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Re: term understanding: voltage reversal.
Original poster: "Malcolm Watts" <m.j.watts-at-massey.ac.nz>
On 18 Feb 2004, at 18:04, Tesla list wrote:
> Original poster: Jim Lux <jimlux-at-earthlink-dot-net>
> At 08:06 AM 2/18/2004 -0700, you wrote:
> >Original poster: "Christoph Bohr" <cb-at-luebke-lands.de>
> >Hello All.
> >I came across something I am no longer sure I understood that right:
> >In pulse cap data sheets there is usually a point called "voltage
> >reversal". To say it less technical I understand that voltage
> >reversal causes stress on the cap and is undesireable if you like
> >longelivity. But what exactly is this voltage revesal in a AC, sync
> >gap, TC? is it:
> >1.: The changing polarity of the carging current, i.e. the fact that
> >I once carge the cap with the one polarity and during the next half
> >sine wave to the other
> >2.: The changing polarity during the HF-"ringing". As the changes
> >happen more often and more rapidly here I feel that this is the main
> >voltage reversal relatet Stress on the cap.
> It is the latter... the ringing of RF
> waveform. http://home.earthlink-dot-net/~jimlux/hv/caplife.htm has some
> equations from Maxwell Labs on how to scale it. (and how to convert Q
> to VR...) For what it's worth, the usual TC probably has a loaded Q
> of 5-10.
In fact that would be true for a continuous output arc as in CW
coils. For a disruptive coil, the Q is essentially the open-circuit Q
of the secondary until ringup is almost complete. At that point the Q
dives but even that statement needs some qualification as the loaded
Q is still about 50% or so of the unloaded Q with air streamers
prolonging energy trades. Scoped waveforms show this all clearly.
High amplitude reversals are the norm for each bang under normal
running conditions for disruptive coils, at least until near-
completion of each initial ring-up. This BTW is why I like to design
high Q secondaries - more volts until a discharge issues forth.
> By the way, when you run the primary without a secondary, the Q is
> quite high, and that may be why this kind of operating is notorious
> for being a "cap-killer".
> For what it's worth, reversal is hard on a cap, but higher voltage is
> harder. The exponent on voltage is much, much higher than that for
> the reversal.