A ferrite ring with a few turns of wire on it, threaded onto one of the thick conductors going to the primary. It is how the driver finds out what the tank is doing, and it is the only sense the machine has.
What it is and why
The thick conductor passing through the ring is the primary winding, and it has exactly one turn: itself. The wire wound round the ring is the secondary, with N turns.
I_secondary = I_primary / NSo a 500:1 ring carrying 500 A gives one amp out. That one amp is then run through a resistor, called the burden, and what the driver actually reads is the voltage across it:
U = I_secondary × R_burdenTwo hundred amps in the tank becomes a few volts on a comparator input. That is the whole device.
Why the driver needs it at all. A Tesla coil's working frequency is not fixed. The arc adds capacitance as it grows and pulls the frequency down, several tens of kilohertz across a single bang. A driver that switched at a frequency you set in advance would be right for the first few cycles and wrong for the rest. So it does not set a frequency: it watches the current and switches in step with it, which keeps it on the coil's frequency wherever that has got to.
The one decision this page is enough to make
Two rings, not one. The second thing the current tells you is whether it is too high, and it is tempting to take both readings off the same ring.
Do not. The feedback path has a phase lead network hanging off it, which loads the transformer's first stage; the core saturates, and the protection gets weaker exactly when it is needed.
How many turns, what burden, and the number that actually compares one installation with another are all in two rings, not one.
What will get you
And only one of the two burdens can be probed with an ordinary probe. The feedback burden has one side on ground and an ordinary probe is fine. The overcurrent burden has neither side on ground and needs a differential probe.
What goes wrong
- The feedback gives zero, or chaos. Wound the wrong way round. Turning the ring over is the test and it takes ten seconds.
- 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.
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.
The two rings on the department diagram sit on the return lead, after the tank capacitor and before the bridge, which is the low potential point named in the warning above.