground, to
> help prevent issues if my primary gets hit.  Followed this diagram:
> http://stevehv.4hv.org/DR_plexi/general%20wiring%20scheme.pdf
>
> I also used a bead on the GDT as suggested in the diagram, which did clean
> up scope waveforms.  Maybe steve can shed some light here, but if I'm not
> mistaken, it allows a streamer to hit your primary without things exploding.
>  I beleive this was how steve made his coils where you can see just a
> primary, and streamers hit the thing without it caring at all.
>
>
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