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Re: Placement of Ballast Resistor
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Posted by KEB on January 15, 2006 at 08:45:16 from (192.91.171.36):
In Reply to: Re: Placement of Ballast Resistor posted by Allan in NE on January 15, 2006 at 08:05:39:
Quick explanation - I"m unfortunately at work this morning & can"t spend much time. First, the high voltage pulse is not DC. The inductance of the coil and the capacitance of the condenser form a resonant circuit which "rings" when it is stimulated by the rapid collapse of the magnetic field in the coil when the points open. A mechanical analog of this is a guitar string which "rings" when it is stimulated by a pick. The wave form is actually a damped sinusoid, modified a little by the non-linear characteristics of the spark gap. The impedance through a capacitor (condensor) is a function of the rate of change of the applied voltage. A fast transient such as the high voltage output of the coil has a very fast rise and fall time and does in fact pass right through the condensor (with some phase shift, which is immaterial to this discussion). If the pulse did not pass through the condensor, it would arc across the points and destroy them in short order. In fact, one indication of a failing condnesor is metal transfer from one point contact to the other as a result of the arc. If you ever encounter a set of points with a pit on one side and a corresponding mound on the other, the first place to look is the condensor. Allan, the primary and secondary windings are in fact connected. Check a coil with an ohmmeter. The coil is a autotransformer, with the primary and secondary windings connected in series. There are only three connections, the "+" terminal which is the common connection between primary and secondary windings, the "-" terminal which is the "bottom" of the series windings, and the high voltage terminal which is the "top" of the series windings. Hopefully, I"ll be out of here in a few hours today. If I get a chance in the next couple days, I"ll post a schematic (or a link to one) showing the current paths.
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