A secondary is not a transformer winding that happens to be long. It is one half of a resonator, and almost everything that goes wrong with one goes wrong because it was wound as though turns were the point.
What it is and why
The winding, the topload above it and the ground below it form a quarter-wave resonator. What comes out of it is not a turns ratio: it is resonant rise, and that depends on the coil's impedance and its Q rather than on how many times the wire went round.
Which is why the number that decides how the rest of the machine has to be built is one nobody looks at:
Z = sqrt(L/C), typically about 50kWhat you decide
The shape of the former
Criterion: the impedance that comes out of it. A height to diameter ratio of 4:1 to 5:1 is a reasonable place to start.
If the coil comes out high impedance, above 60k, the consequences run right back down the chain: lower peak current, longer ontime, and so more primary turns and less tank capacitance, not more.
How to wind it
- Tight turns, one layer. Winding with a space between turns raises the frequency, which is usually the opposite of what you wanted.
- No bubbles in the varnish. Every bubble is a corona site and a future puncture.
- Ring out topload to ground before power. Continuity, with a meter, every time.
What to change if the frequency comes out too high
In order of what it costs you: thinner wire, a wider former, a longer winding, a bigger topload.
The last is not a free move. A bigger topload lowers the coil's sensitivity to the arc, which is why a QCW wants one, but it also drops the absolute frequency, and compensating with turns drops the impedance again. Three axes, and they are not independent.
How to ground it
- Not to mains earth. Not ever.
- A counterpoise: a large metal sheet under the coil. Roofing aluminium beats foil, and round the corners off.
- A short lead, not run alongside anything else.
- The bottom of the secondary goes to both the counterpoise and the strike ring.
The whole argument is RF ground.
What will get you
And metal closer than half the secondary's diameter kills the Q. A chassis, a case, a rack, a radiator: they load the resonator and steal energy into induction heating, and nothing on a scope tells you it is happening.
Measuring it
Whatever the calculator said, the coil you wound has a different resonant frequency. The topload moves it, the bench moves it, the wall behind it moves it, and the lead down to the counterpoise moves it. Three ways to find out what you actually have:
- A signal generator through a resistor. Start at 1k and go to 10k when you want the dip sharp enough to read.
- Ring it down. A nanofarad into the scope, and watch it decay.
- An antenna near the topload, looking for the peak in the field.
Measure it assembled, with the topload on. A bare winding is a different resonator and its number is no use to you.
The numbers
- Impedance: about 50k typical, above 60k counts as high and pushes the rest of the design around.
- Height to diameter: 4:1 to 5:1 to start.
- Q: around 200 unloaded, about 4 with the arc out.
- Topload voltage under load: clamps around 200 to 300 kV as the current climbs.
- Metal clearance: further than half the secondary's diameter.
- A floating measurement error, documented: 224 kHz read against a true 171.
The Q of 4, the clamp and the 224 kHz reading are other people's measurements, not ours.
What goes wrong
- The measured frequency is well above the calculated one. Instruments floating, or the coil measured bare without its topload.
- Flashover across the winding, low down. Racing sparks: a strike rail too close to the primary, defects on the toroid, humidity, or coupling simply too high. The usual advice that tight coupling is survivable holds on the upper pole, where the secondary's field is lower inside the winding; on the lower pole, bringing the windings together is noticeably more dangerous.
- Flashover across the middle, level with the top of the primary. A different mechanism entirely: the second mode.
- The coil performs worse than an identical one with no visible difference. Something metal is inside half a diameter of it.
- The frequency walks down as the arc grows. Working as designed. The arc's capacitance adds to the coil's, and on a QCW it does so catastrophically.
- The topload voltage stops responding to more power. It clamped. Current, not voltage, is the better indicator of arc length from there on.
Where next
- What a topload costs, the trade at the top of it.
- What RF ground is, the connection at the bottom.
- The second mode, the flashover that is not about coupling.
Figures here are collected from published measurements rather than taken on this bench.