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Re: NST's shorting out.
Original poster: Vardan <vardan01@xxxxxxxxxxxxxxxxxxxxxxx>
Hi,
At 05:07 AM 3/19/2006, you wrote:
Hello all,
I just rebuilt my Tesla Coil and have run into
problems at higher power inputs.
I first operated this coil with one 12000v 60 ma
Neon transformer and it operated quite well and
consistently with no problems, giving 31 inch
sparks to ground.. I had a safety gap
and choke directly in front of the NST.
Later I decided to parallel two NSTâ??s
together, and the results were great for a few
tries with a real performance boost ( 50 inch
sparks to ground), but then one NST burned out (lost one side).
If you doubled the number of NSTs without
changing the primary capacitor size, it is very
possible that the primary will go resonant and
over voltage the NSTs. 12kV at 120mA has a
resonant capacitor size of 27.3nF. If you are
running 30nF (from below), then you are right on
top of the resonant value. If nothing stops it,
the voltage might skyrocket to say 80kV!!!
I proceeded to add a second safety gap, this one
to discharge into the primary coil, and set this
gap narrower than the other so it would fire
first. I also added a safety capacitor-- five 8
kv .0047 mfd caps in series, across the NST safety gap.
Here is how a popular protection circuit is designed:
http://hot-streamer.com/temp/NSTFilt.jpg
But if the primary is going hard resonant, the
protection circuit itself might be damaged (but
the NST should be ok ;-)). You need to really
fix the over voltage problem that is causing the mess...
I tried the system again with 2 other NSTâ??s ,
and first it blew out the safety caps (shorted
between two of the caps ? bllew out of one and
into another), shorting the NSTâ??s so nothing
worked. I removed them and tried again. Again,
one NST burned out one side (incidentally always
the second one in the parallel, not the first
one, which I operated as a single before).
Next, I unpotted one of the burned out NSTâ??s
, soaking in mineral spirits, removed as much
of the pitch as needed to take the coils
apart. Tested the unit-- both sides worked
perfectly, and then removed half of the shunt
plates (10 out of 20). I reassembled and had a
current draw of 18 amps on the mains (vs.
original 6-8 amps on a working NST), so I
figured I was getting between 120-180 ma output
from the unit. I only planned to operate this
rebuilt unit for 15 seconds at a time to
prevent overheating and now I only needed one transformer, not two.
I installed into the coil and again, it worked
great for 3 tries. Then one side burned out
again, this time permanently, as the resistance
reading was very high compared to the other
side. Now Iâ??m back to square one.
The NSTs are just the "fuse", the problem is in the primary cap size.
My speculations on the problems, possible causes/solutions:
The fact that this has happened three times
seems to indicate to me that the problem lies
AFTER the NST, with the coil design itself, not
with the NST, and that some strong pulses are
making it past the safety gaps and chokes.
The safety gaps "should" work but apparently they
are not. I would worry if other odd components
are across them or in circuit with them. They might just be adjusted wrong:
http://www.pupman.com/listarchives/2000/January/msg00044.html
But if the primary capacitance is wrong, they will just fire all the time...
Perhaps setting the safety gaps narrower would
give more safety at the expense of performance,
but they are already firing every second or
more now. I had to widen them when using the
two NSTâ??s as opposed to one because they were
firing continuously. Was this a mistake?
Yes ;-)
Primary safety gap is about 7/8 inch, and gap
just before NST is 2 X 5/8 inch with ground tap in center.
I perhaps need a safety cap again, but probably
need 100kv+ to stand the pulses coming
backward. I have seen several opinions in the
past about size (mfd) but think low is better to
prevent secondary resonance in the system. Thoughts?
My rotary gap is a little warped so the gap
distance varies per revolution, but the I keep
the contact gap distances to the minimum anyway
( much less than safety gaps) so donâ??t think this is major.
A rotary gap that is not a synchronous type is
not recommended at all. It will randomly fire at
any spot on the AC cycle. A sync gap is twice as effiecient:
http://hot-streamer.com/TeslaCoils/Misc/syncmot.zip
http://hot-streamer.com/TeslaCoils/Misc/sync_motor.txt
If you make your gap synchronus, you can use a
60nF primary cap with two 12/60's. That appears
to give the best possible performance. 120BPS
would be ideal but it should be timed (synced) with the AC line power.
Perhaps I need to find some ferrite toroids for
much better choke protection. I now use one
inch pvc pipes with 25 turns of 12 ga
wire. These were originally air core coils,
but I filled the air core with steel nails
thinking this should increase inductance and could only help.
You need larger primary capacitance right
now. Probably in the 45 to 50nF range. But the
async rotary gap is a problem. Might work better with a static gap...
I have pasted in the Java program results below
to give a better idea of the coil dimensions if you have time to read it.
Anyhow, I would appreciate any suggestions anyone may have.
Thanks for posting lots of details!! That really helps!
Cheers,
Terry
Thank you.
Martin Fryml
System details:
Capacitor 35 kv .03 mfd Maxwell pulse capacitor
Sunday, February 26, 2006 2:53:53 PM
J A V A T C v.10 - CONSOLIDATED OUTPUT
Units = Inches
Ambient Temp = 60°F
----------------------------------------------------
Surrounding Inputs:
120 = Ground Plane Radius
120 = Wall Radius
100 = Wall Height
102 = Ceiling Radius
119 = Ceiling Height
----------------------------------------------------
Secondary Coil Inputs:
Current Profile = G.PROFILE_LOADED
6.33 = Radius 1
6.33 = Radius 2
31 = Height 1
81.4 = Height 2
735 = Turns
16 = Wire Awg
----------------------------------------------------
Primary Coil Inputs:
8.83 = Radius 1
15.23 = Radius 2
32 = Height 1
35.75 = Height 2
9.6 = Turns
0.25 = Wire Diameter
0.03 = Primary Cap (uF)
0 = Desired Coupling (k)
----------------------------------------------------
Top Load Object Inputs (dimensions & topload or ground connection):
Toroid #1: minor=8.36, major=32, height=88.4, topload
----------------------------------------------------
Secondary Outputs:
115.09 kHz = Secondary Resonant Frequency
90 deg° = Angle of Secondary
50.4 inch = Length of Winding
14.6 inch = Turns Per Unit
0.01775 inch = Space Between Turns (edge to edge)
2436.1 ft = Length of Wire
3.98:1 = H/D Aspect Ratio
9.61 ohms = DC Resistance
26439 ohms = Forward Transfer Impedance
26176 ohms = Reactance at Resonance
19.04 lbs = Weight of Wire
36.199 mH = Les-Effective Series Inductance
33.844 mH = Lee-Equivalent Energy Inductance
38.915 mH = Ldc-Low Frequency Inductance
52.829 pF = Ces-Effective Shunt Capacitance
48.415 pF = Cee-Equivalent Energy Capacitance
90.659 pF = Cdc-Low Frequency Capacitance
9.46 mils = Skin Depth
36.302 pF = Topload Effective Capacitance
55.2 ohms = Effective AC Resistance
474 = Q
----------------------------------------------------
Primary Outputs:
107.14 kHz = Primary Resonant Frequency
6.91 % high = Percent Detuned
30 deg° = Angle of Primary
60.47 ft = Length of Wire
0.523 inch = Average spacing between turns (edge to edge)
2.5 inch = Primary to Secondary Clearance
73.561 µH = Ldc-Low Frequency Inductance
0.026 µF = Cap size needed with Primary L (reference)
340.655 µH = Lm-Mutual Inductance
0.201 k = Coupling Coefficient
4.98 = Number of half cycles for energy transfer at K
22.63 µs = Time for total energy transfer (ideal quench time)
----------------------------------------------------
Transformer Inputs:
120 [volts] = Transformer Rated Input Voltage
12000 [volts] = Transformer Rated Output Voltage
60 [mA] = Transformer Rated Output Current
60 [Hz] = Mains Frequency
120 [volts] = Transformer Applied Voltage
0 [amps] = Transformer Ballast Current
0 [ohms] = Measured Primary Resistance
0 [ohms] = Measured Secondary Resistance
----------------------------------------------------
Transformer Outputs:
720 [volt*amps] = Rated Transformer VA
200000 [ohms] = Transformer Impedence
12000 [rms volts] = Effective Output Voltage
6 [rms amps] = Effective Input Current
720 [volt*amps] = Effective Input VA
0.0133 [uF] = Resonant Cap Size
0.0199 [uF] = Static gap LTR Cap Size
0.0346 [uF] = SRSG LTR Cap Size
133 [uF] = Power Factor Cap Size
16968 [peak volts] = Voltage Across Cap
59982 [peak volts] = Recommended Cap Voltage Rating
4.32 [joules] = Primary Cap Energy
342.7 [peak amps] = Primary Instantaneous Current
41.1 [inch] = Spark Length (JF equation using
Resonance Research Corp. factors)
----------------------------------------------------
Rotary Spark Gap Inputs:
1 = Number of Stationary Gaps
21 = Number of Rotating Electrodes
540 [rpm] = Disc RPM
0.44 = Rotating Electrode Diameter
0.25 = Stationary Electrode Diameter
12.25 = Rotating Path Diameter
--------------------------------------------------
Rotary Spark Gap Outputs:
21 = Presentations Per Revolution
189 [BPS] = Breaks Per Second
19.7 [mph] = Rotational Speed
5.29 [ms] = RSG Firing Rate
30 [ms] = Time for Capacitor to Fully Charge
0.88 = Time Constant at Gap Conduction
1.99 [ms] = Electrode Mechanical Dwell Time
58.6 [%] = Percent Cp Charged When Gap Fires
9943 [peak volts] = Effective Cap Voltage
1.48 [joules] = Effective Cap Energy
247504 [peak volts] = Terminal Voltage
280 [power] = Energy Across Gap
45.6 [inch] = RSG Spark Length (using energy equation)
WITH 2 NSTâ??S the Rotary gap speed
was increased to 525 PPS to prevent overcharging the Caps.