Re: Capacitor value not clear yet.

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

Original poster: "Gerry Reynolds" <gerryreynolds-at-earthlink-dot-net> 

Hi Luke,

 > Original poster: "Luke" <Bluu-at-cox-dot-net>
 >
 > Thank you very much for the replies on the toroid size question.  I will
 > have to digest more of that after I tackle a two fold question that was
 > brought up as a result of your replies.
 >
 > Question 1:
 > I was planning on 36" arcs based on the formula I found somewhere
 > []
 > where k is a value taken from a chart based on secondary dia.  in my case
 > was .85.
 > 1800 watts  (2 nst's 15KV 60mA ea. Totaling 1800 watts)
 >
 > is this figure way off?
 > The formula given to me in the toroid replies was
 > spark length (inches) = 1.7*sqrt input power (wallplug watts)
 >
 > Which is the correct formula to use for an approx.?
 > If I was going to drop to a single 15Kv 60Ma NST could I expect to
generate
 > 3 foot arcs?
 >

The 1.7 sqrt (wallplug watts) is a good formula that is based on emperical
data.  Some don't do as good and others exceed it.  One caveat is that it is
based on real power cord watts and not VA.  The transformer is rated at 15KV
(oc) and 60 ma (sc) only and should probably be considered as a VA rating.
If you run the NST with resonant charging, you can draw more than the 60ma.
If you use a static spark gap, you will probably not get more than 50% of
the VA rating as real power (PF = 0.5).  If you use a synchronous rotorary
spark gap, I've heard of power factors of 0.7 to 0.9.  So, ball park spark
length estimates are:

spark length = 1.7 * sqrt ( 0.5 * 1800 ) = 51 inches  (static spark gap)
spark length = 1.7 * sqrt ( 0.9 * 1800 ) = 68 inches  (SRSG)

 > Question 2:
 > I would like to keep the discussion on cap selection limited to NST's
since
 > that's what I intend to use and to keep from getting too much info for my
 > tiny little brain all at once.
 >
 > I understand the idea of resonant rise in the nst.  Or at least enough to
 > see what you mean.
 > The question is will the LTR cap (lower reactance) draw more current from
 > my NST? Say a 15 60.
 > I was under the impression they were current limited and no more than the
 > rated 60mA would be drawn.  Resonant rise plays a role in this?

Yes

The 60 ma limitation assumes short circuit.  The specs were never intended
to be for a capacitive loads.  The Capacitive reactance will take away from
the inductive reactance in the NST and current limiting ability will be
affected.  If one runs exactly at resonance, the two reactances will cancel
and the only thing limiting the current is the winding resistance.   The
core could saturate and the NST primary current would have a dramatic
increase in current (good idea to fuse the NST primary).  There are other
reasons not to run exactly at resonance that deals with potential
overvoltage of the NST if the safety gap is not set correctly.  Simulations
and derivations has shown that a good value for Cp is 1.6 * Cres.  This will
help prevent overvoltage and core saturation and give you about 120 BPS
(static spark)

 >
 > Someone said that the current in the primary circuit can be in the
hundreds > of amps?  If so where in the -at-#$-at-#-at-~! Does it come from?

This refers to the TC primary, not the NST primary.  The average charging
current is say about 60ma (I haven't computed it for Cp=1.6 Cres) for most
of the time.  The energy is stored in Cp until the spark gap fires. All of
the enery is then quickly transfered to the TC primary inductance (in 1/4 RF
cycle).  At this point in time, there is zero energy left in Cp and max
energy in primary inductance.  If you equate the inductive energy to the
original capacitive energy (assuming no losses), you can find the max
current in the TC primary:

0.5 LI^2  =  0.5 CV^2

therefore Max I = V / sqrt (L/C) and yes can be in the hundreds of amps.

This high current circulates only in the primary tank circuit formed by Cp
and LP and does not go into the NST

 >
 > Ok maybe I opened a can of worms but id like to know what is happening and
 > why the current can be so high and still not hurt the nst and where all
 > this high current comes from.
 >
 > Thanx again
 >
 > Luke Galyan
 > Bluu-at-cox-dot-net
 >
 >
 >
 >