Posted by Texasmark on December 27, 2007 at 11:15:16 from (12.39.110.55):
In Reply to: Another Coil question posted by John T on December 27, 2007 at 07:33:14:
Gotta wiggle in here.
Lotta info there so I"ll just shotgun this.
Probably where the secondary low voltage winding is connected is a matter of convenience for the manufacturer. As stated, it makes little difference when considering the secondary.
Service manuals that I have visited indicate that the points are designed for 5 amperes of interrupt current. That sets the DC resistance of the primary circuit, however you get it, primary coil resistance, and or a current limiting resistor that may be located anywhere, including inside the coil case, which makes the coil case very hot by the way.
From what I know about the subject, the direction of current flow in the plug helps to determine the terminal life and I don"t remember the preferred direction. Hence OEM"s polarize the applied voltage accordingly.
When I was a technician serving in the military, I was instructed on the electron theory. Made great sense for troubleshooting. When I got the different education, where hole theory (what happens when an electron moves on to another location and leaves behind this hole) was the norm, I found it very helpful in "designing" circuits whereas electron theory complicated the process. Prior to that it (hole theory) was extremely confusing and I couldn"t understand why it was taught.
The capacitor/condenser in a conventional ignition system does generate an LC circuit (with the inductance of the primary of the ignition coil, including what"s reflected from the secondary) which has a natural resonance, meaning it can and will oscillate if un-iterrupted (like if someone sticks a diode in the circuit) and the field (oscillations) decays at the rate of the circuit resistance, as said, unless interrupted.
The purpose of the capacitor is not necessarily to resonate into ringing oscillations, but to control the release of the L di/dt energy [measured as 1/2 L(i squared)] from the coil, developing across the contacts as they break. The coil attempts to "keep the current flowing regardless of the circuit resistance". Without the cap. the coil will (theoretically) generate whatever voltage it takes to keep that (instantaneous) current flowing, so when the points open, circuit resistance goes to infinity, and hence per Ohm"s law, voltage across that infinite resistance (the point gap) would "theoretically" go to infinity resulting in a horrible arc and terrible contact degradation.
Well, you can"t do that and have your points last for any length of time, so you install a device that operates exactly opposite to an inductor (the cap). As the points open, with the cap wired across them, the cap starts sucking up the current (starting from zero volts since the points had it shorted out) and the voltage across it starts rising in a sinusiodal fashion (LC circuit resonance).
The cap is sized for two things: Large enough to keep the voltage height and rate-of-rise below the ignition (arc) potential of the point gap (which makes it need to be large) and secondly, to do this fast enough to be ready for the cycle to repeat for the next firing of the next plug (which makes it need to be small). So you have to select the value accordingly.
Transistor ignition systems operate significantly different. The energy to fire a plug has been determined to be a certain number of "joules" (watt-seconds, that is Voltage x Current x Time). A capacitor of a distinct size is charged to a distinct high voltage (several hundred volts)and holds the charge [energy...1/2 C(Vsquared)] usually because of a "charging diode". This charge is held until the trigger pulse to the firing switch (transistor, FET, SCR or what have you) coming off a magnet on the flywheel/distributor, or whatever moves, that went by a coil to produce the pulse.
The pulse triggers the electronic switch component which essentially shorts out the circuit and the transfer to the ign. coil primary is accomplished, usually in 1/1000th the time of conventional systems and at 2 times the value, because of the circuit parameters and component values. Obviously, the latter is much better in firing your plugs.
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