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Re: NST and Bridge rectifier to stop res rise

Hi Richie,
               Thanks for an extremely interesting and thought-provoking 
response.  My suggestion about the bridge rect. was obviously far 
too quickly made and off the cuff. Now I have to think some more to 
see whether some good really could come of it.
      A possible scheme I once came up with involved DC charging 
and the use of some heavy SCRs with equally heavy back-
commutating diodes to carry the opposite half cycle current and 
possibly allow the SCRs recovery time for some chosen number of 
primary cycles. And yes - the ratings of these devices are in the kV 
and kA range. However, the dV/dt ratings are not wonderful. It's on 
the "something to try before I die" list.


Regards,
Malcolm

> Original Poster: "R.E.Burnett" <R.E.Burnett-at-newcastle.ac.uk> 
> 
> Hi, Malcolm and all,
> 
> Your suggestion of adding a bridge rectifier to the charging circuit
> to prevent resonant rise is very interesting.  The addition of the
> rectifier does "de-Q" the resonant charging circuit. However, there
> are a few ways other implications which came to mind.
> 
> If the cap is connected across the rectifier output there will be a
> problem.  The problem arises when the gap fires and the cap voltage
> tries to swing negative,  forward biasing the diode bridge.  This will
> drive the diodes into heavy conduction and spoil the primary ringdown.
> You could put a small RF choke in series with the rectifier output but
> I guess that is defeating the object.
> 
> If the spark gap is placed across the bridge,  the situation seems
> perculiar.  When the gap fires, the spark gap should conduct the
> positive half cycles of the RF tank current,  and the diode bridge
> should carry the negative RF half cycles which would normally also be
> conducted by the spark gap.  At the start of each negative going RF
> cycle, the diode stacks should start to conduct before the gap
> reignites ?  The positive volt drop will be the normal volt drop of
> the spark gap,  whereas the negative volt drop should be the sum of
> the diode forward drops. (ie. A lot less.)
> 
> At first sight, this seems like it would almost eliminate the high gap
> voltage drop on negative RF half cycles !  Therefore almost halving
> losses in the primary circuit.  I wonder if it happens this way in
> practice.  Maybe if the gap only conducts on positive RF half cycles
> it will run cooler, and exhibit a higher voltage drop, negating any
> benefit gained ?  I would like to know other peoples thoughts on this.
> 
> The diodes stacks would have to be suitably rated to hold off the peak
> HV voltage, and also to withstand the peak tank current I think. (I
> hope I'm not stating the obvious here, but the diode chains should
> have equalising resistors and equalising caps accross them.)
> 
> Has anyone tried this out with good or bad results ?  I would really
> like to know whether spark gap losses are reduced significantly.
> 
>      Cheers,
> 
>       - Richie,
> 
>       - In Sunny Newcastle.
> 
>