Almost nobody uses this and it is the cheapest diagnostic on the bench. The shape of the primary current envelope across one bang tells you whether the coil is tuned. One capture, one answer. The analysis is Steve Ward's.
The ideal case, so you know what you are looking at
If the coil had no energy to store, and were not detuned, the primary voltage and current would simply rise linearly. Everything you actually see is built on top of that.
Two things are added. There is a step at the start, because before any power reaches the streamer the tank has to swing up to the minimum secondary voltage, so the envelope does not begin at zero. And the tuning changes the transfer ratio from primary to secondary as the ramp goes on, which bends the rest of the shape.
The three things you might see
- A nearly flat envelope. The coil is tuned for the optimum at the end of the ramp. This is what right looks like.
- A rise that accelerates towards the end, faster than linear. It is tuned badly, and the section below says which way.
- Roughly quadratic. What loneoceans measured on his own QCW.
Why flat means correct
The arc grows, its capacitance adds to the secondary's, and the secondary slides down in frequency.
If the coil is tuned for the optimum at the end of the ramp, then at the start it has not reached that optimum yet: it begins detuned. A detuned system transfers energy less well, so the start needs a higher current. Then the arc grows, the system comes into tune, the transfer efficiency rises, and the current needed barely goes up even though the power into the arc is climbing.
Two effects cancelling. The result is a shelf.
Why a tail that curls up means wrong
If the detuning does not match what the arc actually does, then the arc does not bring the system into resonance, it carries it further away. The transfer efficiency falls towards the end of the ramp instead of rising, and to put the same power into the arc the current has to go up.
Which is why the acceleration appears at the tail specifically, where the arc is longest and its capacitance largest.
Steve Ward: the streamer capacitance detunes the system and the primary current may grow more than linearly near the end of the spark event, because the poor tuning demands higher primary current.
What to do about each
- Curls up at the end. The detuning is too small; you did not go far enough to meet what the arc does. Take the primary lower: add turns, or move the tap out.
- Current is high from the start and barely changes. The detuning is too large; you overshot. Take the primary higher: remove turns.
Three ways to misread it
Look at the start of the bang separately. The rise in the first few cycles is the tank swinging up to the minimum voltage at which a streamer exists at all. It is always there and it says nothing about tuning.
And it will never be perfectly flat on a QCW, because the bus voltage itself climbs several-fold across the ramp. Ward's shelf means "rises weakly", not "constant".
The sting in the tail
What the envelope is a picture of, on a QCW, is the frequency sliding down the ramp against the detuning you chose to meet it — which is its own page, and the reason this capture is evidence about the end of a bang and not about all of it.
The current transformers on the department diagrams are where this capture comes from, and there is a piece on them, including why the one you probe with an ordinary probe is not the same one.