Points and Condensers

[pamcopete]

The points system forms a tuned circuit with the inductance of the coil and the capacitance of the condenser when the points open. This tuned circuit briefly acts as an AC source of current to induce a higher voltage in the secondary, in addition to the voltage created in the secondary by the collapsing magnetic field.

Without the condenser installed, the primary resonates at a much higher frequency that is not able to pass through the effective low frequency filter effect of the coil acting as a transformer, so only the voltage induced into the secondary by the collapsing magnetic field is there to produce a voltage in the secondary, and as a result, that voltage is much lower than the voltage obtained by the effect of the tuned primary. This is why the coil will produce a very low voltage without the condenser, sometimes even too low to jump the plug gap.

The reason that there is more visible arcing of the points without the condenser is because the rise time of the initial voltage is faster than the time it takes for the points to open so the high voltage is there when the points gap is very small. With the condenser installed, creating a lower frequency and slower rise time, the points have more time to open and are at a wider gap for any given voltage, thus limiting arcing for all but the highest voltage.

Transistor driven coils are more dynamic. The initial voltage rise induced by the collapsing magnetic field is very fast because there is no artificial capacitance to lower the resonance of the primary tuned circuit, made up of the inductance of the coil and the inherent capacitance of the coil structure. Both the primary and the secondary windings produce a high voltage from the collapsing field until the primary voltage is clamped by the transistor at about 400 Volts, at which time, the primary gets a second kick of current that is the equivalent of 400 volts across whatever the primary resistance is. This second kick of current in the primary induces even more voltage in the secondary, and explains why there is a higher secondary voltage from the same coil with a transistor driver rather than points



In both cases, the inductance of the coil combines with the capacitance in the circuit to form a tuned circuit. The "artificial" capacitance of the points setup results in a fairly low frequency tuned circuit, whereas the capacitance in the coil itself forms a tuned circuit in the case of the transistor driven system. The capacitance in the coil is very small, so the resulting frequency of the tuned circuit in the case of a transistor circuit if very high, which produces a very fast rise time pulse.

(Here comes the analogy)

This in analogous to a teeter totter that is shorter on one end than the other, lets say by a 10 to 1 ratio.

If you move the short end 1 foot, the long end will move 10 feet. If you try to move the short end very fast the long end will not follow your movements because of the inherent inertia of the board material. This is analogous to the points without a condenser. If you were to then place a weight on the short end that slowed your movements down, then the long end would follow. In other words, you have tuned the board to respond to your movements. The weight in this example is equivalent to the condenser.

The other way to get the long end to respond to your movements on the short end is to just suddenly move the short end from one position to the other. From up to down, as an example, and just hold it there until the long end responds. Not only would the long end respond, but it would overshoot and move more than the 10 to 1 ratio allows. This is analogous to the transistor driven coil. There is no condenser. It just allows the primary voltage to go to a very high value and holds it there, forcing the secondary to respond very rapidly, exceeding the nominal primary to secondary ratio of turns.

If not these theories, then some other theory is needed to explain a couple of things:

1. A points driven coil produces a higher voltage with the condenser in the circuit.
2. A Transistor driven coil will produce a higher voltage with a much faster rise time than the same coil driven by points.

[pamcopete]

Here is a video of a points plate running in a test rig and driving a dual output coil. The view starts with the points, the lights are turned off so we can see the arcing across the points, we then pan over to the spark plugs.



Here is the same sequence with the condenser removed:



Notice the much more active arcing across the points with the condenser removed, and the very weak spark on the plugs.

So, again, the condenser is there to form a tuned circuit with the coil to create a higher voltage at the plug. The fact that the points benefit from this is incidental.

Note: The occasional missing spark is not really missing. It's just the camera framing.