The coil runs, the bridge is warm, and nothing comes off the breakout. Or something does and it is a third of what it should be. This is the order to look in, and the order matters, because the cheap checks are also the ones that are usually wrong.
The three classic causes
Mads Barnkob's list, and it accounts for most of them. It is not in his DRSSTC design guide or its FAQ, and this page used to say no thread had been found for it either. There is one. He offers the three to a first-time builder in August 2020, on First DRSSTC, No Sparks on Breakout, as the common problems that come to mind rather than as a diagnosis. Much of what follows is from the same thread, named where it is used.
- The feedback phase is inverted, 180 degrees out. Almost no energy is transferred. Check by turning the CT over.
- The secondary's ground is bad.
- The feedback signal is weak, usually the wrong core or the wrong material in the current transformer.
None of those is a tuning problem, and all three get misdiagnosed as one.
Then the ones that look like a small coil
Lengthen the breakout rod. On the same thread Uspring tells the builder to lengthen it somewhat, to at least three times its present dimension, which will reduce the breakout voltage and simplify tuning somewhat.
Move the coil away from metal. A chassis or a case near the secondary kills the Q and steals energy into induction heating. Half the secondary's diameter is the minimum clearance, so on the 110 to 160 mm secondary of an ordinary coil that is 55 to 80 mm derived from [this site's own size classes]. A clearance of 100 to 200 mm belongs to a 200 to 400 mm secondary, which is the large and very large end rather than an ordinary one.
Test in the dark. Tesla coil arcs are not very visible in normal daylight, and davekni's own DRSSTC running 4 to 5 kW is by his account faint in evening light just before sunset. In daylight you will undersell your own machine and go looking for a fault.
The primary current flattens too early
If the current stops rising partway through the bang, energy is being lost before it reaches the arc. Uspring's three places to look, in his order of likelihood, the first the one he thinks most likely and the second the one with explosive consequences:
- The primary leads are too thin.
- Excessive collector-emitter drop.
- The ESR of the bus capacitors.
The gauge for those leads is not Uspring's and is worth keeping separate. davekni gives it on the same thread: a few 16 AWG wires in parallel work great, even slightly better than the equivalent copper area in a single conductor, which is just a bit of home-made litz wire.
The bang is too short
A DRSSTC is not a spark gap coil. Cut the bang off early and most of the primary's energy goes back into the bus capacitors instead of into the arc. Uspring reads that builder's scope shots and puts the first at a very short burst of about 50 µs and the later ones at about 120 µs, which he calls still on the short side but it might be ok. That is one coil at one power level rather than a threshold for every coil, and no figure for a mid-sized coil in general is established here. Longer ontime, longer spark, especially at modest peak current.
Tuning, in the order that works
Measure the secondary's resonance first, with the topload and the breakout point in place. Three ways, and the first two are davekni's, in his own words on a thread about a driver killing IGBTs. A signal generator wired through a resistor to a scope input, say 1k to start, with the frequency swept until the voltage drops the lowest; once it is roughly found, raise the resistor to 10k, which will make a sharper amplitude dip. Or a ring-down: a capacitor of about 1 nF across a scope channel input, the secondary's ground lead touched to a somewhat high voltage source, say 170 V, and then to that input, and the ring-down is at the secondary's frequency. Or an antenna near the topload looking for the field peak.
Tune the primary slightly below the secondary. The arc adds capacitance, so the secondary slides down as the spark grows, and the detuning should increase with it. Adding primary turns lowers its frequency, and the useful range is wider than it looks. On that same thread Marek Novotny tells the builder not to worry about detuning the primary well below the secondary, and to try it all the way from that coil's 170 kHz down to 100. That is a range to search. It is not a sweep anyone reported making, which is how this page used to have it.
JavaTC is a good starting estimate and no more. Nearby metal and walls throw it off, so try taps by hand from there.
Uspring's method: at low power, tune for the maximum ratio of secondary current to primary current. That ratio should be independent of the input power, which is what makes it usable at a voltage that breaks nothing. He gives the limit in the same breath: the maximum ratio is not exactly the best for high power with long arcs, because the arc detunes the secondary by adding capacitance to it, and from there it is better to bring the primary's resonance down a bit. Find the maximum first, then detune off it.
Coupling at 0.12 to 0.2 for an ordinary DRSSTC. Higher transfers more and is less sensitive to detuning, until racing sparks arrive. Lowering the secondary raises it, and there is a step on the record, in JavaTC's figures rather than in anybody's measurement: Uspring reads that builder's JavaTC file at a coupling of 0.134 and suggests lowering the secondary a bit with respect to the primary, and the builder lowers the secondary and topload assembly by 2 cm and reports that this increases coupling to 0.172.
Where the current transformer goes
And the things that kill the bridge while you are looking
Two traps that present as intermittent death rather than as no spark:
The bootstrap trap. If the driver stops switching in the high state, the bootstrap capacitor never recharges, the high side desaturates, the shock heating kills it quietly, and then the low side goes in shoot-through with a bang. That sequence is TMaxElectronics' own post-mortem: a driver that simply stops switching instead of pulling both legs of the bridge low, a large bootstrap charge resistor and the drop across the bottom transistor together took the bootstrap voltage below 13.5 V, the top IGBT shock heated and shorted quietly, and the bottom one then had, in his words, shoot through from hell. Keep the low side on during pauses, or use a floating gate supply, or add a fast or Schottky diode across the driver chip's internal bootstrap charging diode, which is davekni's suggestion for having some hope of charging the capacitor in one half cycle at the start of each burst. The resistor that goes with it is TMaxElectronics' own 1 ohm, which he reckons gives five RC time constants in one low switching pulse even after the diode recovers.
Bus resonance. The film capacitors and the layout inductance resonate, and an H bridge is essentially a reverse bridge rectifier, so the ripple appears at twice the primary frequency, the way a 50 Hz line supply bridge ripples at 100 Hz. Land on the resonance and the bus swings hard. The whole of that, including the arithmetic, is davekni's on the same thread: with 4 by 3.9 µF and a 5.83 µs period he comes up with 55 nH for the resonance, which he thinks reasonable for the standoffs, the bulk capacitor busbars and the capacitors' own internal inductance. His fixes are to lower the inductance with copper foil bent onto the existing busbars, to detune by doubling the film capacitance, which puts the resonance at the square root of two times the primary frequency rather than at twice it, or, where a high Q resonance has to be avoided, to add series resistors to some of the film capacitors.
The figures and cases here are from published threads and from other people's benches. The DRSSTC diagram has the current transformers on the return lead, at the low potential end, for the reason in the warning above.