Powering Electronic Match

[nbabusis01]

I am building a model rocket ignition system designed to launch a rocket from a weather balloon. I need to heat up a small piece of Ni-Cr wire (or similar) to glow red hot in order to light the propellant. I can get it to work (barely) by shorting outa 9V battery with a piece of 36g Nichrome 80 wire, but it does not work when shorting 9V's in series. Also tried using the same wire to discharge 4700 uF capacitors alone and in parallel without success. What should I do?

 

[dave9]

What do you mean by "does not work when shorting 9V's in series"? Do you mean you have multiple 9V batteries in series or that you have multiple rocket igniters in series on a single 9V battery? It reads like you mean multiple 9V batteries in series but that should work, while it wouldn't surprise me if multiple igniters in series limits current too much due to the increased resistance so you would also need multiple 9V in series.

How about using a series of 3 x 14500 Li-Ion cells instead of the 9V (alkaline?) ? You then have a higher voltage and a much higher current capacity (reducing voltage droop).

That will weigh a little more but probably only a couple dozen grams more, maybe closer to double the weight of a 9V alkaline once you add a battery holder for them.

 

[nbabusis01]

What do you mean by "does not work when shorting 9V's in series"? Do you mean you have multiple 9V batteries in series or that you have multiple rocket igniters in series on a single 9V battery? It reads like you mean multiple 9V batteries in series but that should work, while it wouldn't surprise me if multiple igniters in series limits current too much due to the increased resistance so you would also need multiple 9V in series.

How about using a series of 3 x 14500 Li-Ion cells instead of the 9V (alkaline?) ? You then have a higher voltage and a much higher current capacity (reducing voltage droop).

That will weigh a little more but probably only a couple dozen grams more, maybe closer to double the weight of a 9V alkaline once you add a battery holder for them.

Thanks for your reply!
I used a ~1 cm length of Nichrome wire connected with two alligator clips to 5x 9V alkaline batteries in series in my most recent test, although I use 9V Energizer Ultimate Lithium for the real thing (they work down to -40C pretty well).

A while ago I did a test at altitude with some store-bought electronic matches, home-made prototypes, and magnesium sparklers set up in two parallel sets of 3 ignitors in series powered by 2x relay-controlled, parallel 9V Energizer Ultimate Lithium (it failed) (picture below).

 

[Bluejets]

Perhaps use a model r/c engine glow plug.
Self contained, compact, cost around $8.00 or so and will run anywhere between 1.0v and 1.5v at around 3A.

 

[WHONOES]

Many moons ago, I used a simple igniter for some "flash pans". These are simply a small amount of powdered magnesium in a container which was usually a piece of cardboard tube with a piece of card glued to its bottom. The igniter was simply a couple of inches of small gauge copper wire invariably taken from a length of mains cord. The wire was generally about 0.2mm in diameter. The power supply was no more than a couple of NICads in series. The igniter needs a lot of current and not a lot of volts. A small 9V dry cell will not suffice. The igniter had to be replaced after each event as it was vaporised.
A pieces of Nichrome wire should work but, you will need to use NIcad or NiMh batteries for the current supply. I WOULD NOT advise using Lipo's.
From my perspective, the sacrificial copper wire would be the best option because once it has fused current ceases to flow.

 

[Ylli]

Why are you making this so complicated? An standard Estes igniter will fire just fine on a single good quality alkaline 9 volt battery. If you really are shooting for reliability, then you need to use a commercial e-match.

re using multiple 9 volt batteries in series... Your load resistance is very small in comparison to the internal resistance of a 9V battery, so the current delivered to the load is really a function of the total battery voltage and the total battery internal resistance. For example, if your load is 1 ohm and the 9 volt battery has an internal resistance of 9 ohms, then
One 9 volt battery will deliver 9/10 = 0.9 amps to the load.
Two 9 volt batteries in series will deliver 18/19 = 0.95 amps to the load
Three 9 volt batteries in series will deliver 27/28 = 0.96 amps
Ten 9 volt batteries will deliver 90/91 = 0.99 amps.

Stick with the one battery.

 

[nbabusis01]

I just ran some tests: 1cm of 36g Nichrome gets red hot at about 5V, 2A (in circuit with 2 alligator clips). The wire consistently glows using 4+ 9V's in parallel, which seems strange since I would expect the heat dissipated per time to be proportional to total wattage, which should be the same in parallel or series, right? I could just send up 6x 9V's (two for redundancy) to heat the wire, but that seems awfully wasteful.
I don't think regular Estes ignitors or sonic ignitors will work. I tried some of these in near-vacuums but they usually just burned through without igniting the propellant or didn't heat up enough. I got one to work when it was epoxied into a container of KNO3+sugar granular propellant mix (not airtightly though), so it seems to be unreliable.
Also, does anyone know where I can get a smaller/more resistive wire than 36g Nichrome 80? (.85 ohm/cm)

 

[Ylli]

You are on the right track putting those low current batteries in parallel to allow them to deliver a higher current. From how high are you launching? You are using Candy fuel? Under low atmospheric pressure, you really need to be sure the igniter is in good mechanical contact with the propellant. Candy propellant is somewhat difficult to light even under the best of conditions.

 

[nbabusis01]

You are on the right track putting those low current batteries in parallel to allow them to deliver a higher current. From how high are you launching? You are using Candy fuel? Under low atmospheric pressure, you really need to be sure the igniter is in good mechanical contact with the propellant. Candy propellant is somewhat difficult to light even under the best of conditions.

Launching from 80-90k ft. I was using candy fuel for the tests (just what I had a lot of at the time) and am using store-bought AP propellant for the real thing. I'm experimenting with ignition mixtures, soon trying nitrocellulose and FFFG gunpowder as incendiaries, and have been suggested to coat the wire in sulfur (not sure how I will do this) to liquify and transfer heat better to the incendiary.

 

[dave9]

I just ran some tests: 1cm of 36g Nichrome gets red hot at about 5V, 2A (in circuit with 2 alligator clips). The wire consistently glows using 4+ 9V's in parallel, which seems strange since I would expect the heat dissipated per time to be proportional to total wattage, which should be the same in parallel or series, right?

Total wattage is not linear like this. First you have picked a battery with high internal resistance, which in itself limits current, but then by putting them in parallel you divide that internal resistance. Second if put in series, the internal resistance of the series increases. Third as the temperature of the nichrome element increases, so does its resistance.

You need larger cells for the most volumetric or weight efficiency. Those within a 9V battery are too small to be good for a resistive heating element. I would conduct experiments with a bench power supply to determine exactly what voltage is needed for achieving an ignition temperature, and measure what current flows at this voltage.

Otherwise you would just throw more batteries at it than necessary, but either way I feel a 3 x series of 14500 cells is your best bet still, or perhaps a 2 x serial by 2 x parallel array of 14500 in a 4x cell battery holder. The exception might be if you are doing several flights and the craft is lost so you cannot recover the batteries, then they are a more expensive option, but I still don't know if weight limitations are a factor. If these things are factors then are you might consider lithium primary cells instead of rechargeable Li-Ion, the number in series determined by your bench PSU testing to determine what voltage is needed, the actual voltage @ current to factor for battery voltage droop at the high discharge rate.

Also, does anyone know where I can get a smaller/more resistive wire than 36g Nichrome 80? (.85 ohm/cm)

You did not tell us where you are located. Here's a place in the UK with up to 50AWG:
https://www.wireandstuff.co.uk/products/Nickel-Chrome-80-20--NI8020--Wire.html

Here's some 40AWG & 44AWG on Amazon US:
https://www.amazon.com/Nichrome-80-40-Gauge-Spools-Wire/dp/B07DLRBPZ5
https://www.amazon.com/Kbees-250-Nichrome-Resistance-Length/dp/B074Q4PLRD

There are many options on ebay searching Nichrome and (wire gauge) AWG:
https://www.ebay.com/sch/i.html?_nkw=Nichrome+44AWG

 

[Ylli]

Might want to check with the model rocketry guys - there has been discussion on igniting boosters at altitude. It can be problematic. Hope management doesn't mind if I post a link to a different forum...
https://www.rocketryforum.com/