A current transformer is a ferrite ring with a few turns of wire on it, threaded onto one of the thick conductors going to the primary. The driver uses it to see what the tank current is doing. On a machine that takes its feedback from current, it is the only sensor the driver has.
What it is and why
The thick conductor passing through the ring is the primary winding, with exactly one turn (the conductor itself). The wire wound round the ring is the secondary, with N turns.
I_secondary = I_primary / NThe ring divides the tank current by its turns count, which gives a current small enough to work with. That current runs through a resistor called the burden, and the driver reads the voltage across it:
U = I_secondary × R_burdenHundreds of amps in the tank become a few volts on a comparator input.
The current through the hole does not saturate the core. A ring with seven hundred amps through it looks like it ought to be deep in saturation, and for an inductor that would be right, because there the flux follows the current. A transformer behaves differently, as Steve Ward explains:
Saturation of a transformer happens because there is too much magnetic flux in the core. Flux is proportional voltstime/number_of_turns. For a simple inductor, the current is proportional to voltstime/number_of_turns... which means flux is proportional to current. Now for the "confusing" part... when looking at a transformer, there is another coil coupled in there (the secondary) and it's current interacts with the primary coil current to give a resulting flux in the core. Any secondary current will generally cancel out the additional primary current, and so despite operating at a high level of current, the magnetic flux in the core is the same as if there were 0 secondary current and only the tiny "magnetizing current" that would be in the primary coil as its energized by the source voltage.
He wrote that about the current sharing transformers in a paralleled QCW inverter (HVF 294), and the same reasoning applies here. The core is sized by volt seconds per turn, not by amps, so a ring you can hold between two fingers can sit on a conductor carrying hundreds of them.
It follows that the way to saturate this ring is to take the secondary current away, and both warnings below come from that. Open the burden and nothing cancels the primary current any more, so all of it becomes magnetizing current. That step is our inference, not Ward's; he was talking about a different transformer.
The driver needs the ring because a Tesla coil's working frequency moves. The arc adds capacitance as it grows and pulls the frequency down, by several tens of kilohertz across a single bang. A driver switching at a preset frequency would be right for the first few cycles and wrong after that. Instead it watches the current and switches in step with it, so it stays on the coil's frequency as that frequency drifts.
What one actually looks like
It is a ferrite ring small enough to hold between two fingers, with a few tens of turns of light wire and two leads. The thick tank conductor goes through the hole without touching anything. The coupling is purely magnetic, so the part can sit on a conductor swinging at hundreds of volts and still hand a few volts to logic.
A working coil usually has two of them, often identical. One feeds the driver's feedback input and the other its overcurrent input. Both see the same current but are used for different things, so do not try to save a part by fitting only one (more on that below).
Where they go is set by voltage. They sit on the return lead, after the tank capacitor and before the bridge, because that is the low potential end of the loop. The other end of the same loop sits at the full tank voltage; the warning below covers what happens if a ring ends up there.
The ring also has a direction. Wound one way round it gives the signal the driver expects, and the other way round it gives the inverse, so a self-oscillating driver will not start the coil at all. Turning the ring over fixes it in ten seconds, and some boards have a switch that does the same. Why this is a sign reversal and not a phase error, and why no amount of lead inductance will correct it, is covered with the two rings.
You can wind your own, and on a bench you often have to, because a measurement transformer is a different part from the two the driver runs on. Winding one is covered with the scope.
The one decision this page is enough to make
Use two rings, not one. The second job of the current signal is to say when the current is too high, and it is tempting to take both readings off the same ring.
Don't. The feedback path has a phase lead network on it, which loads the transformer's first stage. The core saturates and the protection gets weaker just when it is needed.
The burden value is also constrained. Gao Guangyan works an example on his UD2.7 page, designing for a bridge that must not see more than 700 A of primary current:
650 A 700 A, the design ceiling
through 625:1 1.04 A 1.12 A
across R17 = 5.1 Ω 5.30 V 5.71 V [derived, both columns]
against a 7 V ceiling 1.32x 1.23xThis is a worked example, not a measurement. He writes "Lets say we have a 625:1 current transformer" and "Lets say we are building a DRSSTC", and the only figure taken from the actual board is R17, "a single 5.1R 2W by default". Whatever numbers you put in, the pattern is the same: twenty three per cent of headroom at his own ceiling, and the burden uses it up. Also, 625:1 is not 625 turns on one core. It is two 25:1 rings in cascade, and the ratios multiply.
Turns count, burden value, and the figure that lets you compare one installation with another are in two rings, not one.
What will get you
Only one of the two burdens can be probed with an ordinary probe. The feedback burden has one side on ground, so an ordinary probe is fine there. The overcurrent burden has neither side on ground and needs a differential probe.
What goes wrong
- The coil will not start at all. The ring is wound the wrong way round, which flips the sign. Turn the ring over to test it, ten seconds. A board with a phase select switch does the same without the screwdriver.
- The insulation punctured. It was in the high potential position, between the primary and the tank capacitor.
- The protection is weak exactly when the current is highest. One ring doing both jobs.
- New faults appeared after probing. A probe ground on the overcurrent burden, where neither side is ground.
- The protection reads a current the coil is not drawing. Check the burden, the ratio, and whether the threshold was set against either. The trip level is not a knob position.
- The first cycles of every bang look wrong and the rest look right. This is expected. The ring has nothing to report until current flows, so every bang starts open loop.
- The signal clips flat before the coil is anywhere near its limit. The feedback is too strong for this machine. That is a number to adjust, not a fault, and it is the one to compare.
Where next
- Two rings, not one, the same part with the numbers, and the feedback strength figure that matters more than the turns count.
- What a phase lead actually buys, what the driver does with the signal.
- Read the envelope, because the picture this ring gives you is the cheapest tuning tool on the bench.
- Scoping a coil without killing it, for probing these two safely and for winding a third to measure with.
On the department diagram the two rings sit on the return lead, after the tank capacitor and before the bridge, at the low potential point from the warning above.
- Current transformer does the cascade ratio and the burden voltage worked above, and puts a figure on what stray series inductance costs you.