People keep arguing about whether a coil is CW. A slayer exciter is continuous, or it has gaps; a fast interrupter is basically CW, or it is not. Both sides treat it as a property of the circuit. What decides it is whether the discharge reaches a steady state while the coil is on, and counting the gaps does not tell you that.
What the question is really asking
Usually the label is wanted for a different reason: to decide whether to fit a topload. The label does not answer that. What a topload costs bases that decision on how far the arc drags the resonance, which depends on the machine and not on the supply, and the two give different answers on the same builder's bench. Richie Burnett runs his 4 MHz resonator with no toroid at all, having found that large ones did not help a CW design (hfsstc). His 350 kHz machine, fed straight off half wave mains, has a 6 by 1.5 inch toroid and averages 14 inches of spark (sstate). Both are his, both get called CW, and only one has a topload. The regime does not decide the topload.
The regime does decide two real things: what the envelope does to the discharge, and how much heat the machine has to handle. Those are the next two sections.
The criterion
Does the discharge reach its steady state within the time the coil is on? If it does, the physics is CW: a continuous balance of power, impedance and voltage. If it does not, the physics is pulsed, and the shape of the envelope is still writing itself onto the discharge.
We cannot answer that question as written, because we have no figure for the time a corona takes to settle. Nothing here measures it, nothing linked from here states it, and none of the sources we read gives it. A fraction of a millisecond has been quoted with nothing behind it. Treat that figure, and any other, as unsupported until somebody films a discharge against a known envelope.
What you can do is read the answer off the discharge. The envelope leaves three different marks on it, and you can tell them apart from two metres away.
Four supplies, one coil
Burnett ran one 250 kHz half bridge off four different supplies and photographed the result each time (sstate). That is the nearest thing to a controlled experiment on this question. The coil, the primary and the resonator stayed the same throughout.
- Half wave off the mains. The RF envelope is rounded bursts lasting 10 ms with 10 ms gaps. Sparks about 6 inches, very straight and sword-like, with hardly any branching. A muffled 50 Hz buzz. About 160 W.
- Full wave. About 300 W, nearly double, and the sparks got fatter and bushier with "no increase in length". The note went up an octave, to 100 Hz.
- Smoothed DC, which is true CW at about 350 V. About 420 W. The discharge became a bushy cone off the point, like a jet of burning gas. The buzz went, leaving a pure hiss. A lot of ozone built up quickly, and the thin wire at the base of the secondary overheated and blistered its varnish.
- A phase angle controller firing at the peak, so the bus goes from zero to around 350 V in microseconds. About 180 W. Sparks about 6 inches again, but branched like a spark gap coil, spidery, and dancing about.
Three of the four, as he photographed them:
Two things come out of that list, and they point in opposite directions.
First, the length stayed the same. Six inches on half wave, six inches with the phase controller, and no gain when the average power nearly doubled on full wave. That matches an arc grows as fast as charge arrives: a step of leader is the topload voltage divided by the breakdown field of air, and the peak bus is about the same in all these modes. Average power does not give you length.
Second, everything else changed. The same coil at the same peak voltage made a sword, a bush and a spider, and the only difference was the shape of the envelope. That is something you can actually observe on a bench.
It also puts a lower bound on the missing figure. If the corona settled in a fraction of a millisecond, nine tenths of a 10 ms hump would be steady state and the half wave discharge would look like the DC run. It does not. Half wave gives the sword and DC gives the bush. So on that coil the settling time is not small beside 10 ms [derived, from his four modes above]. How much of 10 ms it really is has not been measured, and a lower bound is not a number.
The bigger machine adds something, with a confound that needs stating. Burnett's later 350 kHz H bridge, half wave off the mains with no smoothing capacitors at all, averages 14 inches with occasional hits at 18, at about 1200 W RMS input. On smoothed DC at a measured 1500 W it gives a 9 inch flame instead, hot enough to melt the end of the breakout point into a ball that kept glowing after the power was off. More power, shorter discharge. But that machine sweeps its drive frequency from the supply voltage to follow the arc as it detunes, and on a flat supply the sweep does nothing. So the CW figure comes from a coil that was running detuned as well as continuous: two changes, one number. It shows that the envelope matters, but not by how much.
Reading the cases
- DRSSTC, 300 µs at 200 bangs a second (the interrupter is the note). The bang is over long before anything settles. Pulsed, whatever the bang rate.
- QCW, a ramp of six to twenty five milliseconds (the buck is the ramp), or 4 to 5 ms if the ramp is the rising quarter of a mains period rather than a converter. Steady state is deliberately never reached, since growing a sword depends on that. Pulsed.
- An SSTC straight off unfiltered mains. This is the case people actually argue about, and counting cycles does not decide it. Ten milliseconds is 2500 RF cycles at 250 kHz and 3500 at 350 kHz [derived, 10 ms times the frequency], which sounds like plenty of time to reach steady state. Burnett's coil in that mode still makes the sword, not the bush. The envelope is still shaping the discharge at 2500 cycles, so the gap count is the wrong measurement. Near the edges of each hump the bus is below breakout and nothing happens; we have not established how much of the 10 ms that takes up.
- True CW, meaning a smoothed bus. Bushy, hot, hissing. CW. It costs the most and shows the least for it.
The label does change one thing in the circuit, and it is not the thing people argue about. Asked what a CW SSTC needs, davekni answers that its performance comes down mostly to how much line power you can draw before the breaker goes. So leave the bulk capacitor off and let the bus follow the rectified line: the current then roughly tracks the voltage, and the power factor is far better than with the large bank an interrupted coil needs. He also puts a CW SSTC's coupling higher than a DRSSTC's, around 0.3 and perhaps 0.4 (HVF 2646). On a ramped machine the advice runs the other way: the bus page counts a half wave supply as the worse arrangement, because residual bus voltage branches the start of a ramp. Different machines get opposite advice, and the label tells you which one you have.
The numbers
- The threshold: whether the envelope is still moving while the discharge is still growing. Not a voltage, not a duty cycle, not a count of gaps.
- Corona settling time: not established. All we have is the bound above, that it is not small beside 10 ms on Burnett's coil.
- A 50 Hz half wave: 10 ms of RF and 10 ms of gap, stated as the envelope on his coil.
- RF cycles inside one hump: 2500 at 250 kHz, 3500 at 350 kHz [derived].
- Spark length across all four envelopes on the small coil: about 6 inches, roughly 150 mm [derived], on inputs from 160 to 420 W.
- A DRSSTC bang: tens to hundreds of microseconds, so it is never CW whatever the bang rate.
- What a CW bridge is built from: davekni calls eight FGA60N65SMD in a full bridge plenty capable, and says the heatsinks want close spaced fins and a fan rather than size (HVF 2646).
What will get you
Three more hazards that come from running continuously instead of in bangs, all reported by Burnett on the coils above:
- It burns rather than shocks. He got small RF burns from touching metal near the running coil at fairly low power, and says such burns are very nasty. A pulsed coil gives you gaps to be lucky in; a CW coil does not.
- Ozone builds fast. The smoothed DC run produced a lot of it quickly, in a workshop. Ventilate before the run, not after it.
- Anything resonant nearby joins in. With the breakout point left off, an unused resonator two feet away lit up with a crown of corona of its own. The near field was finding a better place to break out, and it will find people too.
If the half wave comes off a transformer instead of the wall, the single diode pushes direct current through the winding and can drive it into saturation. That is our reading of the circuit, not a reported failure. Straight off the mains there is no such problem, and no isolation either.
What goes wrong
- A topload was fitted and the arc got shorter. This is not a CW question. The topload added stability against detuning the coil was not suffering from, and it cost voltage. What a topload costs covers the trade.
- A coil advertised as CW hums at mains frequency. It is unfiltered. The corona is modulated at 50 Hz on a half wave and at 100 Hz on a full wave, and the buzz only goes on a smoothed bus. Not a fault, and the sound tells you for free which one you have.
- Doubling the input power did not lengthen the arc. Expected. Length follows peak voltage; average power gives thickness and heat.
- The discharge went from a sword to a bush when the bus was smoothed. Also expected. That change is what the CW question is about.
- The devices die after seconds of continuous running. They were sized for a lower duty. Whether an SSTC bridge switches softly at all, which decides the heatsink, is covered on its own page.
- A five minute video of continuous operation. Usually interrupted; fast bangs look continuous to a camera and to the eye. An arc grows as fast as charge arrives puts the boundary near 500 bangs a second, where the channels barely cool between bangs and it reads as continuous light.
- A transformer feeding a half wave saturates. The single diode is pushing DC through the winding.
Where next
- What a topload costs, where the topload decision is made on detuning instead of on the regime.
- The interrupter is the note, on ontime and bangs per second, which put a coil on the pulsed side of the line and make it playable as an instrument.
- An arc grows as fast as charge arrives, for why length followed the peak voltage and not the power in all four of the runs above.
The question comes up in the SSTC department because an SSTC can genuinely be run either way. The other two topologies are pulsed by construction.