27 replies to this topic

[pyroptix]

Hi,

I'll be having a couple performers on hula hoops rigged with 6 small ice fountains (lemaitre's)

It's part of a fire show we're doing in the maldives. I've used plenty of gerbs before with a 9V battery to fire, but only 2 at once before and I'm not sure that it will be enough for 6 of these small devices.

I've looked at series and parallel wiring and i'm thinking series would be the best way....

And unfortunately the organiser of the event has only just been able to fly enough fountains in for the show. have 1 spare to test so i can't test the 6 firing.....

Does anyone have any experienced opinions?
They would be greatly appreciated.


oh, feel free to check out www.pyroptix.com for a better idea of the kind of show i do

[fishy1]

Series is not a good idea, when one ignites it will probably break the circuit, preventing the other five fron being ignited. Also, using ohms law (Rt= R1+R2.... for series circuits), you can see the resistance of the overall circuit will be much higher. This means the current will be lower, and consequently each e-match will have a lower voltage dropped on it, and it's possible not even one will fire, they'll all just warm up. Being cautious, you can expect each device to recieve 1V.

Parallel is much better. The total resistance of the e-matches will be much less (1/Rt=1/R1+1/R2...), so you will draw a much higher overall current, and each e-match will recieve about the current it would recieve if you fired one on it's own. One thing to watch for is though, as the overall resistance of the circuit is far less, the battery's internal resistance will be more important, and it could possibly prevent the e-matches from igniting, or even overheat the battery. Also, you might have to watch any wires coming off the battery if as if they aren't able to cope with the higher than normal current, they might get hot - but this is unlikely.

Your battery might be unable to supply the necessary current, in which case I recommend either a car battery/charger or alternatively wiring in say 5 of your batterys in parallel.#

Edit: test of the ohms thingy.

&#x2126

ah it don't work. &#x2126

Edited by fishy1, 07 March 2007 - 10:26 PM.

[karlfoxman]

I use and many many people do series wiring for commercial igniters. Always fires, I once asked Daryl Flemming how he wanted 4 e's wiring and he said series, ever since I have used series. But this is on much higher current and voltage a 9v pp3 can supply. But then again I dont go over 4 in series, I tend to use series/parallel wiring after that. I have tried 8 e's in series off one channel and they all fired but not saying it will work everytime. If I was you I would use 2 12v leadacid batteries in series to make 24v. That will fire them!

Edited by karlfoxman, 07 March 2007 - 06:40 PM.

[pyrotrev]

It all depends somewhat on the length of cable you're using if you're really limited to 9V. Generally, with a long cable series wiring will give you a better chance, with a short cable parallel or series/parallel will be probably the way to go. Typically igniters will need 0.8A to fire reliably, for a big series string I would advise increasing this by at least 40%. So if you have 6 igniters in series with a resistance of 1.2ohms (how do I get the omega symbol Rich? ), that's 7.2 ohms; then 20m of 13/0.2 twin cable would add another 2ohms or so, and if there's no capacitor in the firing box, you'd better add another add another couple of ohms for the internal resistance of the battery if it's a PP3 alkaline. So we're up to about 11 ohms total circuit resistance, so firing current will be V/R (Ohms law) = 9/11 = 0.81 amps, just OK if they're good quality squibs, but I'd want more for firing typical Chinese ones. If you did them parallel, then the total resistance of the igniters in parallel would be 0.2 ohms, and the total circuit resistance 4.2 ohms. This would give a firing current of 2.2 amps, BUT this is divided by 6 igniters, hence less than 0.4 A per igniter, rather dodgy, particularly if you want simultaneous ignition. As Karl said, it would be good to increase your voltage if you can, particularly if you're using long cables.
The reason a lot of display operators go for series wiring is that it gives a positive indication that all igniters are connected - if you had 6 in parallel it wouldn't be so easy to detect one that wasn't connected unless you have an ohmmeter in your firing panel (essential IMHO). Parallel wiring gives greater reliability in the case of variable igniters (one fires at a much lower current then the others) and clumsy boots/wandering shells (wire gets pulled out/broken).

Edited by pyrotrev, 07 March 2007 - 07:52 PM.

[Andrew]

The ohm symbol Ω is made using unicode; it is an ampersand, followed by a hash, a lower case "x" and the the number 2126 (looks like &#x2126) and the html viewer takes care of the rest. That capital Omega symbol earlier was made by simply typing that code into the post text window.

Are you talking about a PP3 battery, they are crap. They have an unbelievably high ESR, making them next to useless for this type of work (when you are after reliability). 12V or 24V SLABs are very good for e-matchs as their ESR is next to nothing. They can be improved by placing a large capacitor in parallel with the battery, but its much better to have a source with a low ESR and high pd to develop the necessary current. The series/parallel debate is a case of engineering. Both work, neither is ultimately better than the other, its just that different configurations suit different source/cable/igniter restrictions.

The important thing to remember here is the ESR of the battery. Accounting for cables and igniters is all very well but if the ESR of the battery is say 30Ω, it fouls up everything.

Edited by Andrew, 07 March 2007 - 10:05 PM.

[pyroptix]

Thanks for all your feedback,

Your battery might be unable to supply the necessary current, in which case I recommend either a car battery/charger or alternatively wiring in say 5 of your batterys in parallel.#

I think strapping 5 PP3's in parallel is my only viable option. Will doing this help regardless of my circuit choice?

Wiring will only be running around the inner surface of the hula hoops. Each performer will be stepping into the hoops to fire after finishing a previous prop.
So I wont have 20 meters of wire. just 2 meters around hoop... so around 9 ohms resistance = approx 1 amp in series circuit. by pyrotrev's calculations....

So that makes me want to go with series. The Ice fountains are Lemaitre so I'm assuming good quality. hopefully that helps in not having the first match break the circuit?

I wont be able to have 12V batteries and wiring running from off stage unfortunately. It's very late in the preps and we're all being flown out to a remote island for this one.... So flying with lead acid batteries will be a problem. Next time I'll know and have the agent organise a couple in advance.
9V's can be either in the performers pocket or if I'm strapping 5 together I'll put them out there while setting stage so minimal fuss and setup.

So from the sounds of it electrical match's for very small devices are not nessesarily smaller/easier to fire than larger ones?

The important thing to remember here is the ESR of the battery. Accounting for cables and igniters is all very well but if the ESR of the battery is say 30Ω, it fouls up everything.

My understanding of Equivalent Series Resistance is extremely limited.....
Will strapping 5 PP3's together in parralell help overcome the high ESR?

[Andrew]

Will strapping 5 PP3's together in parralell help overcome the high ESR?

Yes it will go some considerable way to providing a source that can give you enough current to blow the matches.

As it's a one shot device. A simpler, cheaper, easier, more reliable way to achieve the same increase in available power, a large capacity capacitor can be placed in parallel with the battery. It also lighter and easier to handle than 5x PP3 and a hell of a lot cheaper. You can get 4.7mF 16V electrolytic capacitors for pence (PP3 is pounds). One in parallel would be able to give 1.9A without ever failing. With typical ESRs ranging form 0.2 to 0.001Ω they are far better than a PP3.



The resistor is there to protect the battery. As the capacitor will be doing the work, the PP3 is only there to charge it and maintain a potential across the capacitor. Make sure the capacitor is connected the correct way round or it'll explode!!

Hope this helps.

Edited by Andrew, 08 March 2007 - 08:46 AM.

[BrightStar]

Make sure the capacitor is connected the correct way round or it'll explode!!

LOL - I once watched as my secondary school physics teacher connected a very large electrolytic cap in reverse to a 12v 30A PSU. As his back was turned, I watched in horror as the ammeter crept up and up. 'Sorry Sir, your demo is about to explode...'

Back on topic, if you are charging a capacitor, why not use two PP3s in series to charge to 18v. This will allow you to reliably fire a few matches in series.

Edited by BrightStar, 08 March 2007 - 12:33 PM.

[pyrotrev]

The ohm symbol Ω is made using unicode; it is an ampersand, followed by a hash, a lower case "x" and the the number 2126 (looks like &#x2126) and the html viewer takes care of the rest. That capital Omega symbol earlier was made by simply typing that code into the post text window.

Are you talking about a PP3 battery, they are crap. They have an unbelievably high ESR, making them next to useless for this type of work (when you are after reliability). 12V or 24V SLABs are very good for e-matchs as their ESR is next to nothing. They can be improved by placing a large capacitor in parallel with the battery, but its much better to have a source with a low ESR and high pd to develop the necessary current. The series/parallel debate is a case of engineering. Both work, neither is ultimately better than the other, its just that different configurations suit different source/cable/igniter restrictions.

The important thing to remember here is the ESR of the battery. Accounting for cables and igniters is all very well but if the ESR of the battery is say 30Ω, it fouls up everything.

Thanks for the info on the ohms symbol. Battery ESR varies a lot, but I was happily suprised with a new high spec. PP3 which did 3.5A into 1Ω - hence my example in the calculation. No doubt a cheap-jack one off the market would be much worse.
Pyroptix - I'd assumed you were using an existing firing box that ran off 9V, if you're making something then as Andrew pointed out, capacitive discharge is the way to go. If you're using capacitive discharge, firing the size of the cap needs to be selected so that the voltage doesn't droop too much during the fusing time of the igniters (which is probably 5msec worst case or so at rated current for a good igniter). 0.8A for 5msec is a total charge of 4 millicoulombs (mC). The equation is C=Q/V, so for a 20% drop from 9V (1.8V) C = 0.004/1.8V = 2,200uF. Obviously if the firing voltage was higher, the cap could be smaller.

[pyroptix]

Thanks Andrew,
The diagram was a great help.

I've been down to radio parts and picked up a few 25V 4.7MF capacitors..... they had no 12V's
The resistors are 33 OHM

Will the single 9V be fine to charge the larger capacitor?
If so I'm assuming it wont matter whether I use a parallel or series ciruit.

I'll be connecting the capacitor/resitor/9v together well in advance of the show (the previous day probably). Assuming I do connect it the right way around ;-) there's no issue with having it charged that long?

I do have an excess of 1.5m gerbs for this show. I think I'll rig 6 up the previous night and test this.... given plenty of time to charge I'm guessing each cap/res/9V would be fine for a few firings.

[Andrew]

Will the single 9V be fine to charge the larger capacitor?

Yes

The charge time to 90% would be a second or so (can't be bothered to work it out right now, I've just been Matlab'ing my arse off). The battery could be installed long before it is required and fired at will. As BrightStar said, you could also use two PP3s in series and charge the capacitor up to 18V,which means more power! But at the end of the day, its more about size/weight and the ability to operate it safely. I'd give it a once over if I were you, just to check at all works as desired.


There are some good PP3s around, with low internal resistances, but you have to pay for them though! For a cheap solution to what I'm guessing is a large number of similar devices, that will only get used once, market actions are fine. I'd rather chuck away (recycle properly) a load of cheap arse batteries than hoard a load of good ones I can't bring myself to throw, even though I know there is not way I'll be able to use them all!

[pyrotrev]

OK you only have just enough fountains, but could you do a test with some spare igniters? or are you already out in the Maldives?

[pyroptix]

As BrightStar said, you could also use two PP3s in series and charge the capacitor up to 18V,which means more power!

I've ended up with 24V capacitors. A single PP3 will charge one?

OK you only have just enough fountains, but could you do a test with some spare igniters? or are you already out in the Maldives?

Head out Tue morn. I'm going to have an excess of gerbs. So i'll get a chance to test on 6 of those.

Cheers for your help guys. I'm now quite confident for the firing, ill let you know how it goes!

[pyromaniac303]

I've ended up with 24V capacitors. A single PP3 will charge one?

It will do, the voltage rating is the maximum voltage you can apply to it without it damaging. As your only using 9v or 18v your well clear of this.

Another thing to be careful of is the difference between mF and μF, you can get mixed up between millifarads (1000 microfarads) and microfarads. Also not helped by suppliers using an 'm' instead of a 'μ' on some labelling, and some tolerances being described with letters such as J and M to confuse even more.

If the label says 4.7mF I'd be very wary and check that its right first, if it says 4700μF then you are definately OK.

[pyroptix]

If the label says 4.7mF I'd be very wary and check that its right first, if it says 4700?F then you are definately OK.

says 4,7?F So should be fine. I'm assuming the comma represents 00's

Thanks!