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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
start.
Malcolm