If I transpose your formula to I = (TV)/L, (0.110s * 12v) / 0.0069H -> I get I=191.3A. What am I doing wrong here?

You are missing the fact that I left out the part about DC resistance limiting the current to 8 amps. I just "derived" a crude formula that pretty much kinda works for small time amounts.

 

my spark is waaay to hot.

I am thinking of bumping my 1.2ohm ballast resistor up to at least 3 ohms.

Or you could use a short "on" time to reduce the current.
Did I say 1 millisecond?

 

Or you could use a short "on" time to reduce the current.
Did I say 1 millisecond?

Touche. I am working on that as we speak. I am aiming for 1ms to get the 4A limit. I will let you know within the hour how it goes.

 

Actually, the math said 2.3 milliseconds to get to 4 amps.

 

Actually, the math said 2.3 milliseconds to get to 4 amps.

In that, case if I get it to spark reliably on 1ms, thats where Im headed.

 

Ok here is the status as of end of day.

I started by seperating the coil and valve triggers. At a minimum if the spark locks, the gas doesnt, although you can still meter the gas out in burst. Still, its much better.

Also, now that triggers are seperate, I implemented the suggestions #12 made about limiting the dwell time. I ended up at about 1.5ms. The spark looks good. It is very obvious the current was greatly reduced as my ballast resistor does not get hot anymore, previously after about 20 rapid fire bursts, it would be very hot.

Finally, I rewired my safety. Previously, it simply blocked the trigger signal. Now it actually turns on/off the power to the coil and valves. Now, in the event of catastrophe, one can turn the safety off (which collapses the field if it is generated) and it can never be energized again unless safety relay is activated again. MUCH BETTER! Not really sure why I didnt do this in the first place, nagging deadlines Im sure.

Now as of yet, I havent put gas to it. I have noted that more problems occur when the gas is on (i guessed maybe the gas reduced the resistance at the spark plug) so we'll see what happens when I apply gas on Monday.

Thank you, all of you!

 

PS #12 - Just for you I calcalated 2.61A using 1.5ms

 

I'm flattered.

Now you have an apparent dilemma. When you lower the "on" time, you lower the current, and the dump energy becomes a lot less. You are tempted to use a capacitor with a lower voltage rating. Don't. At least not while you're in the development stage. If the millisecond timer ever goes wrong, by even a few milliseconds, a .47uf, 400V capacitor will likely explode when nearly 1000 volts hits it.

After you get it all finished and tested, you might choose to lower the voltage ratings, as demonstrated in post #33. Still, keep a lid on the finished product to keep shrapnel to a minimum.

 

Duly noted. I never actually did get around to the RC snubber tho, I beefed up the size of flyback diode instead. Monday we will find out if that is sufficient.

 

The capacitor of which I speak does the job of a snubber by limiting the dv/dt on the drain of the mosfet. It does not have a resistor in series with it because that resistor is there to prevent the switch from suffering (approaches) infinite amps turning on with a charged capacitor connected to it. The charge on the capacitor is drained off between pulses, otherwise, you'd need a resistor in series, thus completing the "typical" snubber.

In other words, you did get around to the RC snubber network, it just doesn't need a series connected resistor.

 

@Cloud9, would it be possible to get a photo or photos of the simulator? Are you using some kind of burst disc/membrane? Or is it like a carbide cannon, with two different size chambers? Thanks.

 

@Cloud9, would it be possible to get a photo or photos of the simulator? Are you using some kind of burst disc/membrane? Or is it like a carbide cannon, with two different size chambers? Thanks.

Absolutely I can get you a pic, maybe even a neato video if I figure out where to host it.

It's a cool project. It is an aluminum frame that consists of barrel, baffles, and a firing chamber. The O2 and LP have individual valves and are mixed in the firing chamber. Its all remote controlled from up to 3 miles away (5/6Watt radios)

Dont let me forget Monday!

 

Its all remote controlled from up to 3 miles away.

Excellent idea!

 

Excellent idea!

Hahaha. Unfortunately the testing is about 3 FEET away

 

If you suddenly don't come back to this conversation, we will know why.
Meanwhile, exchange your tin foil hat for a solid steel hat.

 

Meanwhile, exchange your tin foil hat for a solid steel hat.

No can do, my steel plate is reserved for protecting my lower half from the 6W of RF

 

As requested, pictures!

The top board is control, communications, etc
In the bottom left is the radio
To the right of radio is the safety relay (disables coil/valve power)
To the right of relay is the FET board
To the right of the FET board is coil
At the right edge is the valves
http://postimg.org/image/fi5dm9o1j/

External view
http://postimg.org/image/mvpjexmjb/

 

Cloud9, nice you would have had plenty of room for an ignition module in the enclosure. I was hoping you would reveal more about the actual 'gun'. Any chance of that?

I collect among other things, carbide cannons and carbide guns. Ever heard of the "Smiths carbide machine guns"? Made by the Smiths welding torch company back in the late 1920's early1930's.

 

No, I never heard of them before but I googled them and they look neat. How the heck do they work?

Lots of people make stuff similar to what I am making for war re-enactments and pest deterant (like to scare off birds from crops or runways).

Mine is used to simulate gunfire on a battlefield for training our (US) soldiers. We have another thing that is similar but it is a huge chamber that fills up with O2 and LP and sounds like a bomb or IED going off.

----
The update for today is that we shot it ALOT and not once did we lock the gas valves on. What did happen though, was board lockups and resets. We saw this problem a while back and we fixed it by increasing the secondary resistance if I recall correctly.

Also, there were a handful of misfires, which I think may mean I need to increase my spark strength / dwell time up a bit.

 

Heres a short video, you cant see much, but it sure sounds great.

http://www.filedropper.com/201304221305091

As you can see, it was a PERFECT day in TN!