Re: [tcbouklist] DRSSTC First light and bizarre problem

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: Steve Conner <steve@xxxxxxxxxxxx>


> If HV flashed over to
> the primary it may have jumped over one of the GDTs in an attempt to get
> back to earth.

I found some pretty strong evidence that it did:

http://www.scopeboy.com/tesla/drsstc/firstlight/tracking.jpg

This was totally invisible until I breathed on the secondary- then it 
appeared in the condensation! Anyway the tracking all seems to come 
together at a point where the join between the two plastic insulators is. 
That is a weak point in the insulation and the primary is directly on the 
other side.

As for jumping across the GDTs, the AC supply is through an isolating 
tranny so I was able to ground the -ve rail of the DC bus. But there is a 
fair amount of inductance between the DC bus ground and the driver box 
ground (they both run in separate earth wires back to my control point) So 
a high voltage could still appear across the GDTs. Maybe i should tie the 
grounds together right at the coil?


> Shouldnt the 10 ohm damping resistor be in series with the DC blocking
> capacitor feeding the GDTs ???

I have one 10 ohm resistor on each IGBT gate for the HF damping. This one 
is to damp the low frequency resonance between the GDT magnetizing 
inductance and the blocking cap. It's not a problem when driving IGBTs 
directly but Steve Ward said he had problems with it when using booster 
stages, so I wanted to prototype this method of getting rid of it. It 
seemed to work fine.


Steve