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Re: RCR Filter question, LTR question


Hi Ross,

	The resistors between the NST and the filter caps prevent resonant ring up
and possible damage to the NST.  Since I knew that people would use all
kinds of resistor, NST, cap… combinations, I wanted to damp out the
resonance between the NST and the filter caps.  In my LTR coil, and in most
other instances, "I" do not use these two resistors.  However, I have
modeled and tested the system to be sure that the resonance will not be a
factor.  Without knowing a bunch of details about a particular system, I
cannot be sure removing the resistors will not be "bad".  The formula for
the circuit is not quit easy.  The output side of the filter is seeing a
nasty high frequency signal with zero impedance for all practical purposes.
 However, the input side of the filter is seeing the impedance of the
output of the neons, which makes the "real" formula a mess.  I let the
computer figure it out for me to save my brain for other things :-))
However, your formula  Fc = 1 / (2 x pi x R x C) is fine.  Since the system
is center tapped, the R should be 5K and the C should be 1000pF in your
case.  It is really two RC filters.  The resistor between the NST and the
filter pretty much negates the NST impedance for this filter.
	In my LTR coil I use two 2nF caps and two 5K resistors in an RC filter
with the resistors on the main gap side.  Modeling has shown that to be
very effective.  However, I hesitate to recomend that to all because
hooking that to just any NST may do something bad.  I.e. the caps may ring
say a 30mA neon to destructive levels in some cases...

	The LTR systems rely on the inductive kick effects of the neon's large
output inductance.  In a resonant system, the neon's output inductance and
the primary cap are tuned to cancel each other's impedances out and allow
the voltage to ring up to very high levels (at 60Hz).  Of course, we short
the system out before the voltage destroys anything (at least we TRY to
:-)) by discharging the circuit into the Tesla coil.  However, if a large
inductor has a current passing through it, the inductance will tend to keep
the current constant even if the load varies.  If we draw 60mA out of a
neon and instantly short the output, the current will still be 60mA.  If we
instantly open the curcuit, the current cannot be maintianed and the
voltage will jump to gaint values in an attempt to maintain the 60mA
current.  In the LTR case, there is enough stored energy (like 20 joules)
to keep pushing current into the primary cap with greater effeciency than
in the resonant case.  The advantage is that we can charge larger caps to
the firing voltage.  My single 60mA transformer will charge a 28nF cap to
21000 volts without straining the neon.  That gives it about 2.5 times the
power of a resonant system (the resonant system may be able to fire more
often making up for that somewhat and people tend to open up the gaps and
let the system fire at higher voltages than perhaps is "safe"...).  LTR
sytems are good at not destroying the neon since they cannot ring up to
destructive voltages (but ALWAYS use safety gaps!!).  Typically, if
anything goes wrong, LTR sytems will drop the voltage way back and save
everything.  LTR systems need to use sync rotary gaps since they need to
fire AFTER the peak voltage.  My system's charging voltage actually goes to
around 24000 volts peak but I fire it at 21000 volts.  This late firing is
needed to optimize the inductive kick charging effects.  Apparently, you
can us a static gap but I don't think the effeciency will be nearly as good. 

Cheers,

	Terry

At 12:02 AM 7/11/99 -0700, you wrote:
>Hey Guys,
>
>I am using the typical Fritz RCR filter with (4) 5K resistors and (2)
>1000pF caps.
>I am using (2) 12/60 NSTs in parallel, so I assume that I have 120ma.
>This works out to .120^2*5000 = 72 watts per resistor.   I really hate
>to waste 4*72 watts on heating the resistors...
>
>1)  I have a basic understanding the formulas and schematics for simple
>RC filters, but I know that I'm missing some of the details on the RCR.
>Other than adding to the overall R of the filter, what do the R's on the
>NST side of the filter do?  Does the formula for the high freq cutoff
>still work in this case, i.e.  F(h) = 1/(2*pi*R_total*C)?
>
>2) Can someone please review how LTR systems draw more current from the
>NSTs than resonant-sized caps?  I try to digest all the "theory" posts,
>but somehow missed that one.  I remember that it deals with the energy
>stored in the inductance of the NST secondary, but it didn't quite
>understand it.
>
>Thanks,
>--
>Ross Overstreet
>Huntington Beach, CA
>ICQ 20762411
>
>