Posted by Texasmark1 on July 10, 2016 at 05:09:27 from (198.45.234.19):
In Reply to: Ignition Coil 12 volt posted by gdtractor on July 09, 2016 at 19:48:30:
My take:
Energy in the coil is ½ LI exp2 where L is the inductance of the coil winding and I is the current running through the primary winding circuit at the instance the points open (as Mark said). Energy is measured in watt seconds (Joules)....volts x amps x time it takes to dissipate the VA.
The voltage breakdown is determined by the characteristics of the plug. Once the plug and it's environment determine the voltage breakdown point (on it's way to 18,000V or thereabouts), you have established the V of the VIT energy dissipation equation and the fired plug clamps the voltage across the coil to that value. Since the voltage is now clamped, and the current is determined by the fixed resistance of the circuit (and that clamped voltage value which is less than 100v), the only variable left is "how long will it take" to dump the energy in the coil? The longer it flows, the better chance you have of getting your fuel mixture to explode.
On the question of high resistance in the secondary, the turns ratio of the coil is 6 to 18,000 (or 12-18,000 for 12v circuits) meaning you have 3000 turns of wire in the secondary for every turn in the primary circuit which is where the points opening control the amount of initial current....or thereabouts, depending on how much of the charging current is across the wire and how much is across the current limiting resistor.
On the current limiting resistor, I prefer it external too as the heat of it does not add to the heat developed by the wiring inside the coil and heat kills! The external resistor is built to take what's fed to it for many 10's of years. Also I don't like the "convenience" of mounting the coil to the side of the engine block where a lot of OEMs liked to mount it...if you want to talk about reliability improvement measures, regardless of how many million have been built like that for how many billion hours of usage. Just my preference not to have it there.
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