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Re: Voltage Reversal of Pulse Caps

Hi all,
        I've read some comments on the above topic and would like
to inject a comment or two. Regarding reversals, When you discharge
the capacitor into the primary coil, it firstly discharges to zero
volts and then swings on through to end up with some voltage across
it of polarity opposite to that which it originally had. This voltage
is somewhat less than it started out with because the magnetic field 
generated is shared between the two coils, both of which end up with
a proportion of the original cap energy. What voltage the cap ends
up with on the first swing (with opposite polarity) depends upon
energy absortion by the secondary (good tuning helps and coupling is
also a factor) and how much cap energy is wasted in the primary spark 
gap. This is where low primary Q can waste a lot of energy. This first
reversal is IMO the main determinant of cap life regarding reversals
as it is at the end of the first swing that the capacitor will have
the highest reversed voltage across it. Note that these reversals are
occurring at the resonant frequency i.e. 10's/100's of thousands of
times per second. Two things minimize the number of reversals. One
is the rate at which the secondary gets rid of its energy share in
the output spark, and the other is a function of gap quenching - how
long the gap is allowing the repeated swings to continue. Some recent
single-shot experiments I did with my coil and scope show that a 
healthy secondary discharge minimizes the number of reversals at
fr considerably.
    The above does not take into account opposite polarity transformer
charging which occurs at mains frequency. 
    Any other comments are most welcome.
    
Malcolm