The bridge is four transistors. Something has to tell them when to turn on and off, several hundred thousand times a second, in the right order and with the right gaps. That is the driver, and without it the power section is a box of parts.
What it is and why
It finds the frequency. A Tesla coil's resonant frequency is not a number you can set once, because the arc changes it while it runs. So the driver measures the current in the tank, through a ring called a current transformer, and switches in step with what it finds. That loop, current in, gates out, current in again, is what keeps the bridge on the coil's frequency instead of on one you guessed.
It fires early. By the time the driver has decided and the signal has got to the gates, the current has moved on. So the driver deliberately switches before the moment it wants, by a lead it is given. Getting this wrong in the late direction destroys transistors.
It decides when to run at all. The interrupter, or on a QCW the modulator, tells it when a bang starts and when it ends.
It stops. If the current goes past a threshold, it ends the bang or skips a cycle, because nothing else in a doubly resonant coil limits that current.
And what comes out is not a signal straight to the gates. It goes through a small transformer, because the top two transistors have their emitters sitting on the switching node, which is swinging by hundreds of volts. You cannot wire logic to a point that moves like that, so the signal is passed across magnetically.
The one decision this page is enough to make
Whether the parts doing the deciding are comparators.
A Schmitt trigger's thresholds wander with temperature and supply voltage. The driver needs a predictable threshold in two places, deciding when the current crossed zero and deciding when it is too high, so both want proper comparators. Using a 74HC14 is a design that can work rather than one that will.
Everything else, the lead network, the feedback strength, the protection behaviour, is a decision with a number attached and it is inside the driver.
What will get you
Most amateur coils run something from Steve Ward's open UD line, or a clone. Which revision you are holding changes the part numbers but not the logic, and that matters the first time you follow somebody else's modification: the UD boards.
What goes wrong
- The coil will not start, or runs at a frequency nobody asked for. Feedback. The current transformer's winding direction is the ten-second check.
- Transistors die at one tuning and survive at another. The lead, in the late direction.
- The protection never fires. It is a threshold, not a knob position, and it is not a setting.
- A board that worked stops the moment a probe touches it. See above.
Where next
- Inside the driver, the same board with the numbers.
- What a phase lead actually buys, and why switching exactly at zero is the wrong target is its companion.
- Converting a DRSSTC driver, what has to change on one of these boards to run a ramp.
The driver is the box at the bottom of every department diagram, with the gate transformer above it and the current transformers feeding it from the right.