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Help with ignition coil

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Help with ignition coil

Unread postAuthor: skyjive » Mon Oct 18, 2010 2:04 pm

So I recently obtained a cheap ignition coil with an eye towards using it to generate high-voltage sparks. Having read up on how they work, I understand that the low-voltage primary coil connects to the two screw terminals on the "shoulders". The central port is clearly one connection of the high-voltage secondary coil, but in the diagrams I've seen the other end of the secondary is supposed to be grounded, and I can't figure out how that's supposed to work. Since these things evidently generate some pretty serious voltage I don't want to proceed without understanding what's going on. Any words of wisdom?

Some pics of the coil in question:

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Unread postAuthor: jrrdw » Mon Oct 18, 2010 2:22 pm

Rob that off a Ford hahaha...Hook up the ground from the battery to the ground on the spark plug as well as the coil ground. Put a set of breaker points in between the coil ground and spark plug ground. Close the points to charge the coil and open them to spark the spark plug.
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Unread postAuthor: Fnord » Mon Oct 18, 2010 4:08 pm

To save you some frustration I'd like to point out that if you hook the coil up backwards, you'll destroy it. Make sure you have your +/- terminals figured out correctly. I believe there are some diagrams on the wiki of coil setups.
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Unread postAuthor: Zeus » Mon Oct 18, 2010 5:03 pm

Sorry Fnord, ignition coils aren't polarity dependent, you can't kill them like that. Try a switch with a 1 micro farad capacitor in parallel with the contacts, then connect that in series with the ignition coil. Power it with eight D cells, a sealed lead acid battery, or a car battery.

The arc will be from the + terminal to the center terminal. Arcs are about 1 inch.

Hope that helps, I'm using the same setup for my nearly finished hybrid.
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Unread postAuthor: Technician1002 » Mon Oct 18, 2010 7:30 pm

The ground side of the high voltage winding is most often connected to the - terminal on the shoulder or connected to the can.

The spark current path is either through the can and engine block from the plug on the ground side, or through the condenser at the points to the engine block. The current is low in relation to the primary so this is not critical.
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Unread postAuthor: jrrdw » Mon Oct 18, 2010 7:58 pm

"Keep it simple, stupid."

I forget what member coined that phrase but it fits in real nice right now. Ya'll are gonna confuse him....maybe...
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Unread postAuthor: Technician1002 » Mon Oct 18, 2010 8:05 pm

OK, this page will make it easier..

http://en.wikipedia.org/wiki/Ignition_system

The minus of the high voltage coil is either tied to the can and needs grounded or ties to the - primary wire that connect to the points.
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Unread postAuthor: skyjive » Mon Oct 18, 2010 8:10 pm

As far as the circuit setup and conceptual stuff goes I don't have a problem, my question is regarding the physical attachment points of the ignition coil. The primary (low-voltage) coil attachments are easy, they're the screw terminals on the sides. But there seems to be just a single central terminal for the secondary (high-voltage) coil, so my question is how do I have two separate wires, one high voltage out and one ground, attached at once?

EDIT: Just saw Technician's last post, that's what I needed. Thanks!
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Unread postAuthor: Gippeto » Mon Oct 18, 2010 11:59 pm

On a bit of a side note...

Some of the "things" we rebuild at work are magnetos from natural gas engines. Forcing the spark to jump a 1" gap is VERY hard on the coil, and will lead to premature failure.

The max gap should be 8mm or 5/16" to ensure longevity.

Just sayin' :)
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Unread postAuthor: skyjive » Tue Oct 19, 2010 1:44 am

I plan on using a spark plug with a spark gap of maybe 1/8 inch so won't be an issue.

Just to double-check, does this setup look okay? (sorry for the crudeness of the diagram)

Image

And on a safety note, when I'm taking the circuit apart (with the switch open) the only residual stored energy I have to worry about is in the capacitor, right? So if I disconnect the battery and then short the capacitor to discharge it that should be ok?

Thanks for all the help guys.
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Unread postAuthor: Technician1002 » Tue Oct 19, 2010 2:19 am

Connect the shell of the coil to the battery minus. If the high voltage is connected to the shell internally, this could cause a damaging arc inside the coil. It is the only connection missing. Use a pushbutton switch. The switch needs to be on for just a couple milliseconds.

Spark is made when the switch turns back off. The sudden interruption of the current causes the voltage spike.

Do not use an electrolytic capacitor for the cap. Use a metal film or poly cap with a voltage rating of more than 300 volts.

Never touch the wire between the switch and the coil minus. To do so will give you a first hand lesson in inductive kick.
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Unread postAuthor: Fnord » Tue Oct 19, 2010 9:34 am

Sorry Fnord, ignition coils aren't polarity dependent, you can't kill them like that.

I've personally killed 2 of them like that. It didn't make any sense to me either but it happened regardless. If there's something else I did that could have killed them I have no idea what it was. I was only using a 9vt battery at the time.
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Unread postAuthor: Gippeto » Tue Oct 19, 2010 1:36 pm

A question Fnord...was your spark gap positive ground as well?

Have to check if reversing the polarity of the primary has an effect on the secondary...I know that when converting a tractor from positive ground generator to negative ground alternator, reversing the polarity of the coil is in the instructions.


http://www.steinertractor.com/pdfs/ABC4 ... ctions.pdf

:?
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Unread postAuthor: Fnord » Tue Oct 19, 2010 1:46 pm

Probably. It was around when I first joined so I can't remember precisely.
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Unread postAuthor: Technician1002 » Tue Oct 19, 2010 6:04 pm

Reversing the polarity is for spark plug wear, not coil protection. Metal transfer in the spark is reversed back to normal for the plug.

The coil damage when running on a 9 volt battery is most likely caused by not grounding the coil shell causing insulation failure between the iron core and primary.
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