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Camera Flash Circuit questions

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 Flash Circuit questions

Unread postAuthor: saefroch » Mon Jul 25, 2011 9:50 am

I know I've started threads like this before, but my questions are rather different this time.

1) I rewired my flash circuit, but now it isn't charging. I did make one minor change to it though... I soldered the trigger switch contacts together and put a switch in series after the transformer that provides the high voltage. Could that have caused a problem?

2) I have two intact flash circuits (three, I think if I search), so can I connect the secondary of one transformer to the primary of another and create a high-voltage ignition system? I estimate if the transformers kick the voltage up to 350V I should be able to get close to 80kV, albiet at very low current.
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Unread postAuthor: Technician1002 » Mon Jul 25, 2011 10:03 am

1 Some flash circuits need the pulse of the switch change to start charging. Those are some that you press the button and it continues to charge and auto recharges after each shot. The press and hold to charge the flash ones work fine with a shorted power button. There are two basic types of flash charging circuits.. A press to hold on and a trigger start. The trigger start is triggered by a button press or the flash discharging. One stops when you release the button, the other shuts off when the cap is charged.

2 Due to the impedance of the transformer (Number of turns and inductance) the low impedance of the second transformer will basically place an overload or short on the first and you will get less voltage. Sorry that does not work. They are designed to change high current to low current high voltage. The low current high voltage output simply does not have enough current to operate the second transformer. Nice try.
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Unread postAuthor: saefroch » Mon Jul 25, 2011 10:06 am

Its a press-and-hold... I have a toggle switch for charging, which was working perfectly fine earlier.

Could I make it work by putting a few batteries in parallel?
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Unread postAuthor: Technician1002 » Mon Jul 25, 2011 10:23 am

Check for solder where it doesn't belong and check for overheated parts that may have gotten broken. Bummer you are in the troubleshooting phase. They are cheap.. do you have a spare?
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Unread postAuthor: saefroch » Mon Jul 25, 2011 10:27 am

I do have a spare, but all that soldering again? I'd almost just rather search for another ignition source for now. I have a few options available to me.

Do you think if I opened up a 9V battery and put all those cells in parallel, I'd be able to muster enough current to power the second transformer?
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Unread postAuthor: jhalek90 » Mon Jul 25, 2011 12:05 pm

Nope, it would take hundreds of batteries.... and i still dont think it would work.

That transformer is not designed to do what you are trying to make it do.
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Unread postAuthor: saefroch » Mon Jul 25, 2011 4:25 pm

So unfair... So how does a stungun generate that very high voltage? Just one transformer?
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Unread postAuthor: Technician1002 » Mon Jul 25, 2011 6:44 pm

A stun gun doesn't just use one transformer and they use a capacitor at higher voltage to provide the required high current pulses into the second transformer.

Note in the schematic below that there are 2 transformers in a stun gun. the first is like a camera flash charging circuit. It's job is to charge the high voltage capacitor. That is followed by the trigger circuit and SCR to discharge that cap at high current into the second transformer. This is where the serious voltage is generated. It is not simply two transformers stacked on each other.

This uses the exact same principals as CDI (capacitor discharge Ignition).
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Unread postAuthor: ramses » Mon Jul 25, 2011 7:59 pm

Sometimes the stunguns use a simple spark gap to dump the HV capacitor.

As for more batteries, you are still limited by the power rating of the transistor/diode, and the impedence of the primary of the first transformer. 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).
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Unread postAuthor: saefroch » Mon Jul 25, 2011 10:02 pm

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?
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Unread postAuthor: Mr.Tallahassee » Mon Jul 25, 2011 11:25 pm

If you don't mind an extra pound or two, I made an ignition source capable of remarkably high output voltages. A simple electric flyswatter puts out 1.5-2.5 kv from a capacitor at a decent current. Wire one through a spark gap like this one and it will probably ignite anything you throw at it. Even say, a 30x hybrid. :D It might cost 15 or 20 US dollars.
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Unread postAuthor: Insomniac » Tue Jul 26, 2011 2:11 am

One thing no-one's seemed to mention... The step-up transformers in camera flash circuits are designed for a maximum voltage of 330V. Even if you could pump enough current through to cause a higher voltage spike on a second transformer... it would simply short internally. The windings are not insulted well enough to take high voltage, unlike a true high voltage transformer (often potted in epoxy under a vaccuum, to reduce the chance of internal arcing).

You can take the transformer from a 'wall wart' style power adapter, then manually pulse a 9V battery through what used to be the secondary windings. You will get a short (3 to 5 mm) spark out of what used to be the primary windings, which are now functioning as secondary windings. However, due to the fact the transformer was never designed for these voltages, it will begin to arc internally after a very short time.
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Unread postAuthor: saefroch » Tue Jul 26, 2011 10:03 am

Thanks for mentioning that, I was rather worried that'd be the case. I have a smallish transformer core, IIRC it has 3 loops in the primary and 14 in the secondary, would that be enough to bump up the discharge voltage from my two capacitors and arc ~1mm? I'd be looking at about 1.6kV, but I have no clue if that's enough potential to arc...

EDIT: If I can make do with two turns in the primary (which should be enough with only a short length of copper wire and two capacitors) I can get the final voltage close to 3kV.
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Unread postAuthor: Technician1002 » Tue Jul 26, 2011 12:26 pm

turns ratio is the same as the voltage ratio.. unless the core saturates.

A transformer like that would typically have a maximum of 1 volt per turn for a 50-60HZ application and up to 50 volts per turn in a high frequency pulse application. Assuming a CDI application.. Cap charged to 150 volts at 50v per turn. Secondary with 14 turns will have 50 volts per turn X 14 turns. That is 700 volts. You will want many more turns on the secondary to reach your target voltage.
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Unread postAuthor: saefroch » Tue Jul 26, 2011 12:37 pm

The cap is rated 350 volts, or so it says on the side. Wikipedia says they're usually charged to at least 330v...

Where are you getting the 50 volts per turn from? That seems like a staggering generalization to me... I produced my 3kV from a simple ratio... assuming I have 2 turns in the primary and 14 in the secondary, I should step up the voltage by a factor of 7.5, 7.5*350 = 2,975.

EDIT: Can I wrap the primary on top of the secondary? I cold probably get in a few more turns if I do it that way...
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