Re: Placement of Ballast Resistor

Posted by John T on January 15, 2006 at 08:06:31 from (66.244.90.98):

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

Good Morning Thomas, Great question and thanks for your curiosity. I love your type of questions and these sparky discussions. Im here to share and learn and help educate myself and hopefully, others, and NOT to argue, so I will begin and look forward to learning and discusisng this with others I hope follow.

First of all, I respectfully disagree with your conrentions about the condensor being an "open circuit" during firing. Although the points open which initiates the collapse of the magnetic field and the subsequent induction of low primary voltage over into the high voltage secondary (mutual inductance transformer action), the condensor (which is indeed an open circuit to DC EVENTUALLY) is not IMMEDIATELY an open circuit when voltage is first appied. A condensor is initially and briefly a "closed circuit" during the time it conducts current unti such time the opposite electrical charges have accumulated on its plate surfaces, ONLY AFTER WHICH it appears as an 'Open Circuit" and DC current flow stops, i.e. it stops conducting.

Soooooooo, although the points are open as you say at firing, and its true a condensor eventually appears as an "open circuit" to DC, right after the points open SUCH IS NOT THE CASE. I dont have them in front of me, but have seen oscilloscope traces of what occurs in the Kettering Coil Ignition immediately after the points open, and what happens is theres some oscillations and ringig going on in that LC circuit back n forth from the condensor to the coils primary.

Its indeed true, the coil will still fire EVEN IF THERES NO CONDENSOR WHATSOEVER in the circuit HOWEVER the spark will not be quite as strong since theres none of the ringing and oscillations of current and energy transfer that takes place between the coils primary and the condensor when its in the circuit. If the condesnor were too large, there would be NO SPARK as the current flow through the coils primary would be over damped and slowwwwwwwww to change (no necessary sudden field collapse) while if its too small the points will burn up quickly BUT STILL A SPARK. Its the right sized condensor with the right time constant to make the proper LC circuit which yields the maximum spark energy.

It has been my "understanding" that if an inductive wirewound ballast resistor is placed in between the coil and condensor, that it may hamper the ringing (back n forth energy transfer) described above plus may also waste some of that enerfy as it gets converted into pure heat versus potential magnetic/ inductive energy. Thats how I understand the ballast should be ahead of the coil NOT between the coil and the condensor.

NOTE: If I get time later (gotta go help my 82 yo mom) I will dig up some Kettering Ignition theory and try n join back in the discussion and your GREAT question. I cant wait to read other explanations so we may all learn something here.

Thanks agian Thomas, Best Wishes n God Bless, I will check in again later.

John T (long retired electrical engineer, i.e. a lil rusty lol)

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