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I don't like spark plugs, they put the spark much to close to the chamber wall. The flame front is a modified hemisphere almost from the instant of ignition.
Screws aren't all that tough to do and they get the spark out near the center of the chamber. BBQ piezo or stun gun both work fine and cost darn near the same. A BBQ piezo can be mounted in a nice handle using nothing but generic PVC parts. The stun gun is fun but the temptation to spark it continuously is too strong (at least for me ) and that usually significantly shortens the stun gun's life. Plus the legal issues.
Flint sparkers suck, mostly because you have to dedicate 100% of the abilities of one hand to operate. Holding even a modest sized spud gun with just one hand isn't a great idea.
"Zeus: PVC when used around 100PSI is near it's failure point" What? Pressure rated PVC is rated at (IIRC) 220 PSIG for 3" pipe. Smaller diameters have much higher ratings. Given that a typical "advanced" combustion gun rarely exceeds 70 PSIG or so the 220+ PSIG rating is more than adequate. (Heck, household water supplies often spike at 100+ PSIG, at least mine does, which exceeds the normal operating pressure of even an "advanced" combustion gun.)
Homebrew ignition systems, like a photoflash circuit or a re-purposed automotive type ignition system or a flyback transformer setup are fine but can be more expensive and larger/heavier than a piezo or stun gun. They are fine if you are into the technical aspects but they won't, in general, perform any better than cheaper/easier/simpler alternatives.
Yes. They work very well, but they do take up a lot of space without hacking the battery holder off and replacing with a switch and smaller one. If you get lucky you may find some on eBay et al that aren't potted and can be reduced in size even further.
Otherwise, you can just keep it the way it is and mount it in a project box the way it's supposed to be mounted and used (AA battery and all)...
But as you can see it's about as space efficient as a 150lb box of hammers. In a Miata.
That looks pretty nice though. You should have put a Russian nuclear missile key on there just because its so massive. Not a bad idea... I personally think the ignition give the gun it's character.
Jimmy- I think we are on the same page!
PVC isn't rated to carry compressed air, not to mention alot of people use DWV which isn't rated to anything.
Using any material at more than a third of it's rating is pushing your luck with PVC, it may be rated to 220PSI but it isn't rated for the sudden peak of 60 to 100 PSI.
It's failure mode could be compared with glass, quite strong but when it breaks a great deal of damage can result. For anyone who's tried homebrewing or has had cordial turn to moonshine then release it's pressure violently, you know how powerful it can be.
The ratings of pipe include a safety factor. The temperature is very brief, and doesn't significantly weaken the pipe. The sudden spike is not THAT sudden.
A PVC combustion is well within it's ratings as long as it is constructed well. Clean and glue joints, use barrel supports, don't use cleanout caps, etc.
I've always been a fan of flyback ignition myself. It adds another dimension to the gun, making the driver... I'm using it on a hybrid I'm building
The ZVS or Mazzili driver is the way to go, see linky
http://sites.google.com/site/uzzors2k/f ... ransformer
Since I'm still a flyback noobie I went with a 555 for my first one :3
But of course ZVS is the best. That's a bridge design, is it not?
Zesu, you need to do more reading. Just because a pipe says DWV does not mean it is not pressure rated. Indeed, the vast majority of pressure rated pipe also says DWV.
The pressure spike in even an advanced combustion gun is not a problem for PVC. The dP/dT is slow slow, in the low audio range, that it is comparable to "water hammer" and is not a concern for the safety of the pipe. Pressure rated PVC is often used in home potable (pressurized) water systems. Since those systems occasionally have static pressure as high as 100 PSI and "hammer" pressures considerably higher than that the 70 PSIG or so maximum pressure in a correctly operating (or even the ~135 PSIG max pressure in a gun with a jammed round) is not a significant hazard.
I found one for $15 shipped on eBay and it works great. Uses a triple A battery. I hacked it up and modified it to my needs...but even unmodified it worked very well...makes a strong spark about an inch gap.
I agree the form factor of the stock battery powered igniter is pretty chunky. I tried to accommodate it but ended up cutting down the battery holder/switch and using a remote battery and switch. That made it significantly slimmer. I think it came out pretty nice.
Hate piezos, a stiff lever/trigger on any of them.
Flint sparker seems too clumsy.
Stun guns, well they don't exist round here.
Camera flash, I have no idea why anyone would want to use one of those unless they had to. Slow, overkill, fiddly, eats batteries.
I've got my own take on a stungun circuit I use.
Runs on one or two 1.5V batteries. Charges up extremely fast and puts out a spray of sparks at ~20hz. Tested running non-stop on two fresh batteries for about half an hour before I got fed up with the buzzing.
You could call it an extremely simple ignition coil driver using a miniature ignition coil.
The mains or large lead acid battery powered flyback or full size ignition coil drivers I don't like at all, far larger and heavier than needed.
If you replace the 120UF cap with something smaller, say 1UF or 0.1UF it'll recharge in less than a second and consumes very little power from the battery. Since the big-ass 120UF cap isn't used in the spark circuit anyway (a 0.1UF'ish cap is the energy storage for the spark) and just wastes power, and presents a significant shock hazard, it can be replaced with a much smaller cap.
I'm very aware of this.
The circuit I actually use, uses part of a disposable camera, a 0.22uf metal foil cap instead of the electrolytic and further bumps the voltage through an ignition coil. Then as I say repeats the spark up to 20 times a second.
I'm sure you posted the last time I talked about it but it was a while back.
Still, we agree that an unmodified camera flash circuit is lousy for ignition.
Can't say I recall my testing with a low capacitance capacitor on a ionised spark gap rig worked all that well. However I did move on to high repetition spark circuits fairly soon after since it was clear they were more reliable.
Even using the ionised gap you only have a 0.5cm spark gap at 1atm and with a lowered capacitance it's a much weaker spark too.
I feel a piezo is better than a single spark camera flash board. After all, what's the point?
Well, I guess that depends. A single spark camera board is basically what I made and uses an SCR to dump a smaller capacitor through an ignition coil. You can use a small, easily operated momentary switch to fire the cannon, which is better than the stiff action of a piezoelectric igniter like you've already mentioned, and the recharge time is negligible, not to mention the power consumption is very low.
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