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ignition coil vs. flash circut

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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ignition coil vs. flash circut

Unread postAuthor: FishBoy » Sun Sep 28, 2008 7:11 pm

I was wondering what the best place to get a cheap (or free) car ignition coil would be. Would one of these perform much better than a camera flash circut on a low mix (5-6x) hybrid?
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Unread postAuthor: Hotwired » Sun Sep 28, 2008 7:36 pm

Camera flash circuits are more fiddly. The gap between the sparking points is only a couple of mm using the ionising third wire.

As the pressure goes up you will have to decrease the gap accordingly. You could go nuts trying to get a working gap in a 6 ATM hybrid (requiring a maximum gap 6x smaller).

An ignition coil puts out about 100 times the voltage of a camera flash capacitor when used with a 12V car battery (~30,000v) so the ignition points can be further apart. About 5-6 times further apart because the third wire on a flash circuit boosts the distance its spark could normally jump.

You could also put a higher than normal voltage into the ignition coil to get an even higher voltage spark out.

Downside is that coils tends to be rather more bulky than a tiny circuit, still if you can't get a stungun circuit they'll produce the highest voltages you can get without much effort.
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Last edited by Hotwired on Sun Sep 28, 2008 9:11 pm, edited 1 time in total.
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Unread postAuthor: FishBoy » Sun Sep 28, 2008 7:47 pm

I'm planning on using a sparkplug for ignition, how well would a stungun circut work? (I'm hoping to have a small control box/remote with an arm switch and a fire button)
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Unread postAuthor: DYI » Sun Sep 28, 2008 8:34 pm

It all depends on the mix you're going for. Eventually, it just isn't practical any more to use a high voltage arc ignition. At that point, electrolytic capacitor ignition systems become the cheapest and easiest way to go.

At or below 10x air/propane, high voltage is fine, as long as you're willing to fiddle with the gap length for several minutes trying to get it down to 0.3mm or something...

I'd say combine the two, and discharge a flash circuit through an ignition coil (a very nice and simple circuit that I used extensively before I gave up on high voltage ignition, which happened for me when I started precharging to 250 psi).

Stunguns are fine, but the circuits are very small and fiddly. If they break, good luck trying to solder anything back on, and you can forget about trying to replace parts when they fry - everything's just too tiny.
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Unread postAuthor: FishBoy » Sun Sep 28, 2008 8:42 pm

ok, thanks, but where could I cheaply (preferably legaly) obtain an ignition coil?

(to avoid making a new topic)
galvanized steel pipe connected via threaded fittings could hold 6x, correct?
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Last edited by FishBoy on Sun Sep 28, 2008 8:47 pm, edited 1 time in total.
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Unread postAuthor: DYI » Sun Sep 28, 2008 8:47 pm

where could I cheaply (preferably legaly) obtain an ignition coil?


Get one of the old fashioned ones from a friend who collects old cars. If you don't have any suitable friends, you can buy them at an auto supply store.
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Unread postAuthor: FishBoy » Sun Sep 28, 2008 8:54 pm

ok
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Unread postAuthor: rp181 » Sun Sep 28, 2008 8:58 pm

if you have a old unused hair dryer or toaster, you could use the wire to make a glow plug.
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Unread postAuthor: Hotwired » Sun Sep 28, 2008 9:10 pm

A glow wire is the simplest thing of all being just a wire between two thicker wires from a battery.

Only issues are a potential delay between pressing a switch and having the wire heat up enough.

Still, hybrid cannons don't really require a fast response ignition.
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Unread postAuthor: starman » Sun Sep 28, 2008 11:06 pm

Hotwired wrote:Still, hybrid cannons don't really require a fast response ignition.


Not as much no, however HGDT still predicts a performance gain from multiple spark gaps.
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Unread postAuthor: mark.f » Mon Sep 29, 2008 9:21 am

The older cylindrical ignition coils work the best, IMHO.

Also, if you have the time and the patience, for a 6x mix, I would wire up a 555-chip driven ignition coil. You would route the output of the IC to a MOSFET which would switch voltage to the primary side of the ignition coil.

But, the simplest way to drive an ignition coil is to simply hook it in series to one side of the flash tube in a camera circuit-board. This allows you to use the flash tube as a 'switch' which is pretty reliable. This method also generates rather large arcs which would be good for hybrids.
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Unread postAuthor: jimmy101 » Mon Sep 29, 2008 1:54 pm

The simplest and cheapest method is probably the stun gun. By the time you dig up the igntion coil, photoflash board (or mosfet and 555 timer), a case, wire, battery etc. you'll have spent more than what a cheap stungun costs.

Cheapo stunguns put out about 50KV (figure they put out half what the maker claims). That's a 50 mm (2 inch) spark at atmospheric pressure between sharp electrodes. In a 10X hybrid the gap would need to be about 5mm (1/5") which is pretty easy to do. Spark plugs are typically ~1mm (0.035") gaps.

Pointy electrodes will work better than a generic spark plug. The sharper the points the larger the gap you can jump with a given voltage source. Spark plugs typically have pretty blunt electrodes.
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Unread postAuthor: ramses » Tue Sep 30, 2008 2:12 pm

keep in mind also that with a strait camera flash circuit, the electrodes wear with every firing.

Depending on your spark gap location, you could possibly rig it up so that it discharges through steel wool, creating sparks that quickly fill almost anything. You would have to replace the steel wool after every shot, and the cap might still be charged.

@starman- you can still run a few pieces of nichrome wire in series, though.
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