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Camera Circuit 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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Camera Circuit Question

Unread postAuthor: Hubb » Sun Jul 27, 2008 11:30 am

I've been doing research on camera circuits for ignition. All over the wiki, it says to use an ignition coil. Well, this wouldn't be a problem if I had one. I sure don't feel like buying one.

With that said, I considered hooking up the camera circuit without the coil. Further research said this would work, but would eventually corrode the electrodes.

My question: How long does it take an electrode to corrode? Besides price, are there any more advantages of doing this method?
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Unread postAuthor: Hotwired » Sun Jul 27, 2008 11:52 am

Dumping a 300-odd volt flash capacitor in an open air spark causes visible corroding of the electrodes from the start in the form of roughly 0.5mm wide shallow craters in solid metal and ditto holes through foil.

It would take ages for the electrodes to get corroded too much to use but they will almost certainly need adjusting every now and again so fixing them a set distance is not likely to be a great idea.

As far as igniting goes, a long skinny spark vs a fat plasma ball with particles of molten metal flying out of it...

...well I couldn't say. They both work and that's all that's wanted.
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Unread postAuthor: jimmy101 » Sun Jul 27, 2008 12:49 pm

You might try to use just the HV ignition spark from the flash board. If you don't dump the photocap into the spark there won't be any damage to the electrodes.

Take the HV lead from the trigger transformer and hold it close to the battery plus and minus terminals to find out which of the two it sparks to the best. Then connect to the spark gap. Since the flashtube is no longer connected to the trigger voltage it won't flash. You use the same switch to fire the HV as you would to fire the flashboard itself.
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You want the wire labelled "HV trigger lead", it connects to the reflector on the flashtube.

The HV trigger is probably several KV, but very low current (even for a HV supply) so it's pretty wimpy. It would be best to use very pointy electrodes like drywall screws or pins and a very small gap (1mm?).

Typically, the trigger cap is 0.022uF charged to ~300V, so the cap contains about 1mJ of energy. Figure the energy transfer efficiency through the HV coil is 50%, means you get ~0.5mJ in the spark. Propane + air requires a spark of at least a couple tenths of a mJ so the setup is kind of marginal. There should be enough energy.

If you are thinking about using the trigatron method you basically just remove the wire from the trigatron that goes to one of the photocap's leads.
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Unread postAuthor: Hubb » Sun Jul 27, 2008 1:20 pm

Jimmy, that's one of the places I was looking at. Is there any other methods of keeping the electrodes from corroding without going through an ignition coil?
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Unread postAuthor: microman171 » Sun Jul 27, 2008 2:17 pm

Yeah, I've had bad experience there... The electrodes just burn up and without adjustments it is unlikely to work twice. The ghetto cheap way I have is nichrome wire wound into a spring and having 12v 1a passing through it.

EDIT: Man I have just proof read this and it was horrible! I'm sorry.
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Last edited by microman171 on Mon Jul 28, 2008 1:28 am, edited 1 time in total.

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Unread postAuthor: rp181 » Sun Jul 27, 2008 2:40 pm

i had a new board, it had alot of transistors on it. I somehow accedently made the cap dump into the secondary transformer, gave a nice loud spark. Maybey you can try to do that intentionally. (after 2 shots, the transformer gets hot, and it has to cool.)
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Unread postAuthor: jimmy101 » Sun Jul 27, 2008 5:15 pm

hubb017 wrote:Jimmy, that's one of the places I was looking at. Is there any other methods of keeping the electrodes from corroding without going through an ignition coil?

About the only thing I can think of is to use nichrome wire, the stuff heater coils are made of. Very hard stuff and it has an extremely high melting point so the spark might not blow bits out of it as easy. And, it doesn't oxidize like steel and copper does.

You can buy nichrome wire at a decent electronics parts store (RadioShack doesn't qualify as decent), or salvage it from an old hair dryer, waffle iron, coffee pot, clothes dryer...

Another possiblity would be to replace the 120uF phototcap with a smaller cap. A 120uF cap at 300V contains 5.4J of energy. That's about 10,000 times the energy needed to ignite propane+air. So, you might try replacing the 120uF with say a 1uF cap. The votages will be the same but the spark will only be ~50mJ. (You'll need a cap rated to at least 320V for most photoflash boards.)

The trigatron circuit is pretty cool when it works, gives a very satisfying BANG when it goes off. But, like you said, the electrodes don't last very long.

Edit: engrish
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Unread postAuthor: m4xr8d » Sun Jul 27, 2008 7:36 pm

While everyone has given you great ideas, no one has suggested in making a replaceable spark gap. I've yet to see a single suggestion here, on the wiki, or even in the archives of this. Simply take a small section of 12/2 romex(the stuff they wire residential homes with) and and create your 3-wire spark gap. You've got a +(black), a -(white) and a ground(bare copper.). Bare copper wire is your HV trigger. If your smart you'll take the reflector that was on your flash tube and solder it to the copper wire, this will increase the ionization area. Just bend your + and - wires to fit in the reflector's surface area and just sort of close to the HV trigger and minimal adjustment. Simply replace when it stops working. You'll wear your camera flash out before this spark gap.
12/2 romex can be found on a job site once the roof is dried in. Electricians cut lengths of this off while doing a rough-in.
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Unread postAuthor: TurboSuper » Sun Jul 27, 2008 8:10 pm

@m4xr8d: I think you just described a trigatron, which would cause the electrodes to corrode.
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Unread postAuthor: m4xr8d » Sun Jul 27, 2008 8:47 pm

@turbo: I'm well aware of what I described. While it is a trigatron, I was describing the construction of inexpensive, replaceble(if need be), and minimally adjusted spark gap that no one has used in any of their camera flash circuits here. While one person has been close, adjustments were eventually needed.

Please note several keywords such as

inexpensive
replaceable
minimally adjusted
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Unread postAuthor: SpudFarm » Sun Jul 27, 2008 8:56 pm

well the TIG welder wolfram points are pretty damn hard to melt..
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Unread postAuthor: TurboSuper » Sun Jul 27, 2008 10:01 pm

m4xr8d wrote:@turbo: I'm well aware of what I described. While it is a trigatron, I was describing the construction of inexpensive, replaceble(if need be), and minimally adjusted spark gap that no one has used in any of their camera flash circuits here. While one person has been close, adjustments were eventually needed.

Please note several keywords such as

inexpensive
replaceable
minimally adjusted


What I don't understand is just how you can say that noone had suggested it, when the post directly before yours suggests exactly that.

It sounds like a good idea, but I think the whole issue was whether or not the electrodes would last long enough to make it worth it. If it really will last as long as you say, then problem resolved :D
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