A ring on the primary conductor, and the whole driver's view of what the coil is doing. Take both jobs off one of them and the protection goes soft exactly when you need it.
What it is and why
The conductor itself is the one-turn primary; the winding on the ring is N turns of secondary. The current in that winding is N times smaller, and a burden resistor turns it into a voltage the driver can read.
I_secondary = I_primary / N, U = I_secondary × R_burdenThat is the whole device. Two of them appear on a DRSSTC because the driver wants two different things from the primary current: the phase of it, to know when to switch, and the size of it, to know when to stop.
What you decide
Two rings, or one
Criterion: whether anything is loading the transformer that also has to carry the protection.
The feedback path has a phase lead network hanging off it, and that network loads the first stage. The core saturates, and the protection gets weaker exactly when it is needed. The UD boards keep the two apart, and that is not an accident.
How many turns, and whether to cascade
Criterion: the burden you want. More turns means less secondary current, which means a larger burden for the same voltage.
When one core is not enough, two go in series: 175:1 is done with two cascaded cores, with an intermediate tap at 100:1. The tap is useful, because that is where the second signal comes from.
The burden, and the phase error it brings with it
An ideal current transformer is a current source and gives no phase shift into a pure resistor. A real one does, and it is a lead, not a delay:
phi_error ~ R_burden / (w · L_magnetising) [radians]Count it with a minus sign: when you add up the total lead, this term subtracts. At 51 Ω it is a few nanoseconds, up to four per cent of the whole lead; at 3.6 Ω it is a fraction of a nanosecond and irrelevant.
The lead coil's own DCR sits in series with the burden and belongs in the formula: phi = arctan(wL / (R_burden + DCR)).
Feedback strength, which is the number to compare
How many turns is not a meaningful figure on its own. This is:
feedback strength = |Z| / N [volts per amp of primary current]where |Z| is the whole burden network, resistor and lead coil together. From it falls out the current at which the signal hits the clamping diodes:
I_clamp = V_f · N / |Z|Working boards sit around 0.02 to 0.04 V/A.
Where to put it
Where the potential is low. There is a second option, a ring at the base of the secondary on the ground wire: the signal there is the cleanest available, but it is a different feedback, on secondary current, and it is not advised for an SSTC, because when the arc strikes ground the ratio of the currents changes and the protection fires late.
Wind it the wrong way round and the feedback gives you zero or chaos. Turning the ring over is the test, and it costs nothing to try first.
What will get you
And only one of the two can be probed with an ordinary probe. The feedback burden has ground on one side of it. The overcurrent burden has ground on neither, and needs a differential probe. Note also that a signal generator delivers orders of magnitude less current than a primary does, so to test a ring on the bench, wind ten to a hundred turns through it to get a signal at all.
The numbers
- Ratio in use: 100:1 at an intermediate tap, 175:1 through two cascaded cores.
- Burdens on a UD board: feedback 51 Ω 2 W, overcurrent 5.1 Ω 0.5 W.
- Feedback strength on working boards: 0.02 to 0.04 V/A.
- Phase error at 51 Ω: a few nanoseconds, up to four per cent of the whole lead, and it subtracts. At 3.6 Ω it is negligible.
What goes wrong
- The coil will not start, or the frequency is nonsense. Winding direction. Turn the ring over.
- The protection is weak at exactly the current that matters. One ring doing both jobs, with the lead network saturating its core.
- It tracks at low power and stops tracking at high. Clamping, from a feedback strength three or four times what it should be.
- The transformer failed with a hole in its insulation. It was between the tank capacitor and the primary, at kilovolts.
- Bench tests on a signal generator show nothing at all. Not enough current. Wind more turns through the ring.
Where next
- Inside the driver, which is what these two signals go into.
- Overcurrent is not a setting, the protection side in full.
- Choosing a phase lead inductor, the network that loads the feedback ring.
The two rings on the DRSSTC diagram are drawn separately and labelled separately for the reason at the top of this piece. On the QCW page, taking the tank capacitor out removes the overcurrent one and leaves the feedback.