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random stun gun sparking

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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Unread postAuthor: starman » Thu May 01, 2008 11:30 pm

You mentioned you were getting "blue" light around your screws. That means you are arcing from your wire to your screw in some fashion. Anywhere you see an arc more than likely means that you aren't making good metal on metal contact there. Remove your spark cabling and inspect...and take the opportunity to rewire and terminate. You can use much smaller gauge wire...the 12 ga is way overkill.

Also try bypassing one of the gaps, maybe the middle one...see if your success rate climbs.

Next project, build a spark strip inside the chamber. While usually effective, the screw spark gap method is definitely entry level.
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Unread postAuthor: spudmatic » Fri May 02, 2008 4:23 pm

To follow up on starman's question is it 350v or 350k volts? And if you trigger the gun does it jump the default when it doesnt jump inside the chamber?
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Unread postAuthor: spudmatic » Fri May 02, 2008 4:30 pm

I went through your same problem when I built my first stun gun cannon and I am using 150k volt with 4 sparks. I strugled with it for a bit and figured out that my gaps where actually bigger than what I thought. I would suggest putting your screws as close together as possible even if that means bending them.
Any gases still in the chamber after 1 shot should not effect it that much because I do not have a chamber fan and have gone through about 3-4 shots consecutively without a drop in performance. So in conclusion I would said make those gaps as small as you possibly can.
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Unread postAuthor: pat123 » Fri May 02, 2008 4:55 pm

Pat:
When the stun gun sparks it has no problem igniting the mix and the sparks are set at about 1/8inch for each.

Ok i'm confused then what is the problem if it has no problem igniting the mix?
edit: :oops: sorry it doesn't spark every time got it please disregard my post
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Unread postAuthor: jimmy101 » Fri May 02, 2008 5:46 pm

starman wrote:Next project, build a spark strip inside the chamber. While usually effective, the screw spark gap method is definitely entry level.

Don't know about that. There is no evidence that a spark strip outperforms standard spark gaps. Indeed, basic physics suggests that two gaps on a spark strip wont be as effective as one gap out in the center of the chamber.

Spark strips have several problems;

1. Most people put the gaps pretty close together. A pair of gaps 1/4" apart are really not any different than a single gap. Once the two flame fronts reach each other it might as well have started as a single front from a single spark gap. Two fronts that are radiating from starting points 1/4" apart will meet when only a very small amount of fuel has been burned.

2. The gaps are usually mounted up against the chamber wall. That means the early flame front is hemispherical instead of spherical. Heck, a sphere intersected by a cylinder, with the edge of the cylinder at the center of the sphere, isn't even as big as a hemisphere. So, two spark gaps on a strip arent even as good as a single central gap.

3. They are trickier to install. It's a real arbiet to bolt the far end of the strip to the chamber wall.

The best place for a spark is dead center in the cylinder (plus or minus a bit). For multiple sparks, they should still be located along the central axis, but distributed evenly through out the chamber.

You want to keep the spark gaps away from the chamber wall.

The most sophisticated, and effective, spark setup is probably something like Latke's chamber short extended to include a more sparks.
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Unread postAuthor: dongfang » Fri May 02, 2008 6:09 pm

Hi,

My guess at your problem is:

Either the combustion in your cannon generates water, which condenses on the spark gaps and causes conduction without sparks.

Or, the exhaust gases from the first firing don´t get properly vented out, and there is too little oxygen for the next firing.

Remedy for both is more venting. You could try overkill it and use a vacuum cleaner. Does that help?

PS: In case you have fueled the gun, and it won´t fire, do NOT use the vacuum to re-vent it. The motor in the vacuum has a sparky commutator...

Regards
Soren
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Unread postAuthor: Big D » Fri May 02, 2008 11:57 pm

The stungun is 350KV, forgot the K.

Dongfang:
Im not sure about your second part applying for me because oxygen shouldnt affect whether the gun sparks. The first maybe be true because ive never checked for that.

The problem is not getting the right mix of propane to air, i have that calculated out and it ignites everytime i can get the stungun to work. Its just, the stungun does not want to spark everytime for some reason. :?

Thanks for the help everyone. :)
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Unread postAuthor: dongfang » Sat May 03, 2008 6:01 am

Hi Big D,

I did mean moisture = water. Not oxygen.

Water is a combustion product of propane.

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Unread postAuthor: starman » Sat May 03, 2008 9:09 am

jimmy101 wrote:
starman wrote:Next project, build a spark strip inside the chamber. While usually effective, the screw spark gap method is definitely entry level.

Don't know about that. There is no evidence that a spark strip outperforms standard spark gaps. Indeed, basic physics suggests that two gaps on a spark strip wont be as effective as one gap out in the center of the chamber.


I was coming from more the aesthetic aspects of the screw vs spark strip as well as his apparent conductivity problems with his screw contacts. This especially when trying to do 3 gaps. Yes a proper spark strip is more difficult to implement as you describe, thus my comment about screw gaps being entry level.

We don't disagree.

Here's my latest spark gap effort.

Image
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A little overly complicated parts-wise but very reliable, gets the spark up in the center of the chamber. I'm trying some other ideas at the moment to get the parts count down.
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Unread postAuthor: jimmy101 » Sat May 03, 2008 2:59 pm

Starman

That "chamber short" is very nice. The wing-nuts are pretty clever. Do you put a dab of lock-tight on'm to keep'm from moving?

I would say that your setup isn't a "spark strip". Seems to me people usually mean something along the lines of copper clad board when the talk about a spark strip. One of Latke's strips:
Image

Latke has muzzle velocity data comparing a 3-gap strip with a 2 screw gaps. The strip gave muzzle velocities of 391+/-63 FPS. The screws 365 +/-68 FPS. The difference is not statistically significant (T-test>17%). He used a rifled barrel which might be why the shot to shot variability is so big (roughly 17% SD). He had one shot with the strip that gave a velocity of 540 FPS, that's 150 FPS faster than the groups average of 391 FPS. If you omit that one data point then the two sparks setups give identical results. With or without that odd data point, it looks like 3-gap strip = 2 screw gaps.
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Unread postAuthor: starman » Sun May 04, 2008 10:15 am

jimmy101 wrote:Starman

That "chamber short" is very nice. The wing-nuts are pretty clever. Do you put a dab of lock-tight on'm to keep'm from moving?


I did indeed...:)
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Unread postAuthor: STHORNE » Sun May 04, 2008 10:32 am

very nice starman....I'll post a pic of my latest spark gap/strip creation.

here it is...

the spark gaps are perfectly in the center of a 4" chamber...
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Unread postAuthor: Major Woody » Thu May 15, 2008 5:34 pm

I am having a somewhat similar problem with my gun.
I crammed the guts of a 100kv stungun into an ignition handle made of 2" PVC and ignored the many suggestions I had read about using heavily insulated wire (such as flyback wire from a TV tube) to connect from the stungun's coil to the terminals inside the gun. I ended up getting arcing all over the place. The arc jumps right through the insulation on the wire (I was using 16 ga. automotive wire) to the nearest metal or to the other wire). Getting a spark inside the chamber is hit and miss. Watching the spark go right through the wire insulation is interesting. I actually fried my flashing LED just troubleshooting the arcing.

Now I have an old dead TV set to get the flyback wire out of. Hopefully that will fix the problem.
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