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When the primary loop is closed by the points electrical energy "slowly" builds in the coils. Let me put this in simplified mechanical engineers language. Thanks to all for the interesting descriptions but what I see is one plug attached to each end of the secondary coil. I should have looked at the wiring schematic. It would work the exact same way if driven with +12V and 0V excursion limits ( happens in the motorcycle as it would with +6 to -6V excursion swings. The transformer doesn't care about a true ground reference, only the peak to peak excursions. However later you learn that it is the peak to peak excursion that is important (actually RMS, but that is getting even more complicated), particularly in transformer operation. Usually A/C training begins with a model where ground is a reference and voltage is labeled for a positive cycle followed by a negative cycle. And, this eases understanding of DC power distribution. Lastly, and ignore this if it is too confusing, many assume ground or 0V is a reference point. But, even discounting the Back EMF from the coils to points, the fact that the power going to the coils is not a steady voltage, but changing, makes the electrical model AC instead of DC. In this way, the wire between points and coil actually have A/C current on them. The power travels back toward the points and is damped by the condenser. There aren't any DC transformers, because unless the magnetic field lines of flux are moving across the secondary windings, no voltage is induced in the secondary windings.īy the way, when the coils spark the plugs, the collapsing lines of flux also cross the primary windings, and induce a voltage there, as well. The coil's primary, transfers power to the output coil's secondary, in the same way a transformer transfers power, with changes to the magnetic lines of flux. Positive voltage to the coils (points closed) charge the magnetic field, remove the voltage to the coils (points open) and the magnetic field collapses. In essence, the points (in operation) turn the DC from the battery into pulsed AC power going to the coils. This usually means it will work better/best for the most predominant pulsing system in the automotive world, which is a positive pulse model. Although a trigger circuit can be designed to be insensitive to pulse polarity, there is usually a goal to minimize parts count and cost of the circuit. Timing light trigger designs vary between models and brands. Further, one side has had a nice stable reference voltage before rising edge occurs, whereas, the other lead has a very short period of stability before the rising edge if signal, in fact, only the width of the spark pulse. On the other lead the falling edge is the more rapid transition, and the rising edge is the slower transition. However, the rising edge on one lead is the beginning of the event with a more rapid transition than the falling edge. The ignition spark pulse has both a rising edge and a falling edge, which does occur on both leads. Timing light triggers can be sensitive to, and differentiate between, sensing a positive pulse or a negative pulse. Therefore, one lead has the opposite polarity of the other as far as current flow is concerned. When the voltage rises enough for the arc to form at both spark plugs current flows in the output coil using the cylinder head as a conductive path between the two spark plugs. The ends of the wire must become connected in order for current to flow through it. The two spark plug leads on the coil attach to each end of a coiled wire inside the coil housing. The rules are quite different.) The coil does need a circuit conductive path for current to flow.
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(FYI, A coil's output work on an AC model, not a DC model. The coil needs no ground reference, and that is irrelevant for current flow.