RE: Cap charging

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

Original poster: "Luke" <Bluu-at-cox-dot-net> 

Wow! Confusing.

So lets say the capacitor is a lot smaller than res size.
Can the cap ever achieve more than the 21KV peak of the NST in one half
cycle?

If the cap were much greater than the res size I would assume it would
not reach a greater value because its rc time constant would dictate
that it would never get fully charged any way.  Is that correct?

So I am a bit confused as to how the cap can reach a voltage higher than
the 21KV NST voltage in one half cycle.  I understand it is due to
resonant rise.  Is there an analogy one could use to help as an aid in
picturing what happens?  Math is cool and I am all for it so if that's
what it takes go for it.  But I was also hoping there would be some easy
way to picture the concept of how it happens.  It just confuses me that
if there were never more than 21KV there in the first place how did the
cap get charged higher (in one half cycle)?

Luke Galyan
Bluu-at-cox-dot-net

-----Original Message-----
From: Tesla list [mailto:tesla-at-pupman-dot-com]
Sent: Wednesday, January 21, 2004 8:28 PM
To: tesla-at-pupman-dot-com
Subject: RE: Cap charging

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