Correct. The total circuit impedance is determined by the total resistance, the inductive reactance of the coil, and capacitive reactance of the capacitor.
For any non-electrical types, impedance is essentially a fancy term for the opposition to current flow in an AC circuit, where the effects of capacitance and inductance (capacitive reactance & inductive reactance) are present. This is why test equipment , like a multimeter or oscilloscope, might call out a spec for "input impedance" as opposed to "input resistance", since they usually provide AC measurements as well as DC ones.
Inductive reactance is represented by the equation X
L = 2*pi*f*L where X
L = the inductive reactance (in ohms), f = frequency, and L = inductance. Capacitive reactance is represented by the equation X
C = 1/(2*pi*f*C), where X
C is the capacitive reactance (in ohms), f = frequency and C = capacitance.
Before everyone reading this starts to nod off (unless they already have), the main point here is the location of the "f". Note that for inductive reactance, as the frequency goes up, so does the impedance. In contrast, for capacitive reactance, as the frequency goes up, the impedance goes down.
Electrical components contain elements of each to varying degrees. For example coils have often have a fair amount of resistance in addition to inductive reactance. Other times, one element is so dominant, that the others are considered insignificant. For example a switch or wire has capacitance, but the value is so small that it's effects are probably negligible for applications such as this, leaving the resistive element as the primary factor in determining circuit impedance. There was a question in an earlier post concerning the calculation of primary circuit resistance (wires, switches, grounds, etc), so this is what my suggestion was trying to address.
When the points close, you really only have to deal with the coil and related wiring (RL circuit) - since the capacitor is essentially shorted out. The voltage transition as the points close looks like a relatively high frequency input to the coil. Since the coil is an inductor, if the f is high, so is it's inductive reactance, so it is essentially trying to resist a change in current. This is why if you ever see a plot of current vs. time for a coil charging, it starts out low, and gradually builds up until finally leveling off. The rate of the build up is a function of the inductive reactance, while the steady state after that is due to the resistance in the circuit.
Granted, once you throw both the coil and capacitor (aka condenser) into the mix, things get a little more complicated. Once the points open, you now have an RLC circuit. The capacitor sees the rapid voltage transition caused by the field in the coil collapsing as a high frequency input. The "f" in the capacitive reactance formula is on the bottom, so high frequency = low impedance. The capacitor temporarily acts almost like a short circuit, keeping the voltage potential across the points low enough to suppress any arcing.
At the risk of beating a horse that is not only dead, but hauled away and sold to the glue factory, the question I still have concerns this business about the capacitor forming a tuned resonant circuit, which supposedly increases the primary voltage thus increasing the coil output. I see it in some documents, while it is totally ignored by others. I am just curious as to what real effects it has.
Jerry, your brother sounds like a really sharp guy!! (Must run in the family) Maybe he could create a Saber, or Matlab model of a breaker point ignition system and let us know what the real answer is.
Lastly, in case anyone is interested, I happened to stumble across this site which has a high level schematic of an HEI ignition module. It appears that they use a Zener diode to suppress the peak coil primary voltage.
HEI InfoBest Regards...
