More equipment dies at the bench than in the arc, and the reason is always the same: a scope's ground is bonded to mains earth, and the thing you are probing is not.
The one rule
There are two ways to satisfy that, and builders use both.
- Isolate the circuit under test. Ward's own test setup lists an isolation transformer "for powering the bridge electronics", wired for a 1:1 ratio. Mads Barnkob puts the same item under what is nice to have, "to be able to ground the negative rail of the power circuit when testing" (DRSSTC FAQ). On HVF 646 Teravolt makes the case on price: an ordinary dual-winding mains transformer "will be cheaper than a diferencial probe".
- Or leave the coil where it is and probe differentially. In the same thread Mads Barnkob describes his own bench: "I use my DSO1054z with a range of different Pearson current monitors and a Tektronix P5100 differential probe for up to 1100V isolated measurements. I also have a 2ch Tektronix A6909 isolator for up to 600V."
Once the electronics are isolated the probes themselves need not be exotic. Ward asks for "at least 20MHz bandwidth, and a good set of 10:1 voltage probes so that you can view 200-400V signals safely". Those are two builders' benches rather than a recommendation from this site, which has none of its own to recommend from.
Probing the driver
Mads Barnkob's UD3 bring-up is the same rule counted out on a full bridge. The four gate signals go to four channels, three of them through differential probes and the fourth through a normal 10x probe, because "differential probes are needed, to avoid short circuit of the full-bridge through the ground clips of normal probes" (UD3 installation, configuration and test).
So a gate and its own emitter are one measurement, and two of them in a leg are not two measurements you can take at the same time with ordinary probes.
Both driver pages lean on this section for their own probing warning, and the traffic is worth having in both directions: what the board is doing while you probe it is what a DRSSTC driver does, and the same board with the part values on it is inside the driver.
The probe is part of the circuit
Ward found sag and ringing on a gate to emitter trace that were not in the gate loop at all.
There are very large, high-frequency currents flowing in the primary circuit, and thus any loop of wire suddenly becomes an antenna with a voltage induced by the dB/dt.
The give-away is the frequency. The droop and the ring sit at the primary's own resonance, "the natural Fo of the primary system", which is nothing the gate drive knows about. His fix costs nothing: "squeeze down the loop area of the probe tip and the ground clip, preferably by twisting the ground clip around the probe body itself". The trace he publishes afterwards has neither the droop nor the ring.
That matters here rather than on the gate page. Reading a gate waveform files sag under too few turns on the transformer and low frequency ringing under too much leakage inductance, and the cure in both entries is to rewind the thing. Neither entry carries the probe, because the probe is this page's business. Twist the ground lead against the probe body and take the trace again before you take a transformer apart.
Ward also runs the probe lead and the scope's own mains cable through ferrite toroids against common mode noise, and is honest about what that buys: "You can operate without the filters completely, but you may get waveforms that look noisier than they really are."
And a probe can start a coil as well as lie about one. The symptom "it starts only while a scope probe touches the comparator input" is on every bang starts open loop, where the probe is an antenna into a comparator whose idle state nothing defines.
The two burdens are not the same
On a UD2.x board there are two current transformers with two burden resistors, and only one of them will take an ordinary probe.
- The feedback burden has one side on ground, so an ordinary probe goes across it. What that reads is the current, because the phase lead coil sits in series and the same current runs through both. It is not what the comparator reads: the driver's input sees the resistor and the lead coil together, so the waveform on your screen and the waveform the loop is switching on differ in phase.
- The overcurrent burden has neither side on ground. Clip a probe ground to one end and you have joined that end to the scope's earth, which is a connection the board was not built with. What the comparator sees afterwards is not what it saw before, and this site's own symptom list carries "new faults appeared after probing" for exactly that. It needs a differential probe.
Do not assume there are two rings before you have looked. The values, and the reason the two are kept apart on the boards that keep them apart, are on two rings, not one, which also records the UD3 putting gate drive synchronisation and current limiting on one input by design, on a single burden.
Looking at the bridge output
Two probes, one on each output of a full bridge, both ground leads on the same node, the negative bus. Subtract in the scope. That gives you the differential without a differential probe, and it obeys the warning above because the two ground clips are on one point rather than on two.
It is also how the loop margin gets measured, on a bus low enough to be harmless: the margin is in the loop.
Looking at a gate
Probe it through an isolated supply. How much bus voltage the picture needs before a gate resistor set on it means anything is on reading a gate waveform, and that is where the figure belongs.
Two things Ward reports from his own small half bridge are worth carrying while you bring a coil up on a bench supply, and both are that coil's rather than thresholds for yours. Under 35VDC the feedback on it was too weak to hold a strong oscillation, so a refusal to start at the bottom of the range is not necessarily a fault. And under about 50VDC, in his words, "IGBT damage is very difficult to achieve".
The current transformer, while you are in there
Where the ring goes, and why never between the primary and the tank capacitor, is on what a DRSSTC current transformer is, and what it senses.
A ring is not always the right instrument
Everything above assumes you want a current transformer. For measurement rather than for feedback there is a real choice against a resistor, and it was put to somebody replacing shunts with CTs on a topload current measurement. Uspring's two objections are worth having before you make the swap (HVF 117):
Do you have an idea about their bandwidth considering e.g. ground strikes? … An advantage of using resistors is, that you can catch possible polarity dependent effects of the arc. A CT might filter that out.
Both are about the same property from two sides. A ring will not pass DC and its low end rolls off, so anything the arc does slowly or asymmetrically is exactly what it removes, and a ground strike is the event most likely to contain both. A resistor has neither limit and every other disadvantage, chiefly that it is in the circuit and at whatever potential the circuit puts it at.
So the working answer is that the ring is right for what the driver needs and the resistor is right for asking what the arc did, and a measurement that is trying to settle an argument about arc behaviour should not quietly use the instrument that filters the evidence out.
And the question that provoked that exchange never got answered, though it answers itself. He was asking how to spend a second scope: on the primary and secondary base currents, or on one channel for the bridge output voltage. The bridge output is a square wave that sits at plus bus or minus bus and nothing else, so the only thing in it is where the edges fall, and an external trigger input records exactly that. He says so himself, in a parenthesis he appends to the question rather than treats as its answer: "Might be able to have a 1-bit value recorded of that anyway via the ext trigger input." The currents are the analog ones, where the shape is the whole point and a bit per sample tells you nothing.
So spend analog channels on things whose shape matters and take the square wave as a trigger. That reading is ours: nobody in the thread replied to the question at all.
Making your own measurement transformer
Not the driver's ring: a separate one whose only job is to put primary current on a scope. On HVF 1277 Daniel Uhrenholt asked for one covering 100 to 200 kHz after selling his Pearson, and got two answers worth copying.
Hydron's is the cheap one. "A couple of ferrite toroids (specification un-critical as long as they're not too low-Al) some wire and a resistor or two", cascaded so the ratios multiply, and in that band it "will work extremely well". He is equally plain about the price of it: no Pearson-grade electric field screen, "so it will pick up noise from fast switching transitions", and no good low frequency response. On HVF 914 he adds that his own DIY cascade "picks up a LOT of noise when not band limited to 20MHz, whereas the pearson picks up very little". Uhrenholt had already built one, two cores at roughly 1000:1, and reports the failure that matters most to a coiler: the waveform "looked awful in the start of a burst, compared to a Pearson monitor", which is the part of the bang you were probably trying to see. Hydron suggests nanocrystalline cores over ordinary ferrite, offered as something to look at rather than as a settled answer.
davekni's is the careful one, and what makes it worth copying is that he gives the whole chain instead of a part number. An E80 ferrite core carries a single-layer secondary of 40 turns of 24AWG. Over that goes "just over one turn of copper foil, insulated at the overlap (no shorted turn), and connected to ground as a shield", and over that a one-turn primary of 0.2mm copper foil, made as two layers of 0.1mm. Both windings run almost the full width of the centre leg, which is what keeps the leakage inductance down. That stage then feeds two ordinary toroidal transformers, each with a 2-turn primary and a 25-turn secondary, into a 5 ohm burden, and the sensitivity he quotes for the whole thing is 1V/100A.
He reports it "works well to 2600A" and designed it for 3500A eventually. He also says he never explicitly measured its frequency response, so read the sensitivity as measured and the bandwidth as untested.
A Pearson is still the instrument to have if one turns up, and the way to get one is patience rather than money. Hydron's 2788s are specified "for use over 6 orders of magnitude (300Hz to 300MHz)", and he came by them slowly: "I got lucky on eBay a few times and collected a number of Pearson CTs (5 in total, averaging about 60£ each)". Uhrenholt, pricing a new one in the same thread in 2020, had "£300+" with shipping, import tax and handling into Denmark.
And this is what such a transformer is for at the sharp end. Ward sets the current limit with it rather than by calculation: "I typically set the limiter too low and watch the primary current on an oscilloscope (using a home made CT). I then operate the coil with free air streamers only if possible. I then set the limiter so that it just barely does not trigger with free air streamers. Then I check that the limiter does engage with ground arcs as it should" (general DRSSTC design guide). Why that is the right way round, and what the threshold is not, is on overcurrent is not a setting.
What can hurt you rather than the equipment is on its own page, and it should be read first.