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First spudgun :) Needing ignition help

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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First spudgun :) Needing ignition help

Unread postAuthor: Silentone » Mon Oct 20, 2008 12:27 am

Hi guys,

Ive done some reading, I have built myself a spud gun, Its all set and ready to let rip.. But the ignition system doesn't always spark, I have taken apart 2 camera and used the capacitor from one and soldered it onto the other PCB.

Ive grinded down two bolts to make the weakest point between the two bolts. (See pictures) But it doesn't seem to always spark there?

Any ideas how i could improve this to spark all the time?

ImageImage

ImageImage

ImageImage

ImageImage
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 20, 2008 12:35 am

Easy, your bolts are far too close together. You need around 3-5mm of separation in order to give the spark enough gap to "jump" over.
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Unread postAuthor: Silentone » Mon Oct 20, 2008 12:51 am

jackssmirkingrevenge wrote:Easy, your bolts are far too close together. You need around 3-5mm of separation in order to give the spark enough gap to "jump" over.


How many volts should it be for it to jump? As when i had bare wires and held them apart nothing happened until they where just about touching :(

Ill try moving the bolts apart now

EDIT: It wouldn't spark at all.. i'm using a 1.5v battery and the 2 caps from the camera flash.. Should i make it 3 caps?
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 20, 2008 1:07 am

Hmmm, it should be working. Is there anywhere along the circuit where the wires are close enough to spark before it reaches the bolts? Perhaps you could separate the wires, sometimes the spark will jump through thin insulation.
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Unread postAuthor: Silentone » Mon Oct 20, 2008 1:35 am

jackssmirkingrevenge wrote:Hmmm, it should be working. Is there anywhere along the circuit where the wires are close enough to spark before it reaches the bolts? Perhaps you could separate the wires, sometimes the spark will jump through thin insulation.


Only on the PCB, Is there any way to test? Because when i moved the bolts apart. I charged up the capiciters and the LED light was red meaning its all charged, I then pressed the switch but nothing happened.. So its not finding anywhere to discharge?

Im not really sure what todo. Would you like some more photos?
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 20, 2008 1:52 am

Perhaps you could reconnect the flash bulb and see if that works, just to confirm that you haven't damaged the circuit?

To be honest capacitor-based sparkers aren't really my thing, I prefer piezo ignitors - cheap, easy and mostly issue free ;)
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Unread postAuthor: Silentone » Mon Oct 20, 2008 2:57 am

jackssmirkingrevenge wrote:Perhaps you could reconnect the flash bulb and see if that works, just to confirm that you haven't damaged the circuit?

To be honest capacitor-based sparkers aren't really my thing, I prefer piezo ignitors - cheap, easy and mostly issue free ;)


Do you modify your piezo before you use it? Because the one i had had a very tiny spark.

When i took the flash bulb off there was 3 wires.. one on the left and right of the bulb, and one in the middle.. I presumed the one in the middle was just a earth.. Should i have removed it? Or any idea what that third wire would have been for? It was attached to the casing that the flash was in
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 20, 2008 3:10 am

No need to modify the piezo at all, just hook up the wire to one of your bolts and the cap to the other bolt and it works perfectly as a simple maintenance free ignition system.

*clickyBOOM!*, easy ;)
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Unread postAuthor: Silentone » Mon Oct 20, 2008 4:01 am

jackssmirkingrevenge wrote:No need to modify the piezo at all, just hook up the wire to one of your bolts and the cap to the other bolt and it works perfectly as a simple maintenance free ignition system.

*clickyBOOM!*, easy ;)


Sweet, So.. What about charging the cap? As im not too smart on all the electronics you see :(

So it would go something like?

ImageImage

How would i charge the cap?
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Last edited by Silentone on Mon Oct 20, 2008 5:32 pm, edited 1 time in total.

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Unread postAuthor: jackssmirkingrevenge » Mon Oct 20, 2008 4:44 am

No need to use a capacitor, just hook up the two points of the piezo to your bolts.
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Unread postAuthor: TurboSuper » Mon Oct 20, 2008 4:45 am

Your problem is simply that the caps don't have enough voltage behind them. They only have about 300V or so- not nearly enough to jump a gap.

You'd need to either wire the 3rd, really high voltage lead on the board in between them to ionize the air, or put some steel wool in between the electrodes. Or you can wire the caps to an ignition coil to step the voltage up to a few kV.

I'm with JSR though- go with a piezo. You don't need any fancy electronics whatsoever, just wire the two leads to your spark gap and you have an insta-kaboom :D

These Mickey-Mouse camera ignition circuits are usually more trouble than they're worth, the only real advantage I see to them is they make that cool sci-fi whining noise when the caps charge.
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Unread postAuthor: mark.f » Mon Oct 20, 2008 9:13 am

Actually, they're good for making hybrid ignition systems, since you generally need more voltage. A camera capacitor dumping through a Nissan ignition coil made one of the biggest sparks I ever generated...
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Unread postAuthor: jrrdw » Mon Oct 20, 2008 9:26 am

Go with the 100,000v stun gun circuit from www.bcarms.com :D
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Unread postAuthor: jimmy101 » Mon Oct 20, 2008 10:26 am

Like turbo said; you doubled up on the wrong capacitor. The big-ass cap is charged to 300~330V by the circuit board. That voltage will only jump a gap of about 0.2mm (~1/100"). There is a lot of energy in the cap but the voltage just isn't high enough to spark. (Two photocaps would contain about 12 joules of energy when fully charged.)

The high voltage (about 10KV) needed to trigger the phototube is created by dumping a second, much smaller cap (see photo), through the HV trigger transformer. The output of that transformer is carried by a wire to either the metal reflector around the flashtube or to a wire wrapped around the flashtube. That wire is the only source of high enough voltage to jump a gap. You can try to use that wire as the spark source. The other side of the gap would be wired to the circuit boards ground contact. Though the spark is up around 10KV the current is extremely low because the cap that supplies the power is only something like 0.02 MFD, total power when charged to 320V is 0.5millijoules (just barely enough to ignite propane+air).

So, it is that small cap that you might try doubling up using the cap from another board.

Or, dump the big-ass photocap through a car ignition coil. You might think you could dump the big-ass cap through the trigger transformer but I suspect you would destroy the transformer with that much power going through it.

Or, try a trigatron, which uses the two leads from the big-ass cap plus the HV lead in a three electrode spark gap. That'll give a huge spark, but is a PITA to keep working becuase the alignment is tricky and the electrodes get burned up.

Or, like others sugested, just use a BBQ piezo or buy a stungun.
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Unread postAuthor: SEAKING9006 » Mon Oct 20, 2008 10:30 am

jackssmirkingrevenge wrote:No need to modify the piezo at all, just hook up the wire to one of your bolts and the cap to the other bolt and it works perfectly as a simple maintenance free ignition system.

*clickyBOOM!*, easy ;)


I'd remember that video anywhere. :shock:

It was made about a year or two ago in response to a thread I made on spudtech about the construction of cartridge based combustions. About a month or two later, I went through a year and a half hiatus. And now, here I am again, only now I don't fiddle with combustion designs.
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