RE: NST current metering surprises

		Hi Gary,

		(is your old email address no longer valid?)

		True, it will probably bounce, or so I'm told.

		> Original Poster: "Lau, Gary" <Gary.Lau-at-compaq-dot-com>
		> I finally got around to measuring my NST primary and
		>currents, with the help of a hamfest 20A AC ammeter and a
		>borrowed 250mA AC ammeter.
		> Briefly, my coil uses a 15KV/60mA NST.  Until recently, I
had  been
		>using a .012uF cap, selected to be mains-resonant with the
		>secondary, and I got 51" sparks.  My 1.6K/113W protection
		> resistors ran hotter than I thought they should.  Now I
run with a
		>.022uF cap with even better 58" sparks, and even hotter
		>but now I'm tripping my 20 Amp circuit breaker after about
a minute
		>of runtime. This with 88uF of power factor  correction
		> Today I metered the NST secondary current.  This is an
unmodified NST
		>whose short-circuit current measures 72mA (-at-140VAC).  With
		>.022uF cap, it was pushing a whopping 230mA!  Excluding
power due to
		>bypass cap discharging, that accounts for over 84 Watts,
finally enough
		>to explain the hot resistors.  When I went back to my
.012uF cap, the
		>current fell to 130mA.

		Interesting. The question is if the measured 230mA are
		continiously present. What I mean is, this might be a flux,
		because your meter canīt follow the true current flowing,
		if you are seeing inductive kicking effects mentioned on the
		List. The 22nF cap is twice the MRC (mains resonant cap)
		size, so I doubt you are seeing a current increase due to XC
		cancelling out XL. I suspect that the 230mA current is a
(lot of?)
		spike(s). This WILL help you charge the (too big) cap, but
		(most likely) isnīt your average rms current. I think you
		need a scope (and TFīs fiber optics probe) to really see
		is going on. Another possibility might be electrostatic
		building up around the meter itself (due to the high voltage
		present), but this doesnīt really explain the difference you
		in the second experiment (with stable current readings). Are
		meters, you used, frequency independant RMS meters? If they
		are simple meters (for pure sinusoidal measurement), any
		distortion of the waveform will change their accuracy quite

		The meter was a simple analog type.  I thought about the
effects of the movement being at such a high potential, but concluded that
since it's AC, it should cancel out, though I could be wrong.  There's no
doubt that the waveform is non-sinusoidal and that the calibrated RMS scale
will be off, but I'm hoping that it's at least in the ballpark.  If there
are tall, narrow current spikes, I would guess that the meter would
under-indicate the true value as the inductance of the movement, plus
movement mechanics would form a low-pass filter.

		> One further interesting observation.  With the .012uF cap,
		>output and primary current increased monotonically with
		>setting.  With the .022uF cap, there was a distinct null in
		>and somewhat in performance, at about 60% variac setting,
		>at this point, the primary current became rock-steady. The
		> of the arc also became much more stable.  I'll bet the
		>was a non-chaotic 60 or 120Hz (didn't have the scope out).

		That could be, but what exactly do you mean by the words:
		"there was a distinct null in current". Surely the current
		actually "zero" below 60% of the variac setting.

		Sorry, a poor choice of words.  The current didn't actually
get nulled to zero.  What I meant was a distinct, stable minimum in primary
current, I think in the 9 Amp region.  Increasing or decreasing the variac
setting resulted in higher, chaotic readings.

		Regards, Gary Lau
		Waltham, MA USA