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SpudBlaster15 wrote:

saefroch wrote:Might it be possible to dump a photoflash capacitor into another transformer coil to produce a high-voltage current for ignition? Or would that probably just cause arcing all over the secondary?

Yes, it has been done a number of times with an automotive ignition coil. You'll typically get an output voltage of ~30kV from a 300-350V input.

The setup is quite bulky, but it's highly effective for low to moderate mixes.

That's a negative.



Coil, capacitor, charging and pulse circuit are present on my hand.

Bottom wires are 1.5 to 3VDC battery power

Top wires give off ~25KV pulses with that particular setup.

If anyone hasn't yet seen it, spark demo: [youtube][/youtube]


The main space saving is in the coil.

Yes a full blown ignition coil the size of a pint can will be capable of higher voltage output.

However to get a high spark repetition rate in a small circuit you don't want to go mad on the voltage because that needs more energy, higher current rated components, maybe even heat sinks. All of which is a major drag.

So if we don't want to try for 100KV we can do fine with a miniature ignition coil which is practically identical to the HV coil found in a stun gun and in fact commercial gas ignition devices. In the clip it's doing about 25KV.

You can still try for higher voltage output with a small circuit like this, you just trade off spark rate for higher voltage.

I've tried up to 45KV with the same circuit by changing the spark gap that pulses to the HV coil but the spark rate is lowered significantly.

If you're still going after using a camera flash circuit, have you thought of using the circuit as intended but replacing the flash tube with a spark gap? I can't remember where but I did see it done somewhere.

[basic camera operation]
When you take a picture, the shutter triggers a secondary transformer to apply a high voltage to the center of the flash tube to help ionize the gas and create the flash.
[/basic camera operation]

Alone, the 330v won't be enough to jump a very large gap. But if you help it a little by applying an ionizing voltage in the middle, it will work. If you take the three wires that originally went to the flash tube (one for each of the capacitor terminals and one for the center terminal, sometimes connected to the reflector) you should be able to make a double spark gap, although it might take more adjusting than usual.

Not sure if you still want to go this route but it's a possibility.

Insomniac wrote:One thing no-one's seemed to mention... it would simply short internally.

ramses wrote: If you managed to get it to work, you might just end up saturating the core of the second transformer (or arcing through the insulation).

aEx155 wrote:If you're still going after using a camera flash circuit, have you thought of using the circuit as intended but replacing the flash tube with a spark gap? I can't remember where but I did see it done somewhere.

That can be done, but the gap is very small, and there are spark rate and electrode erosion issues. Not to mention it hurts quite a bit more than anything else if you happen to touch it.

@hotwired, where did you get that potted coil? what size capacitor are you using instead of the main photoflash one? And because it's semi-relevent to the OP, what exactly is the coil that appears in this video?

I tried to build a switch, replacing the flash tube with a simple ceramic tube, almost the same size. Contacts are very close together, but with the trigger wire wrapped around the switch and everything connected, it won't trigger. I might try to just use a flash tube, but I'm working on the actual spark plug in what free time I have right now.

ramses wrote: @hotwired, where did you get that potted coil? what size capacitor are you using instead of the main photoflash one? And because it's semi-relevent to the OP, what exactly is the coil that appears in this video?

The coil was from a quad bike CDI unit.

The circuit of the unit was not of use to me so I smashed the coil off it (entire thing was in a plastic shell filled with epoxy) and added my own pulsing circuit and capacitor.

The capacitor is a 0.22uf metal foil capacitor rated to 600VDC. The circuit can actually charge up to 600V but in the current configuration it's set to charge up to and pulse 250V to the coil. For 600V I'd use a much smaller capacitor to avoid decreasing the spark rate.

The coil you see in the "doublecoil" clip is not a coil as such. It's a whole circuit and a coil within a box that runs off a 9V. It's commercially sold as a gas ignition unit for BBQs. Several of the Spudfiles sponsors sell such circuits from different manufacturers but they all work the same way.