RE: Re[2]: RC filter

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Lau, Gary" <gary.lau@xxxxxx>

The NST current measurement reading I cited was made by using the fiber
optic current probe designed by Terry Fritz, fed into a digital scope
that can render an RMS reading.  The waveform of NST currents feeding a
TC is not remotely sinusoidal, so I can't say how inaccurate a
measurement made by using a simple analog meter movement in series with
the NST secondary would be.  But if this is all simply to determine what
wattage resistors to use for the RC filter, it's just academic - just
use a 50 or 100W ceramic (NOT metal-cased) wire wound resistor.

I doubt that a useful reading would result from primary-side
measurements.  Again, the waveform is not sinusoidal (power factor
correction won't help this), and core saturation will result in
erroneous readings.  A better low-tech approach would be to put a
small-wattage resistor in series with the NST secondary with a
thermocouple or thermistor glued to it, run the coil for several minutes
continuously until thermal equilibrium occurs (CAREFULLY floating the
meter, since the resistor will be at HV potential).  Then apply a
variable DC potential to the same resistor until the same temperature
rise occurs, noting what current was needed.

Regards, Gary Lau
MA, USA

> Original poster: Sebastiaan Draaisma <sebas@xxxxxxxxxxxxxxxx>
>
> Chiang Mai, Tuesday, March 15, 2005
>
> In reply to: RC filter
>
> Posted on: Tuesday, March 15, 2005
>
>
> Hello Gary,
>
> Thanks again,
>
> This is very interesting all trough I'm wondering how you measure the
> actual output of the NST, I don't have, and am not familiar with a
scope.
> Can it be done by measuring the primary with the power factor
correction
> connected to it, measure the actual power consumption and convert that
back?
>
>
> For example I measure on the primary of my 230V - 15kV 30mA NST a
current
> of 3.26A, which would equal 750W and convert that back to the
secondary
>
> 750W / 15000V = 50mA. Or would this not work?
>
>
> Best regards,
>
> Sebastiaan Draaisma
>
>
> Tesla> Original poster: "Lau, Gary" <gary.lau@xxxxxx>
>
>
> Tesla> Hi Sebastiaan:
>
>
> Tesla> Sorry for the delay - I found this reply languishing in my
Drafts
>
> Tesla> folder...
>
>
> Tesla> The easy answer is that the choice of R-C filter caps does not
affect
>
> Tesla> the needed value of the tank cap to achieve pri-sec resonance.
>
>
> Tesla> Estimating the power dissipated in the filter R's is more
challenging.
>
> Tesla> There are two components to this (as far as I'm aware).
>
>
> Tesla> One component is, by Ohm's law, the current from the NST
squared, times
>
> Tesla> the resistor value.  But the catch here is knowing what the NST
current
>
> Tesla> under actual operating conditions is.  I have a 15/60 NST, no
>
> Tesla> modifications to the current limiting shunts.  But I've
measured in
>
> Tesla> excess of 150 mA RMS when running.  Such high currents are
possible
> when
>
> Tesla> Variac-ing up the NST primary voltage and when using high tank
cap
>
> Tesla> values, but is not well understood or predictable.  For my
coil, I use
>
> Tesla> 1.7K series resistors, so .15*.15*1700 = 38.2 Watts per
resistor.
>
>
> Tesla> The second component is determined by realizing that the power
in the
>
> Tesla> bypass caps is dissipated in the resistors each time the main
gap fires.
>
> Tesla> The larger the bypass cap is, and the higher the main gap BPS
is, the
>
> Tesla> more power is dissipated here.  For my coil, I have 450pF caps,
and the
>
> Tesla> bang voltage is about 25kV.  Each of the two bypass caps is
charged to
>
> Tesla> 25kV/2 or 12.5kV, 0.5*C*V*V = .0351 joules.  With my 120BPS
sync gap,
>
> Tesla> this will dump 120*.0351= 4.2 Watts into each resistor.  If I
were to
>
> Tesla> use a static gap, this would have a higher BPS, but possibly a
lower
>
> Tesla> charging voltage; not sure if the power would be higher or not.
>
>
> Tesla> So I'm guessing my resistors each dissipate about 42.4 Watts.
>
>
> Tesla> Regards, Gary Lau
>
> Tesla> MA, USA