A standard DRSSTC driver will not run a QCW. Here is what actually has to be touched and why, so that nobody has to work it out by removing components until something changes.
The list
- A starting oscillator, or self-oscillation. To reach and hold the upper pole. Mandatory if you are going up there.
- Pulse skip instead of cutting the bang on overcurrent. Cutting a ramp in the middle gives a pop instead of a sword. Effectively mandatory.
- A smaller burden resistor, so the feedback strength lands in its working window of 0.02 to 0.04 V/A. Mandatory.
- The phase lead coil to match the new burden. The lead is set by the ratio
L/R. DropRand you must dropLwith it. These two go together or not at all. - Bleeder resistors on the buck's output. Without them the ramp does not come back down, because a buck can only add charge. Mandatory.
- An amplitude limit, so the bridge is not taken past its voltage. Worth having, and it is not protection: if the buck's switch fails short the bridge gets the full bus regardless of what the firmware thinks.
Three ways to hold it, and why this page is about the third
- A damper,
R+L+Cacross the primary capacitor, which suppresses the lower pole. Passive, does not touch the driver, has to be tuned in place. - Primary tuned slightly above the secondary, so the driver settles up there on its own. Free, and it puts the pole rather high.
- A starting oscillator or a PLL, which forces it regardless of Q. The cleanest, and it means changing the driver.
A PLL is usually a 74HC4046 with the VCO centred on the upper pole and the capture window trimmed so the lower pole is not inside it. Then there is nowhere to fall to. Two things to watch: capture time at the start of the bang, which wants the VCO pre-biased, and loss of lock when the arc strikes ground, where an XOR phase detector is the more robust choice.
Self-oscillation, from the people who did it
The recipe came out of Mathieu thm's build thread and was confirmed by davekni:
Mathieu thm: removed R7, put a 50k resistor between pins 3 and 7 of the TL3116, and changed C33 for a 300 pF one. davekni: looks ideal, and recommends staying with self-oscillation.
What is happening there. The comparator's hysteresis feedback runs in two parallel branches, a DC one through R7 at 100 kΩ and a pulse one through R26 and C33. Removing R7 and running a resistor from the output back to the signal input turns the comparator into an oscillator: it starts swinging on its own instead of waiting for the current transformer.
The capacitor sets the frequency, roughly:
f [kHz] ~ 139000 / C [pF]And put it slightly above that pole rather than on it, which is the part usually left out and is not specific to any one coil:
lower pole < upper pole < self-oscSet that way, the pole you want sits between the oscillator and the pole you do not, and has to be crossed to reach it — the upper pole becomes a fence. Set the oscillator below the upper pole and both poles are on the same side of it, with nothing in between.
The capacitor is what sets it, and on one machine the difference between two neighbouring values is the difference between a fence and no fence:
poles at 317 and 519
250 pF -> 556 kHz
317 < 519 < 556 fence
270 pF -> 515 kHz
317 < 515 < 519 no fenceTwenty picofarads, and the oscillator moves from just outside the pair to between them. At that frequency there is nothing for the upper pole to protect: the driver is already on the wrong side of it and the lower pole is downhill all the way. That is the whole argument for the smaller value, and reading it as "515 is close enough to 519" gets it exactly backwards — being close is what makes it useless, because the sign of the gap is what does the work.
Two smaller notes from the same work: it only functions with the phase lead network in circuit, because otherwise the loop does not close properly; and a 1N4148 is preferable to a Schottky in that path, because more voltage on the node makes for cleaner oscillation.
Which protection mode, and the word that means two things
A UD2.9X offers both, and the manual's own wording decides it. Ordinary OCD: the pulse ends and goes back to steady state until the next interrupter pulse. Pulse skip: it drops a single oscillation without killing the pulse completely.
On a DRSSTC pulse skip is the kinder of the two — losing one cycle out of a few hundred microseconds costs nothing and the bang survives. On a ramp the arithmetic inverts. "Until the next interrupter pulse" is until the next bang, so ordinary OCD gives a clean, predictable end to the ramp. Pulse skip keeps the burst running through whatever caused the trip — and if that was the driver losing the upper pole, the bang finishes on the lower one, which is the mode the machine was not built for.
So start with ordinary OCD and add pulse skip afterwards, if at all. The board and its wording are WaskaLabs' UD2.9X.
Be careful with the designators
The good news is the other face of the same fact. The comparator core did not change from UD2.1 through 2.5 to 2.7; Ward wrote that the main logic and drive circuits are unchanged. So the recipe transfers legitimately between revisions and onto clones. What transfers is the logic, not the part numbers.
The one that is not on the list
Nothing above turns a DRSSTC into a QCW by itself, because the buck is not in the driver. The modulator is a separate converter with its own switch, its own choke and its own gate supply, and it is the department in one part.
Why the upper pole has to be forced at all, rather than preferred, is on the ramp's frequency page: with plain feedback this coil settles on the lower pole at both ends of a bang, so the changes below are the difference between the machine working and not.
The QCW diagram labels its driver "driver and lead" for the reason in the second bullet, and gives the modulator a line of its own running up the outside to the buck.