Re: cap firing voltage scope measurements question

Hi John,

> Original Poster: FutureT-at-aol-dot-com 
>  All,
>  I'm seeing some strange results in my TC:
>  Here are some cap voltage measurements under different conditions:
>  I set my input variac at 60, and the scope indicated the output voltage
>  of xfrmer was 15.2kVpeak, without the TC connected.
>  At the same variac setting, but with TC connected, but with the 
>  spark gap removed, the cap voltage reads 22.3kVpeak.  I suppose
>  I'm getting some resonant charging here with this particular ballast
>  setting.
>  At the same variac and ballast setting, but with the gap firing 
>  (120bps), and the TC producing sparks, the cap voltage reads
>  30.4kV.  This means I'm obtaining a 2X voltage increase during 
>  ~1/2 cycle, I thought this degree of resonant build up was possible
>  only when using DC charging?
>  Does this seem normal?  I would have expected the voltage to be
>  lower with the gap firing since there's less time for resonant voltage
>  build-up.  In there a gap in my understanding?  Or is my scope
>  reading wrong?  The 30.4kV makes sense from a Joule perspective,
>  since it comes out to 840 cap watts or so, and the meter reads 1000
>  watts, which seems about right. 
>  Mystery, or insanity?
>  Thanks,
>  John Freau >>

My reading of that is when the gap fires near the transformer voltage 
maximum, the supply is effectively slammed across the ballast forcing 
it to store energy rather rapidly in a period when it wouldn't store 
any without the gap short. The stored energy adds to the transformer 
energy to recharge the cap. This mechanism in a resonant charging 
situation does allow a theoretical 2x voltage maximum.
     The same mechanism is what allows a gap to be opened once firing 
in some systems to a point where it would otherwise be unable to