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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
input power.

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 there first.

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
wasn't thinking.

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.

Cheers Joe


Happy and safe coilin' to you,

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