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Re: [TCML] Racing sparks - question
Joe Mastroianni wrote:
Hi Bert, Thanks - I think I understand (I hope). When you over
couple the frequency response of the system exhibits a double-pole,
and so you have two frequency components, one in the primary and a
different one in the secondary, and they're going to exhibit
"beating" phenomena. That's the mental picture I have now - tell me
if I'm wrong.
This is always confusing to new coilers. Coupled tuned circuits are
deceptively complex, especially since, with most Tesla coils, we're
dealing with their transient response. Unfortunately, most textbooks
tend to deal only with (simpler) steady-state response. In fact, the
general case for lossy coupled resonant circuits was only recently
correctly solved by a TCML list member (Antonio Carlos M. de Queiroz).
Earlier closed-form solutions either were for the lossless case, or had
previously undiscovered errors in the formulae.
Unfortunately, the mental picture you have is not quite right. Even if
the primary and secondary circuits are initially tuned to exactly the
same resonant frequency (when the systems are isolated from one
another), coupling the circuits introduces a complex, multi-cycle energy
transfer process whereby the "bang" energy cycles from one LC circuit to
the other then back again. Although each energy transfer may take
several cycles, the process can be quite efficient at transferring most
of the initial energy from one circuit to the other.
One way to view it is that the "amplitude" of the oscillations in each
circuit are being "modulated" as energy transfers from one LC circuit to
the other. In fact, it is the change in amplitudes that cause the lower
and upper frequency "humps" to appear. This is analogous to the
frequency spectrum for AM radio, where audio modulation creates upper
and lower sidebands on each side of the carrier frequency.
Some of that beating is going to show up as overvoltage between the
secondary and the primary, some is going to be "seen" or "reflected"
or "miller effected" (whichever it really is, in this case, I don't
know). I suspect this is why I see various "burping" or sputtering
in my SRSG when I'm overcoupled, and I imagine the "racing sparks"
have to be big voltage differences in the secondary itself which are
showing up due to this undesired "beating".
A SRSG also adds another wrinkle, especially if it is not adjusted
properly. If the gap "misses" (fails to fire) on one electrode
presentation, mains resonance (at 50 or 60 Hz) can cause abnormally high
voltage to build in the tank cap so that the NEXT time the gap fires you
get a significantly larger "bang" size than normal. Missing,
sputtering, and periodic flashovers (associated with abnormally large
bangs) are all typical symptoms of SRSG problems. However, similar
behavior is sometimes seen when the system is not properly tuned.
In my mind, I imagine there's a response surface for energy transfer
from primary to secondary and resultant spark production. Simply
increasing the energy transfer between primary to secondary doesn't
guarantee you've achieved the highest voltage at the top most point
of your secondary. (Presuming maximum spark production means maximum
top-of-secondary voltage plus current.) Due to these other
problems, increasing coupling actually must decrease the voltage at
the top of the secondary, and so the key is to figure out where the
maximum is. And for any given system, it's going to be somewhere
south of the maximum parameters, k, input V, resonant frequency,
You have the right concept. Optimizing Tesla coil performance is really
a balancing act. Spark gap Tesla coils operate best with coupling
coefficients between 0.10 to 0.22. However, the ability to operate at
higher coupling coefficients is usually limited by the insulation
strength of the secondary winding. Highly-coupled coils often ride the
edge of disaster. Your coil will have a "sweet spot" where longest
sparks are obtained _without racing sparks_. Unfortunately, this can
only be found experimentally for a given coil.
Unfortunately, I only have one big knob to turn once the thing is
running. So I reference everything along that axis, even though the
other parameters. I guess the hardest thing to internalize is that
the maximum spark length will occur somewhere other than maximum
In a well running coil, spark length continues grows with increasing
input power. If spark length flattens out, or even begins to shrink with
additional input power, this usually means your tank cap is too small
for your power supply, or (more typically) your spark gap is overheating
and failing to quench properly. BTW, gap overheating can also occur if
your secondary fails to break out, and the secondary's reactive energy
is being dumped back into the primary circuit. Since the main lossy
element in the primary circuit is the spark gap, overheating shows up
But what I was saying in my post, was that by doing
the necessary "tuning" one was indeed reducing the energy transfer.
But that might actually give bigger sparks...I didn't say because I
Ideally, you want to tune the coil with a comparatively low coupling
coefficient (say 0.10 - 0.12), and then begin ramping up power, fixing
any tuning, gap, or insulation problems you encounter before further
increasing power. Only after you can run under full power for an
extended time with no problems should you begin to increase coupling.
Once you begin to see racing sparks, reduce the coupling until they
completely disappear... and you're there!
One thing I would like to know, as a radio operator, is what is the
SWR in the secondary of a coil?
Extremely high (at least prior to breakout). Most well-constructed TC
secondaries have Q's in the range of 200-300. So, at resonance, very
little input energy is required to maintain 200-300X the reactive energy
circulating in the secondary and toroid system. Once you get lossy
streamers, the Q drops into 10's depending on the loading. Repeat if you
make any significant changes to the system.
The reason I say this is because the arcing between primary and
secondary, the sputtering SRSGs, all the other bad problems of stuff
exploding and burning up - sure seems like unreasonably high SWR to
me - just like when you're tuning an HF power amp.
Yes - lots of reactive energy is transferring between the primary and
secondary - the peak power can be megawatts or 10's of megawatts...
With the k going high - are you actually increasing the SWR in the
secondary, which is reflected back to the primary?
Yes. The nearby physical presence of a high-Q secondary will cause a
significant shift in the resonant frequency of a mistuned, lower-Q
primary so that it becomes closer to that of the secondary. The greater
the coupling, the greater this effect.
In an HF amp, you blow up your finals with the high backpressure of return energy.
Seems the same in a DRSSTC. With the SRSG, you luck out because the
"finals" are a spark gap, which might misbehave but is pretty
Fortunately, spark gaps are tough enough to hide a multitude of sins.
Energy transfer effects to and from the high-Q secondary can be quite
profound within solid state coils, particularly in DRSSTC's where
reactive energies are substantial. Close coupling often leads to complex
reactive energy transfers back and forth between primary and secondary
systems WHILE the DRSSTC primary circuit is ringing up - this shows up
as relatively large low frequency ripples in the primary current
envelope during ringup. Careful over current protection and sensing of
primary current zeroes is critical to prevent switching transistors from
losing their precious smoke during this time. The trick is to ramp up
the reactive energy quickly so that it can be converted to thermal
energy (sparks!) instead of circulating in the primary system... :^)
Anyway, thanks for taking the time to write. I hope what I've said
makes even a little sense, and please correct me where I am wrong. I
have little experience in coil engineering - more in radio and even
more in microelectronics. So I'm rather hungry to learn.
Happy and safe coilin' to you,
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