Discussion in 'Firing Systems And Fusing Fireworks' started by SydB, Jul 4, 2018.
Wierd, so much effort and knowledge yet he got the conclusion of three ingintors in series slowly having the current ramped up a bit wrong.
He concluded that slowly increasing the current could lead to one ignitor going open circuit, thus not firing the rest. Yet what his test showed was that the right ignitor went fine, then the middle one went fine, then the last one went fine, but blew open circuit.
The conclusion I drew from this was that he was pushing 800ma by the time he got to the third ignitor, hence it blew open. If he'd added a fourth ignitor, I'm willing to bet the other three would have gone fine, and that the 4th would have blown open.
Why do you think this?
The current it blew was surely due to the differences in the igniters.
Your conclusion would suggest the larger the current the more explosive it blows?
So which one is it?
Could the resistance increase when the igniter goes open air?
Exactly - when all three are in series there is around 500ma before one blows. The last one to blow will always be the one that needs the most current to blow. In this case 800ma - and it's plain electronics that if you increase the current across a piece of wire, it will get hot, and when you increase it enough, it will melt (nothing to do with the pyrogen)
Yeah but that has nothing to do with it blowing open.
Say you have three igniters all needing slightly different currents, 500ma 600ma 700ma. That has to do with the internal resistance of the piece of wire and just because it takes 700ma to blow doesn't make it any more likely to blow open than the 500ma. It just means it needs more current because it is more resistive than the others.
The current it takes to blow will be different for every igniter.
The current remains the same across them all, even the blown ones, it's just the heat it has to get to to ignite the pyrogen.
When an igniter has fired and gone short circuit, what and where is the actual short? Unless the solder melts and flows between both contacts, I don't get it. From memory, every time I test a spent igniter it has gone open circuit. But then I use a lot of power, thick wire and generally fire in parallel.
Supplying a constant current source gradually increasing, as Clive did, is not usually how igniters are fired, of course, and I think is completely artificial. I'm trying to understand the actions during ignition. With most wire, the hotter it gets, the higher the resistance, and the relationship is exponential (I think something like nichrome does not behave quite the same, though). If higher power means that it is more likely an igniter will end up open circuit then I guess it is more likely that the later igniters to fire in a series will fail open circuit (if the others have spent shorted). There is a voltage drop across each unspent igniter because each has significant resistance so that when one of them fires, there is less power in that ignition compared to the next igniter to fire.
However, when we fire, we usually whack a much higher pulse current than necessary down the line (high voltage) which, I suspect, will cause every igniter to begin to fire before any of them have had a chance to go open circuit (a kind of latency). At lower voltages or higher wire resistance, the outcome of series igniters may be more unpredictable. That's my guess.
I don't think it matters if they blow open or closed or that they need slightly different currents to fire. When the output of a Cobra mod for example is 3-5A, all three will fire within a fraction of a millisecond with plenty of current to spare.
I'd guess that most inserted into cakes without the sleeve break open because of the heat in the lift charge TBH.
Technically it is not a short circuit, it is still a resistive load which is the small wire which is under the pyrogen still intact after it has heated and ignites the pyrogen.
So the resistance before firing and after firing is pretty much the same?
In theory yes, I have never checked though!
His test is not that artificial, he just slowed time down to see what would happen. In reality a load cannot just go from no current to full current, it would ramp up (very quick mind) in the same way the potential would ramp up.
Igniters will not fail due to the number infront, this is a false belief. While the current may have minor fluctuations, if a firing system is supplying a current orders of magnitude higher than what the whole circuit needs that it will be fine.
What will cause igniters to fail is mixing manufacturers for example. Say one manufacturer has a 300ma sure fire and the second has 800ma then obvious if the 300ma go open circuit in the microseconds as the circuit is made then the 800ma will not fire. This has been an issue discussed many times. However, this is not to say it will happen every time, if the 300ma stay intact then it will be fine.
These are not precision devices and as such results will always vary.
High speed footage would disprove this, igniters all fire at different times and not at the same time. As the current climbs the igniters will pop as their individual sure fire currents are reached.
At lower voltages cable resistance would cause an issue but the reliability of the igniters would be no different as long as they have the correct sure fire current applied.
So you are saying that an igniter in series never ends up open circuit as a result of firing unless it is the last one to ignite?
No, I am saying that they normally stay closed but the ones to go open are random.
Then once an igniter has 'popped' open 'as the current climbs', why would any that have a slightly higher fire current fire at all? This is what I am saying. There must be some latency between beginning to fire and the igniter going open circuit. On one hand you are saying that ramp up timing is unimportant, igniters can randomly go open circuit and on the other you are saying that igniters in series will fire at different times. If this were so, why would anyone connect them in series? My point is that the start of ignition of all igniters in series must occur before the first one goes open circuit, otherwise there would be non-ignitions. Does that make sense?
I think the video shows a random event and there is the belief that it is actually a result of the higher current due to it being the last igniter to fire due to the higher current it needs.
I am saying that this random event is not reflective of what 'normally' happens. If you redid the experiment you might find none go open circuit or the first one might or possibly the second. Who knows!
When you wire a show you generally use the same manufacturers igniters if wired in series. This is important as they will have around the same firing current rating, thus making them fire closer together and less likely causing it to go open circuit before the others have fired. That is not to say there is no chance, it may still happen. I have certainly derigged shows where this has happened so I would always tend to quickmatch fireworks together rather than use an igniter if possible.
Just because the igniter has fired does not mean it will go open circuit, so there will still be a chance for the others to get the current they need.
In special fx we would never wire in series due to this possibility of it being unreliable, parallel is the only true way of making sure each igniter will get the current it needs.
However, firework companies don't like this because you can't just test the whole circuit from the firing module as in parallel only one igniter has to have continuity to show it....meaning others may not. It also complicates the wiring on site.
That's the way I see it too.
Put 5A through three igs in series and they all get 5A at the same time and blow at the same time. Doesn't matter if one blows at 500mA, one at 600mA and one at 800mA because they are ALL getting 5A.
Ok, I don't have time to get my DSO out and do it properly but here is a simulation of a circuit. The 1R is the igniter and 12V power supply is our firing system. The green line is the current trace through the resistor.
When it fires, voltage and current ramps up as in physics you cannot just suddenly move something from 0% to 100%.
Igniter1 fires at 500ma, Igniter2 fires 700ma and igniter3 fires at 900ma.
If igniter1 fired and went open circuit igniter 2 & 3 would never fire as they are later in time. The amount of current you want to use 5a, 12a, 100a, 10ka, is irrelevant - its all about the time it takes.
If igniter2 was the one that popped open then igniter1 & 2 would fire but not igniter3...etc
and here is the graph with 3 resistors as igniters just incase you think my simulation is missing igniters and inaccurate, the current curve ramps slower with the additional resistance but the conclusion remains the same (notice how the current curves of all the resistors over lap).
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