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Re: [TCML] Racing sparks - question
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.
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".
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, etc.
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. 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.
One thing I would like to know, as a radio operator, is what is the SWR in the secondary of a coil?
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.
With the k going high - are you actually increasing the SWR in the secondary, which is reflected back to the primary? 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 industructable.
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.
On Aug 8, 2010, at 6:17 PM, Bert Hickman wrote:
> Adjusting coupling actually changes the RATE that energy transfers between primary and secondary LC circuits. While reducing the coupling also slightly reduces the amount of energy transferred, this is a secondary effect due to system losses. Increasing the coupling reduces the "ringup" (or energy transfer time) from the primary to secondary, and vice-versa. Coupling that is too high can also cause coil performance problems due to quenching failures in the primary gap. For an excellent description of coupling and its roles in TC operation and quenching, see these sections on Richie Burnett's site:
> A given secondary/topload can safely absorb energy at a limited rate. When this rate is exceeded, flashovers begin to occur between various regions of the secondary. These flashovers may span only a few inches, or may occur across the entire winding. Although racing sparks originally plagued only spark gap switched systems, it can also appear in certain configurations of SSTC's as well.
> When coupling is too high, abnormal high voltage differentials are created which can induce racing sparks to jump across portions of the secondary winding or even across the entire winding. There is some evidence that racing sparks are created when higher frequency modes of oscillation are excited within the heavily-coupled resonator. Coils with a uniform protective overcoat tend to survive higher coupling better than uncoated coils. Coated secondaries will capacitively "spread out" and distribute the energy at the the ends of racing sparks over a larger area instead of poking holes and arcing/melting at specific points on the winding.
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> Joe Mastroianni wrote:
> > So as we are not quantifying spark production and measuring our
> > success by objective energy output measurements - if it looks cool
> > and nothing detonates and everybody is safe, then we have succeeded
> > as coilers - far as I can tell in this eclectic hobby of ours.
> > I guess reducing the k works, though to me it's the same as running
> > it at a lower voltage on the variac.
> > Cheers, Joe
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