I'm experimenting with capacitive discharge circuits, and am wondering the difference
between an open core transformer (a single bar of laminations) and a transformer
with a closed magnetic path (E-I core)?

When a cap bank is discharged into an open core transformer, is the energy on the secondary
higher or lower than a closed core? Does it make any practical difference for a single shot?

I don't have scope, so I am measuring spark length on a gap in the secondary output. I can't
detect a difference, but my test apparatus is less than uniform. I've read the document here
at AAC on transformers, but still scratching my head...

 

When a cap bank is discharged into an open core transformer, is the energy on the secondary
higher or lower than a closed core? Does it make any practical difference for a single shot?

The energy on the secondary is very close to the energy on the primary. I think that a single shot can transfer energy as well as those handling an continuous pules train. The main thing to look out for is whether there is significant loss in the coupling between the primary and secondary

 

I seem to recall most ignition coils have a single bar of laminations.

I'm winding my secondary tightly over the primary to more closely couple them, but would rather
use a bar, or open core design for simplicity. If I'm not going to loose energy in a single shot
then that's what I'll do.

 

I seem to recall most ignition coils have a single bar of laminations.

That was true in the old days. But modern ignition coils use an E-I type core like a mains transformer, to get the higher energy for the computerized ignitions. It started pretty much when GM switched to what they called HEI; High Energy Ignition and the old round coils couldn't keep up.

The "laminations" in the old round coils were sometimes not flat laminations but round wire in a group.

 

That was true in the old days. But modern ignition coils use an E-I type core like a mains transformer, to get the higher energy for the computerized ignitions. It started pretty much when GM switched to what they called HEI; High Energy Ignition and the old round coils couldn't keep up.

The "laminations" in the old round coils were sometimes not flat laminations but round wire in a group.

Thank you. I seem to recall those E/I, GM coils. I also have taken apart a GM coil-on-plug, and discovered it had a tiny E/I core. Boshe makes a coil-on-plug with a ferrite core.

On automotive systems, higher inrush probably had higher emf, more radio noise, etc., so until those problems were solved, single core ignition coils were good enough. Still, I have driven those old oil filled ignition coils with capacitive discharges of 600v,
and they can really throw a spark, 50mm or more, without burning up. Not much current in the output however.

I've done some reading, and learned that E/I transformers have higher inrush current, especially with grain-oriented steel. I think this might be desirable for capacitive discharge, although I don't know how to test such a thing for total energy, as spark length only tells part of the story, voltage...

 

Bringing up the Bosch version makes an interesting point. This field isn't my specialty but I have heard in some ignition systems the energy going into the spark is really a gradually decaying oscillation, ie AC not DC. You'd HAVE to have that approach if the coil was ferrite, or else (because of limitations in the core material) the decay would be too fast and the energy transmitted would be way too small! I can tell you that some of the "ignitors" on the small jet engines work this way (the person who showed me this used to work at Champion), now they aren't "timed" but they're basically spark plugs too. I wish I could "shed more light" on the topic because what I'm saying is actually adding to confusion, clearly the parameters you put into designing the windings and core and the design of the accompanying waveform and circuit are tightly related!

 

Bringing up the Bosch version makes an interesting point. This field isn't my specialty but I have heard in some ignition systems the energy going into the spark is really a gradually decaying oscillation, ie AC not DC. You'd HAVE to have that approach if the coil was ferrite, or else (because of limitations in the core material) the decay would be too fast and the energy transmitted would be way too small!

Yeah, I seem to have seen somewhere that each instance creates a ringing. Perhaps there is some parasitic capacitance inherent in the design of transformers. It's above my present understanding. I have the idea that adding a cap in parallel with the primary coil has the effect of stretching out the spark duration, and this somehow enables better energy transfer (I suppose it's the area of study referred to as L/C.) And then there is the effect of the circulating flux within the core itself. How the arc is sustained with all this ringing going on is a mystery to me. Maybe someone here can add insight.