whats the difference between an AC Ignition coil and a DC one?

That's true, Steve. But is the base material conducting or is it the surface contamination?

Paul

When the voltage burns through it in multiple places I would say the problem is the base material. It doesn't insulate high voltage very well. I have tried JB weld for this same application. It burned through and arced from the moment the current hit the repair.

I suppose some would accuse us of hijacking Gunner's post, but some may learn from our discussion.

Probably any regular epoxy would fail when the voltage is sufficient. We have a customer that must test his circuit boards at 6kv. His new design arc'd between leads of through-hole resistors. A quick fix using corona dope was tried. Didn't work. More layers of corona dope. Still arced. Had to do a board redesign to reorient the resistors to give about 3/4 inch spacing between leads. That to cured the problem.

I don't know what is in corona dope, but in the 1950's we used to repair TV flyback transformers with it. Why it didn't work on the resistors is unknown.

Paul

Gunner Asch has brought this to us :

I didn't know that any chargers put out filtered DC. I would expect them to be half or full wave rectified only.

I can't tell you the difference. Maybe there isn't any. A coil is nothing more than a large step up transformer. It requires a change in current to operate. It can't work on straight DC. The points or CDI provide a change in DC current. A pulse of DC current into the coil provides a pulse of a spark out.

Epoxies are routinely used at 50KV and more.

There's an interface between the epoxy, dope or whatever and the circuit board. Arcs can sneak along this interface. If a slot is routed thru the board and epoxy fills the slot and covers HV conductors on both sides of the board, then the "short path" along the board surface is blocked.

One of my former jobs was testing the insulation on some motor housings, we tested at 20kv normally. I got to "play" with other insulating materials as well.

Most corona dope is nothing but a thick enamel or solvent dissolved polystyrene material. About 4kv per mil was the rating on the stuff I used. The problem in your case may have simply been that the hole spacing was tight enough that no insulation would have worked unless you could make the arc path itself longer.

One of the other folks in our plant used to say that the testing I was doing was scary. Then I mentioned that the powder gun he used daily ran at a 40kv charge. His eyes turned into saucers then.

It's a polyurethane varnish. Google on "corona dope", go to the GC/Waldon website, and look at the MSDS.

Joe Gwinn