Re: Placement of Ballast Resistor

Posted by Gene-AL on January 16, 2006 at 14:47:48 from (65.4.38.60):

In Reply to: Placement of Ballast Resistor posted by Thomas Donahy on January 15, 2006 at 07:27:04:

Thomas,
My take on your original question: The return path from the sparkplug ground for the high voltage current has two low-impedance paths: One through the battery, switch, and ballast to the coil; the other through the condenser to the coil. The high-voltage arc current is very small (a few milliamps) compared to the primary current, and would have little, if any, effect on these two low voltage paths. Placement of the ballast resistor before or after the coil would seem not to matter, except for 12V switched starting.

It might be useful to consider the current in the two joined circuits which form the Kettering ignition system. The primary coil winding could have currents approaching 8-10 Amps with the points closed which will, through inductive field collapse in the coil primary, maintain that same current through the points at the instant they start to break open. Without a condenser in place, some of the stored energy will dissipate in a hot arc across the points, saping total field energy available for the high voltage ciruit which triggers at the same time. The arc will also erode the point contacts excesively.
However, with the capacitor properly in place and initially at zero volts charge (the points had it shorted prior to this time), the high initial primary current can continue at point opening as the coil's field collapses and the condenser charges. The current then falls at a rate set by the capacity of the condenser. At point opening, all the coil voltages reverse in polarity and spike, then will oscillate (ring) back & forth (AC), falling to zero in a fraction of a second.

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