Posted by Gerald J. on December 27, 2007 at 09:25:26 from (4.254.71.11):
In Reply to: Another Coil question posted by John T on December 27, 2007 at 07:33:14:
1. The internal connections you mention are the two most common connections. It would be practical to connect the low end of the HV winding to the metal case.
Surely the inductive kick (Ldi/dt) of the primary winding can add or subtract from the voltage created in the high voltage winding from the collapse of the magnetic field of the core. But is it significant? I think that a couple hundred volts from the primary turns compared to 15,000 or so on the high voltage winding isn't significant. So I think it doesn't matter which of the three connetions are. That its more important to match the output polarity so the hot electrode emits electrons better.
The high voltage generated in the coil gets clipped by the break down of the gaps in the secondary circuit, first the one at the rotor and then the one at the plug inside the cylinder. The function of the coil is to make available enough voltage to cause a spark at the plug under all operating conditions which has to include the supply voltage, the time of the dwell (during which the primary current rises relatively slowly), the varying caps at the rotor and each plug and the compression pressure in the cylinder at the time of firing. The pressure in the cylinder will vary a great deal from throttle closed to throttle open and even with throttle open will vary with the engine speed becuase of flow limitations in the entire intake system. So the voltage the gaps clip the voltage to depend on all the engine operating conditions.
The voltage created from the primary is V = Ldi/dt, that's a differential equation that has a time varying solution and the instantaneous solution is dependent on the polarity of the current. The polarity of the magnetic field in the core depends on the direction of current through the primary, so the polarity of the voltage generated in the high voltage winding when that magnetic field collapses is dependent on the direction of the primary current.
This gets more complicated by the addition of the condenser that makes that primary current oscillate giving a series of voltage pulses of both polarities that makes that (sans condenser) wimpy yellow spark turn to a fat blue spark.
I'm thinking that the first gap breakdown limits the voltage so that the opposite polarity pulses don't spark at the plug because of that observed difference in plug firing voltage according to polarity. I'd rather check that with a scope than a pencil lead, I'd see more detail.
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