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Simple ignition question

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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Simple ignition question

Unread postAuthor: Jelly » Fri May 28, 2010 4:44 pm

Hey! First post! Anyway, I built a basic combustion cannon and I'm having trouble with the ignition. I'm using a piezo grill sparker, and until recently I just had the wires that came with it taped to a couple of screws. This worked fine until the tape wouldn't hold and the wires kept falling out, so I bought 6 feet of 12 gauge stranded copper wire to solder to them instead (making the sparker 3 feet from the gun).

LONG STORY SHORT, with the wires hooked to the ignitor, I can get a spark between them, but when hooked to the screws, no spark. How do I fix it? Am I using to much wire?
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Unread postAuthor: Technician1002 » Fri May 28, 2010 5:14 pm

You are using too much wire. The igniter produces a short duration high voltage pulse. Shunt capacitance from large diameter conductors and length tends to shunt the voltage. Adding the spark gap in the chamber may add enough additional leakage path to drop the voltage low enough to prevent a spark. The fix is shorter wire with a smaller center conductor and the tips of the spark gap need to be closer together.
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Unread postAuthor: clemsonguy1125 » Fri May 28, 2010 7:36 pm

Shortening the wire should help but if it doesnt work you can use one of the holes mount a lantern sparker, or upgrade to a stun gun
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That is all.
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Unread postAuthor: jimmy101 » Sat May 29, 2010 10:58 am

Is the wire "zip-wire" like used for appliances with two insulated conductors in one cable?

Typical appliance cord, especially several feet of it, won't work well. Like tech said, either the capacitance of the cable pair or breakdown of the insulation will absorb all the piezo's energy.

Separate the two wires so they don't approach each other any closer than a 1/2 inch or so.

12G wire is pretty overkill for a sparker. 24G or so will work better. HV at tiny currents really doesn't care about the gauge of the wire all that much and small gauge (large diameter) wire actually hurts a bit in a HV circuit.
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Unread postAuthor: Jelly » Sat May 29, 2010 12:06 pm

Thanks guys. I read somewhere that 16g or smaller was ok. I guess that was wrong. I also thought being able to trigger the gun from a few feet away could be useful. Oh well.

So I guess I'll either go out and buy a stun gun or try cutting off some of the wire.

Come to think of it, what brands of stun guns should I keep an eye out for on ebay?
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Unread postAuthor: jimmy101 » Sat May 29, 2010 12:42 pm

Don't give up so fast!

If you used zip cord just unzip it and try that. The gauge isn't all that important. It is easy enough to see if the spark gap starts to work without even fueling the gun. Also, the spark gap should probably be less than 1/8" (~3mm). Even 1/16" or less will work to ignite the fuel.

If this is your first gun then remote firing is a reasonable thing to do.

As to a stun gun, I would just keep looking until you can find one that is no more than $20, including shipping. Don't fall for the $5 BS in the add then get stuck with a $25 shipping charge.

Stun guns typically have a fairly limited life. As fun as it is to fire it to see the big-ass spark, each firing basically degrades the stun gun. The stun gun should never be triggered, especially when there is only a large spark gap, for more than a second or two and there probably should be a minute or two between firings.
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Unread postAuthor: Gun Freak » Fri Jun 11, 2010 10:03 pm

Dude, you joined just to answer a question that was already answered... lol fail.
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Unread postAuthor: Insomniac » Fri Jun 11, 2010 10:23 pm

Gun Freak, it's a bot or a spammer... parrotted jimmy's post to make it look like a 'real' post, and then put spam links at the bottom.
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Unread postAuthor: Technician1002 » Fri Jun 11, 2010 11:08 pm

I PM'ed PCGUY the info on the post.

Edit, it looks like him and his post is gone.
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