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Nichrome Ignition

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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Unread postAuthor: Hubb » Fri May 20, 2011 7:43 pm

So, in summary, a strand of nichrome (or similar) wire could be used, but certain parameters must be met, and these parameters may or may not be worth the effort.
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Unread postAuthor: Technician1002 » Fri May 20, 2011 9:16 pm

Nichrome wire heating due to capacitor discharge is the same idea used by Polaroid in the 1960's when they had their winklight flash. Before strobe tubes were a dime a dozen and used in disposable cameras, Polaroid used a high voltage (low by today's standards) to charge a capacitor to about 45 volts and then discharge it into a 12 volt light bulb to make a short duration burst of light.

All I proposed was the same thing with a camera flash and wire to make an instant heating hot wire ignition so the ignition is not pull the trigger and wait for it to heat.

http://www.flickr.com/photos/californiagirl27/2800914287/
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Without common inverter power supplies, they used an expensive 45 volt battery to charge the cap.
Batteries and bulb;
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Unread postAuthor: saefroch » Mon May 23, 2011 2:07 pm

If anyone's interested, I made some progress on the platinum wire ignition, and I got some very interesting results. Long story short, it works, but the wire is obliterated. All that remains are two scorch marks on the solder connections to the wire after discharging two photoflash capacitors through a section of wire about .1" in length, and .003" in diameter.

I'll see if I can try different materials and get any different results.
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Unread postAuthor: jimmy101 » Mon May 23, 2011 3:27 pm

I think you are using a couple thousand times more energy than needed. A photocap, assuming 120MFD at 300V, is 5.4J. Two would be nearly 11J of energy.

It takes about 0.0005J of energy, in a sufficiently small volume, to ignite propane in air.

I would omit one of the photocaps and only charge the cap up to about 50V. That'll give 0.15J of energy.[/u][/i]
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Unread postAuthor: saefroch » Mon May 23, 2011 5:29 pm

I agree heartily, but I'd like to try to get some resistance-based ignition setup with my capacitors to work.

Anyone tried a nichrome setup yet?
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Unread postAuthor: Technician1002 » Mon May 23, 2011 7:22 pm

It is OK to use the full voltage if you substitute the cap for a lower value. For example try using a run cap from a small AC motor so you use about 4MFD instead of 50. Another possible cap to use is one from a microwave oven. It is rated for a higher voltage, but has smaller capacitance.

Start with a LONG ignition wire and work shorter so the amount of metal to heat is excessive to prevent exploding wire and then shorten it until the length properly heats on the discharge.

On safety, avoid contact. The voltage is dangerous.
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Unread postAuthor: saefroch » Mon May 23, 2011 7:50 pm

Resistance increases with length, and so would heat output. I'd think that the length doesn't matter in this case, since the discharge is complete and quick. When I obliterated the platinum, the capacitors had been fully discharged.
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Unread postAuthor: Technician1002 » Mon May 23, 2011 9:28 pm

Resistance increases with length, but we are working with a limited number of watt seconds. Increasing the length increases the mass to be heated. The goal is to match the heat required to make the wire glow with the amount of heat provided by the capacitor. Too small of a wire will over heat and too big a wire will under heat.
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Unread postAuthor: saefroch » Tue May 24, 2011 5:41 am

Watt*seconds = Joules. That just bothers me, because it's like saying I have a projectile at 1 kg/m<sup>2</sup>*s<sup>2</sup>. It obfuscates the idea of energy.

The equations seem to agree with me: P=I<sup>2</sup>R, and P=kA(T/d)
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Unread postAuthor: Technician1002 » Tue May 24, 2011 6:16 am

Watt seconds has been used for electrical energy. Joules has been used for stored mechanical and electrical energy.

Photoflash equipment is often rated in Watt Seconds.

Watt Seconds is a direct extension of the Killowatt Hour as a measure of a unit of electrical energy.

A 50 Watt Second flash will expose the film the same as a 50 watt xenon lamp on a 1 second exposure, a 500 watt at 0.1 seconds, 5,000 watt for 0.01 seconds or typically 50,000 Watts for 1 ms. 50,000 watts into the flashtube for 1 second would break the tube.
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Unread postAuthor: saefroch » Tue May 24, 2011 5:04 pm

saefroch wrote:Watt*seconds = Joules. That just bothers me, because it's like saying I have a projectile at 1 kg/m2*s2. It obfuscates the idea of energy.


Maybe it's just me, but the way that last paragraph is constructed is horribly confusing, because it seems obvious to me that subjecting a flashtube to such large amount of energy in a short amount of time (high power) will make the equilibrium temperature massive, so it's no surprise the tube will fail...
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Unread postAuthor: Technician1002 » Tue May 24, 2011 7:33 pm

A 50 watt second photo-flash is relatively common. Typical flash duration is 1ms for a pulse of about 50,000 watts. The flash tubes survive fine. You can hear the audible pop when they fire.

Here is an even larger one.. 150 watt seconds;
http://store.uniquephoto.com/e/index.php/quantum-qft5d-r-qflash-150-watt-second-flash-requires-battery-adapter-qft5d-r.html
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Unread postAuthor: saefroch » Tue May 24, 2011 8:31 pm

Strand of copper tested, nice ignition-like flash once, then it seems it became a powerful insulator, I was unable to dump the capacitors again. Ideas?
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Unread postAuthor: DYI » Tue May 24, 2011 8:40 pm

...I was unable to dump the capacitors again. Ideas?


Considering the relatively low voltage you're using, I'd say that the "flash" you saw was boiling and combustion of the copper at the contacts, creating a fine oxide layer which is acting as an insulator. I don't know why you guys insist on using "fast" cap discharges when a strand of nichrome matched to an available small lead-acid/LiPo battery will be so much easier and capable of generating the same temperature for a longer time.
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Unread postAuthor: saefroch » Tue May 24, 2011 8:43 pm

Not "guys," just me :)

I'm curious about this, and have little better to do with my capacitor discharge unit.

EDIT: I think I fried my 125V switch...
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