Re: NST current metering surprises

[Terry Fritz <twftesla-at-uswest-dot-net>]

Hi Gary,

Now this IS strange!!

If you crank your variac up to 140 VAC and you neon sucks in 20 amps, you
are nearing 3.0KVA input.  Assuming you are getting somewhere around 17500
VAC out with 230mA that works out to 4.0KVA out.  Possibly a problem there.

At 230 mA output your neon should be internally heating 15 times normal.
They can be pretty tough but...

With 230mA at 120 BPS with a 22nF cap, the voltage works out to 87000 volts
(I think).  But, that would be 10000 watts...

My first thought would be is something wrong with the readings?  Could the
high voltage or high frequencies be affecting the meter?  Perhaps the
addition of a cap across the meter leads would cause a drastic difference
if high frequencies were getting into it.

I have run with a similar setup and everything seemed normal.  Nothing like
this...

I have no explanation, but this would not be the first time something new
has been found with these Tesla coils...

Cheers,

	Terry


At 09:48 PM 6/29/99 -0400, you wrote:
>I finally got around to measuring my NST primary and secondary 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 NST secondary, and I got
>51" sparks.  My 1.6K/113W protection circuit resistors ran hotter than I
>thought they should.  Now I run with a .022uF cap with even better 58"
>sparks, and even hotter resistors, but now I'm tripping my 20 Amp circuit
>breaker after about a minute of runtime.  This with 88uF of power factor
>correction caps.
>
>Today I metered the NST secondary current.  This is an unmodified NST whose
>short-circuit current measures 72mA (-at-140VAC).  With the .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.
>
>Experimenting with the power factor correction caps, I couldn't tell if they
>had any effect with the .022uF cap as the meter pegged both with and without
>it.  Perhaps I should have measured how long it took my breaker to trip,
>with and without the PFC.
>
>With the .012 uF reso-cap, primary current was significantly less, dropping
>from 14-20 to 11-14 Amps without and with the 88uF PFC.  Secondary current
>was unchanged with or without the PFCs at 130mA.
>
>I stated the primary currents as ranges because the reading is extremely
>unstable, no doubt due to the chaotic firing of the static gap.  It's
>curious that the secondary currents seem so much more stable.
>
>One further interesting observation.  With the .012uF cap, spark output and
>primary current increased monotonically with variac setting.  With the
>.022uF cap, there was a distinct null in current and somewhat in
>performance, at about 60% variac setting, and at this point, the primary
>current became rock-steady.  The sound of the arc also became much more
>stable.  I'll bet the charging was a non-chaotic 60 or 120Hz (didn't have
>the scope out).  
>
>With both the primary and secondary currents so much higher than simulations
>predict, I now have to question the validity of that model.
>
>Regards, Gary Lau
>Waltham, MA USA
>