Ballast Resistors
... my point is that DC meters are designed to measure constant voltages and currents. The ignition voltage and current is not constant, so the readings are not accurate in the way the meter was designed to be used. They are averages of the time varying current and voltage. The readings produced are based on the design of the meter for a digital meter, and can differ from meter to meter. That is why I used the word loosely.
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The experiment, based on the average values seems, to be a good approximation as long as we understand that we dealing with average values. I am just drawing the distinction that the peak currents are likely in the ignition system is higher than the average current measured.
Not disagreeing, but I think the important point can be lost among the details.
In this case, we are absolutely not concerned about any instantaneous voltage at any time.
Partial duty cycles and variable wavefroms of varying shapes are all fine.
Why do we have a ballast resistor, and what are we trying to accomplish?
The ballast resistor is there to reduce current flow under conditions of high demand, to reduce the chance of the coil overheating, and also to act as part of the LRC circuit involving the ballast resistor, the coil, and the condenser, which provides its own frequency-based impedance to current flow.
The coil is drawing current a certain amount of the time, as described by the dwell angle, which is really just another way to describe its duty cycle.
For the six, the dwell angle is about 40°, out of 60° of distributor rotation per cylinder.
For the eight, the dwell angle is about 30°, out of 45° of distributor rotation per cylinder.
For both of these, the dwell angle (number of degrees the points are closed) is 2/3 of each timing interval, so it represents a 66.6% duty cycle.
Both coils are spec'd to draw 3A switched on constantly, 1.9A in use at idle.
Coincidentally, 2A (1.9A) is 2/3 of 3A.
Now, when you run a bunch of electric current through a whole lot of thin wires, they heat up.
There are formulas for exactly how much they heat up, depending on the diameter of the wire, its exact composition, and electromagnetic forces that may result from, say, being all wound up in a tight coil. All of this is for straight DC, completely disregarding frequency-related impedance effects.
So, if you run a whole bunch of current through a whole lot of wire (there's what? A hundred or more feet of wire in a coil, right), and that wire isn't laid out in the open, like strung along telephone poles, but instead is all wrapped up tightly in a can, the heat generated by that current has a hard time escaping, and the coil will heat up.
The longer you apply the current, the more the coil will heat up.
If you interrupt the current, the heat has a chance to dissipate a bit, but if you leave it on steadily, it will build up more.
So, at idle, with the coil "on" for longer stretches between sparks, the coil will heat up more, even though it does have longer periods to cool off in between. At higher RPMS, there is less time for heat to really saturate the coil, so it heats up less. Also at higher RPMs, you are likely to have greater impedance, so that will further restrict current flow and heating (I don't know the inductance of the coil, so I can't calculate just where the impedance is lower or higher in this circuit).
So, if the ballast resistor heats up more with increased current flow through the coil, and by heating up, its resistance increases, it restricts current flow through the coil at a time when the coil is only too happy to pass current and overheat.
If the ballast resistor cools down at higher RPMs, partly due to the increased impedance due to the increased frequency, it will allow more current to flow, helping to overcome the increased impedance and keep enough current flowing to run the engine.
What's my point? Actually, I forgot.
No I didn't. It is that what you want to do when measuring the resistance, current, and voltage here is not to measure the voltage at an instant in time during a long series of high-frequency oscillations (at 4,000 RPM in a \6, you are firing 200 times a second or 200Hz), but to capture the overall averaged values, which are what would be heating (or overheating) the coil, points, supply wires, etc. You can run a huge amount of current through a small wire if you do it briefly enough (there are formulas for this kind of thing). So long as you don't exceed the time required to soften the insulation or melt the wire, you're fine.
So in this case, where you're trying to prevent the coil from overheating, it's the big, mushy amount of current flowing that matters, not the little variations, whose effects will create little bits of heat which will be absorbed.
Analogue meters will serve this purpose. As Mike said, digital meters are running some sort of basic (not necessarily digital) programs that are likely reading the measured values at fairly high frequencies and then averaging them out according to certain rules, which may or may not provide a true dead average of the energy passing through, but an analogue instrument will do this automatically.
There will be a short quiz.
– Eric