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[on:]

Hello guys. Sorry i`m very new to all this, so my question may be lame. sorry again if i`m breaking any forum rules.

I`m trying to build an electric confetti launcher for disposable cannons like the ones here http://www.confettimaster.com/streamer.electric.confetti.cannons.html

Basically each disposable barrel holds a sealed co2 tank. The output plug holds two wires connected to something like nichrome thread. When you drive enough current through those wires the nichrome warms up, burns the seal and the canon go boom. There are nothing sophisticated there, i tried shooting them with a simple 12 V battery and everything worked fine.

Now i`m trying to use arduino to set up a wireless launcher and since i don`t have any education on electronics i cant calculate a proper way to connect the cannon to arduino.

My guess is that i need some kind of mosfet to drive enough current but i have no guess how to choose the right one.
For example, i have some IRF530A - they can hold up to 14 A current. Is that enough? I`m going to use 12v lead-acid batteries, they can give extreme high ampreage.
Besides, is it ok to connect mosfet directly to the cannon, or perhaps i need to put some resistors somewhere in between? Or some optoisolator to break the circuit?

[Reply #1 on:]

If it was me, I'd start by measuring the current required to fire the thing, just by connecting it straight to the battery with an ammeter in series with your load.

Then, I'd probably go looking for mosfets that could handle a bit more current than I absolutely needed. I think it's the RMS value rather than peak/surge you need to look at. If you run them at their absolute limits for too long they might not last as long as you'd like.

I would definitely consider using an opto-isolator between the output pin and the mosfet, just for that extra level of protection in case something goes horribly wrong. Connecting all those parts up is dead simple, a quick snoop around Google and you should be able to find loads of circuit diagrams. A 100kohm resistor pulling the mosfet's gate to ground might also be needed, to make sure it stays off when it's not supposed to be doing anything.

Hope this helps!

[Reply #2 on:]

Thank you. In fact, i don`t have any real problem connecting everything, i actually have a working assembly just in front of me. The only thing is that i use a simle LED instead of cannon to test circuits and radio.

The thing i`m trying to find is some kind of rough  estimated values. Running multiple tests on those cannons is kinda expensive ;-)

Problem is, i can figure no way to actually measure the current.  You plug the thing in, then it burns the seal, and if you hold it just a sec longer - the wire breaks.

My guess is to switch mosfet open for about 1 sec - does that qualify for peak surge?

Perhaps there is a way to use some other parts instead or combined  with a mosfet? Maybe some relay? There are really no space issues, so it will fit just fine.


 

[Reply #3 on:]

I think it's the RMS value

RMS (root mean square) is a measure of A/C, so isn't a consideration here.

[Reply #4 on:]

I think it's the RMS value

RMS (root mean square) is a measure of A/C, so isn't a consideration here.

Quite right, I was thinking of the wrong damn component :-P "continuous" is what I should have written. On that note, I was checking the datasheet again for my favourite N-channel mosfet for playing with pyros, the IRF540N. At 25 degrees C, it reckons it can handle 33A continuous drain current. That, or something like it, would probably stand up to repeated abuse

[Reply #5 on:]

I was checking the datasheet again for my favourite N-channel mosfet for playing with pyros, the IRF540N

Rofl. i actually use wireless controller for pyrotechnics as a radio transmitter\reciever now. going to upgrade for something zigbee-like later.

[Reply #6 on:]

So, to close the issue.
I need to find some mosfet wich can sustain 30+ A continuous current, and, just in case to use optron between mosfet and arduino, is that correct?

Can arduino handle optoisolator directly from control pin? Or there should be another transistor between them?

[Reply #7 on:]

You should use a multimeter to find the resistance between the two leads of the igniter. With that knowledge, and the fact that you're kicking it off with 12V previously, you can calculate the amperage needed (Ohm's law) and find a suitably-rated transistor/mosfet.

You do not need an optoisolator here. There should be a resistor between the output pin of the Arduino and the gate of the mosfet (to limit current out of the Arduino pin to 20ma -- more Ohm's law stuff) but that's all you need. Assuredly a little googling of "arduino mosfet" will give you plenty of examples.

[Reply #8 on:]

You could measure the cold resistance of the nichrome wire to estimate the startup surge current. Presumably that is the worst case.

Do they need to be driven from 12V? From your description of the wires burning out immediately, I wonder if they might be designed to work at a lower voltage.

[Reply #9 on:]

I tested it yesterday. Everything works just fine, except the test canon scared the shit out of my cat ;-)

The resistance of canons varies from 1,5 to 3 ohm, and as far as i know the resitance of nichrome raise with temperature -> max current is roughly 8 a.

So i used irf530 plugged directly to arduino output to send 1000ms pulse. Since the canon does not burst instantly i will increase the duration of impulse to 1500-2000 ms to be sure it works even in cold weather.

Thank you all very much.