A secondary that flashes over across the middle of its winding is usually blamed on coupling. Sometimes it is coupling. But there is a mechanism that produces exactly that failure with the coupling untouched, and it explains why a small secondary works in one topology and burns in another.
What it is
A resonator has more than one mode. The first is the one everybody tunes: a quarter wave, with the voltage maximum at the top. The second has a node partway up and a maximum in the middle, level with the top of the primary.
Anders Mikkelsen's chain, in four steps:
square wave drive -> strong third harmonic
-> excites the SECOND mode of the secondary
-> voltage peak in the MIDDLE of the winding,
level with the top of the primary
-> flashoverThe damage happens where nobody is looking, because the top of the coil is fine.
What goes wrong, and how to tell it from coupling
- A flashover across the middle of the winding, level with the top of the primary. This mechanism. Racing sparks from coupling strike the lower third instead, and they have been caught as low as
k= 0.185. - A small secondary that works in a DRSSTC or a QCW and burns in a bare SSTC. Double resonance moves the mode away from three times the drive. The single resonant machine has nothing pulling it clear. This is the answer to the question that comes up constantly: why somebody's tiny coil is fine and yours flashes over on the same form.
- The arc squeals. Not a mechanical noise. It is a beat: the second mode sits near the third harmonic, and the beat between them lands near the primary's own resonance. A squeal is a diagnostic, and it says the second mode is being fed.
- The coil starts on a frequency you did not ask for. An antenna feedback pickup placed near the winding can lock onto the second mode rather than the first.
What you do about it
The practical handle is the height of the primary, not the coupling coefficient.
- Raise the primary and the switching cleans up.
- Raise it too far and you get the flashover anyway, from ordinary proximity.
There is a position where both are satisfied, and finding it is a matter of moving the primary rather than of retuning anything. It reproduces on old RSSTC hardware as well as on new, so it is not a property of any one driver.
The other handle is the coupling capacitor's value, which moves where the beat lands.
What the evidence does not support
That the beat frequency figure is settled. With a 1.0 µF coupling capacitor the beat is reported at around a third of the working frequency. Our notes on this carry an instruction to check that number at the source thread before publishing it, and that check has not been done, so treat it as indicative and not as a value to design against.
That thin secondaries are simply worse. The mechanism is a ratio, not a quality: the closer the second mode sits to three times the working frequency, the more completely the third harmonic feeds it, and the smaller and thinner the secondary the closer that ratio gets. A thin secondary in a machine that moves the mode is fine.
The numbers
- Where it strikes: the middle of the winding, level with the top of the primary. Racing sparks strike the lower third.
- What excites it: the third harmonic of a square wave drive.
- Coupling at which racing sparks have been caught:
k= 0.185, which is a different failure and belongs to the other list. - Beat frequency at 1.0 µF: around a third of the working frequency. Indicative, unverified, see above.
Where next
- Winding and measuring a secondary, the rest of what goes wrong with one.
- Coupling: what it buys, the other flashover mechanism and its ceiling.
- An SSTC primary is a different part, since the handle here is where that winding sits.
Attributed to Anders Mikkelsen, with the squeal observed by davekni and ZakW.