RE: Cap charging

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Lau, Gary" <gary.lau-at-hp-dot-com> 

Hi Luke:

You're asking all the same questions I did way back when...  There is much 
confusion about what actually occurs in a static gap - it's actually much 
more complex than a rotary!  Comments interspersed:


 >Original poster: "Luke" <Bluu-at-cox-dot-net>
 >
 >Can someone tell me if I have this right?
 >
 >Let's assume a static gap and an NST of 15KV 60mA for this question.
 >
 >If the gap is set to fire at 15KV the gap will fire at each peak of the
 >cycle (120bps).

No, this is what many folks assume, and the truth is counterintuitive.  In 
your example, you made no mention of cap size.  This profoundly affects 
what occurrs.  If the cap were correctly sized, what you describe would 
happen in a perfect world, but taxes will be abolished and world peace will 
prevail before that actually occurs.  In practice, the gap firing voltage 
will be reached slightly before the peak charging voltage is achieved, or 
possibly after that point.  In either case, this is not a problem, and 
energy in the cap is never "wasted" if the gap does not fire at the peak 
voltage. Any leftover energy in the cap beyond the peak of the charging 
cycle is used in the next charging cycle, even though that occurs in the 
opposite polarity.  I have some simulated wave forms showing the charging 
bahavior for various cap sizes and gap voltages; see 
www.laushaus-dot-com/tesla/gapsim.htm.  Much can be learned by looking at these 
waveforms.

 >If the gap is set to fire at 20kv the cap will continue to charge until it
 >reaches 20kv and then poof the gap fires.

Depending how close to the mains-resonant value your cap is, it may charge 
to many 10's of kV, until either the gap fires or something breaks!  It may 
take more than one half-cycle, this is what resonant rise is all about.

 >Do I have a correct analogy for how this happens?
 >I think of it like the NST being a battery and the cap being another
 >battery that will charge the cap.
 >So the NST will charge the cap to peak, then on the next cycle the NST's
 >polarity will be opposite so the cap and NST can act like two batteries in
 >series (like         + bat -  + bat - ) so now the NST will combine with
 >the cap will produce more and the cap will store that charge.  This will
 >continue with each cycle until enough voltage is built up to fire the gap.
 >Is this correct so far?

No, the NST is never in series with the cap as far as the gap voltage is 
concerned.  Voltages in excess of what the cap may charge to in one half 
cycle occur by means of resonant rise.  This is why a static gap will 
typically fire slowly even when the variac is turned way down.  And its why 
there's not a critical setting of the gap requiring the peak voltage to 
occur at the gap firing point.  There is no such sweet spot.


 >Now if the gap is set to fire again at 20KV and the cap is of a small value
 >so it charges up very quickly can the cap charge to the 20KV in one half of
 >the ac cycle?

Yes, it's common that static gaps fire multiple times per half cycle.

 >A post I found in the archives made it sound like it
 >could.  This confuses me.  Because in only one half of the ac cycle we
 >essentially have a dc voltage that is simply rising in value to a max.  and
 >if dc is applied to a cap no matter how high the dc voltage is the cap will
 >never reach a voltage over that.  So if in the case of a small cap that
 >charges rapidly can reach say the 20kv in one half cycle of the 60hz cycle
 >how does that happen?  If it cannot and it indeed does happen over
 >successive cycles then I would like someone to at least let me know I am on
 >the right track.

The peak voltage of a 15kV transformer is just over 21kV, so a small cap 
will certainly achieve that and fire a gap at 20kV, several times during 
the half cycle.  Or were you asking about a situation where the gap was set 
higher than the peak transformer voltage?  If so, the ability for the cap 
to charge above 21kV will depend on how close to mains-resonance the cap 
is.  If it's very small, it may not go much above 21kV.

Regards, Gary Lau
MA, USA