40 AWG Nichrome wire for igniters

[mpbenda]

I have the chance to get some 40 AWG Nichrome wire. Can this be used for igniters by itself or do I need something else.

[stefanj]

You can make old-fashioned nichrome-only igniters. There are instructions in scanned-in catalogs / instruction sheets up on Ninfinger.

Short version: Cut off about 2" of nichrome. Wind the middle around a toothpick or paperclip to create a little coil. Bend so "legs" are parallel. Insert into nozzle. Pack a little ball of wadding into place with a pen tip.

Or maybe use an igniter plug, if you have spares.

They're pretty reliable, but you pretty much need a 12v launch system.

[mojo1986]

As the AWG number goes up (e.g. 32 to 40) does the diameter of the nichrome wire increase or decrease? Anybody know?

Joe

[Ltvscout]

As the AWG number goes up (e.g. 32 to 40) does the diameter of the nichrome wire increase or decrease? Anybody know?

Joe
The diameter decreases as the AWG number increases.

[pantherjon]

It is intuitively backwards...lol..Wire gauges: Bigger number=Smaller thickness

[Ltvscout]

It is intuitively backwards...lol..Wire gauges: Bigger number=Smaller thickness
Kind of like shotguns. The lower the number (guage) the bigger the shell.

[mojo1986]

Yeah, I remembered it that way but needed confirmation. Now, one more question.............

Which wire will heat up faster, small or large diameter, for a given voltage? Intuitively, it would seem that the small diameter wire would be better for an igniter (for one thing, there is less mass for the electrical current to heat up). And this seems to be consistent with an Estes igniter, which is two pieces of larger diameter wire welded to a short length of fine diameter wire. Have I answered my own question? More importantly, correctly?

Joe

[Solomoriah]

For a given metal composition, the thinner the wire, the higher the resistance. In general, yes, you have it right.

[pantherjon]

Yes

The thinner wire has more resistance so will heat up more quickly then the thicker wire.

[Mark II]

Yes

The thinner wire has more resistance so will heat up more quickly then the thicker wire.
A follow-up of my own: what about voltage and amperage requirements? Do they go up with the thinner wire, or go down?

If this sounds like a dumb question, I apologize. I must have been playing hookey from school on the day that they explained the basic physics of electricity. :o

Mark

[tbzep]

IIRC, lower voltages need the thinner wire to heat up enough to function without having to hold the button down forever. THe Solar ignitor bridge is much thinner than the original Astron wire because it was to be used with 6v instead of the 12v for the Astron.

I think I read somewhere several years ago (maybe R.M.R.???) that somebody built a high voltage capacitor based system that vaporized the wire so quickly it didn't have time to heat the pyrogen enough to ignite it. Does anybody recall that?

[Carl@Semroc]

With lots of assumptions:

6 v. Lantern Battery
18 feet of 18 gauge wire
Ignition at 600 degrees F

Length of nichrome "bridge" is .25"
30 gauge needs 1.2A to reach 600 degrees, circuit provides 4.2A (if battery can hold voltage)
32 gauge needs .9A, circuit provides 4.0A
38 gauge needs .4A, circuit provides 2.8A (close to Estes igniter)
40 gauge needs .3A, circuit provides 2.2A

All would work with margin to spare, but the lower amps (higher gauge wire) are easier to get with a drained battery.

When you increase the nichrome to 2" long, like the "old days", the results go down (better) on amperage, but then where the igniter heats up or burns through becomes more of an issue.

In general, the shorter the nichrome length and the higher the gauge (both within reason) the less drain on the battery and thus longer life and higher reliability.

Exactly where the Estes and Quest igniters have evolved over the years.

Disclaimer: I am a biologist, not an EE and this was done very quickly because I had a late checkout from the Holiday Inn Express last night. :D

[moonzero2]

You can make old-fashioned nichrome-only igniters. There are instructions in scanned-in catalogs / instruction sheets up on Ninfinger.

Can you help me by pointing me in the right direction on where I can find these instructions on Ninfinger, as there is so much information there I just can't seem to find it.
Thank you.

[Shreadvector]

Instructions attached.

[Ltvscout]

Instructions attached.
One thing that I see that isn't noted is a guage of nichrome to use. Is there a size best-suited for use as igniters?

Also, I'm surprised you haven't tossed a plug in here, Fred. Don't you sell nichrome wire? No problem adding a link if you'd like.

[Shreadvector]

Hmmm. I guess I never added a signature file. Let's see if I can copy one of my usual sigs.

[Shreadvector]

One thing that I see that isn't noted is a guage of nichrome to use. Is there a size best-suited for use as igniters?

Also, I'm surprised you haven't tossed a plug in here, Fred. Don't you sell nichrome wire? No problem adding a link if you'd like.

Estes sold 30 and 32 guage wire. 30 was great for a car battery as it got molten and produced a massive amount of heat energy before melting through.

32 ga. was best for a lantern battery as it required less power to turn red hot. It helps to connect the clips as close to the nozzle as possible to allow it to heat up faster and to prevent melt through outside the nozzle.

The coil heats up first and helps with whatever guage you use.

For MicroMaxx just bend into a V shape since there is no room for a coil/loop.

[moonzero2]

This is GREAT information,... Thanks to everyone!

Sounds like this will work great,... but I have one more question,... so is there no need for pyrogen?

[Bob Thomas]

Some vendors spool up wire ends up available sometimes on e-bay and they say they are great for igniter purposes. BUT, some are too thin. I got a sample of similar nichrome - listed as 40. Too flimsy looking for me to give a serious attempt at. I'm afraid it will probably melt too quickly and before enough heat has been transferred to the combustible. Tried to form into a coil but they had a habit of pulling out of the bend, curling or crossing (shorting) outside the dummy nozzle. Too much aggravation - maybe I'll try again if you're successful.

As a youth I launched with the PowR Pad (9v I think) and replaced broken estes igniters with split matchheads and waste wire from used ones. Sounds kind of reckless now that I think back, but they worked.

[moonzero2]

Estes sold 30 and 32 guage wire. 30 was great for a car battery as it got molten and produced a massive amount of heat energy before melting through.

I use a 12v battery as my power source so it sounds like I need 30 ga.

That 40 ga would be wayyyy to small.

I'll have to look for some 30 ga.

Thanks again for everyones input. That's what makes this forum so GREAT!!!!!!!

[Shreadvector]

I use a 12v battery as my power source so it sounds like I need 30 ga.

That 40 ga would be wayyyy to small.

I'll have to look for some 30 ga.

Thanks again for everyones input. That's what makes this forum so GREAT!!!!!!!

No need to look far. I sell nichrome wire on eBay all the time and I also sell direct. See link below signature for eBay auctions.

[moonzero2]

I won your auction for the 30 ga., and paid lastnight via PayPal.
Thank you.

[Shreadvector]

I won your auction for the 30 ga., and paid lastnight via PayPal.
Thank you.

Dropping the envelope at the Post Office today.; It's Sunday so it won;t start moving until tomorrow. First class moves relatively fast - probably expect delivery Wed-Fri.

[Jeff Walther]

Ignoring melt-through, to maximize the power delivered to the segment of nichrome wire do the following:

Make the resistance of the length of nichrome equal to the resistance of all the other wire put together.

Heh. Impractical, yes? Although, once a launch system is built, it shouldn't be too hard to measure the resistance of all the wiring with a decent ohmmeter.

It is basically a two resistor system with a battery. So you have V (voltage of battery, assuming it doesn't drop due to loading), R1 and R2. R1 is the resistance of the nichrome and R2 is the resistance of everything else.

So the total resistance of the system is R1 + R2 and therefore the current through the system is I = V/(R1 + R2)

The power in either resistor is R * I^2 so the power in R1 is R1 * I^2.

Substituting for I, we have: P = R1 * V^2 /(R1 + R2)^2

P expands into V^2 (1/R1 + 1/(2 * R2) + R1/R2^2)

To find the maximum we take the derivative of P wrt R1: dP/dR1 and hunt for the spot where the derivative is equal to 0.

This gives us V^2 ( -1/R1^2 + 1/R2^2). At the zero point R1 = R2. So the maximum power point for R1 (the nichrome) occurs where R1 = R2.

Physically, if you lower the resistance of R1 below R2, then more current flows, but more of the power is lost in R2 instead of R1. R1 has a smaller share of a larger pie. Contrariwise, if you raise R1, more of the power is lost in R1 relative to R2, but (I) sinks so the overall power goes down and R1 just has a larger share of a smaller pie.

Consider this example. You have a 12V battery. Your launch system has a resistance of 6 ohms. You select a piece of nichrome with a resistance of 6 ohms. Total resistance is 12 ohms (6 + 6) and the current is 1 amp. Power in the nichrome is R1 * I^2 which is 6 watts.

If you lower the nichrome resistance (thicker wire) to 5 ohms, now P = 5.95 watts, because I = 12/11 and R1 * I^2 is 5 * 1.19 = 5.95 watts.

If you raise the nichrome resistance (thinner wire) to 7 ohms, now P = 5.96 watts, because I = 12/13 and R1 * I^2 is 7 * .852 = 5.96 watts.

Resistance increases as wire guage thickness decreases, so thinner wires have higher resistances. So for the same length of nichrome, a thinner piece will have a higher resistance.

Maximizing power at R1 may not always be the best strategy though for three reasons that I can think of:

1) As Carl pointed out as the resistance goes down the total current needed by the system goes up. Thicker wire => lower resistance => greater current flow. If your battery can't deliver the needed current, it's voltage drops. So lower current capacity systems may need thinner wire than the optimum to reduce the current draw to something the battery can handle.

This should be simple to determine. Attach the clips of your launch system to an ohmmeter. Remove the battery and short the battery terminals together--use an extra bit of thick wire if you need to. Press the launch button and measure the resistance. This is your launch system resistance.

Now find the specifications for your battery and how much current it can deliver. A battery of cells in series delivers the current of only one cell's capacity. So four AA batteries in series (6V battery) has a current capacity of one AA cell.

Divide the voltage of your battery by its current capacity. If your launch system resistance is lower than the result, you need to add resistance to the system with the igniter. If your launch system resistance is higher than the result, then you can choose whatever igniter will maximize your performance.

2) If you deliver too much power to the wire it will melt, break, open the circuit and stop accepting power.

3) I have assumed that power is the thing we should maximize, but it ain't necessarily so. For example, if we wish to maximize the rate of temperature increase of the wire then what we really need to maximize is the power/heat capacity of the wire. Heat capacity is the temperature increase of the wire per unit energy put into the wire. So a wire with a low heat capacity will get very hot from the same amount of energy which will only slightly warm a wire with a high heat capacity.

Normally heat capacity is a function of the material used and so would be a constant because we're always talking about nichrome wire here. But in this case we would consider the heat capacity of the whole piece of nichrome wire, which depends on its length and width. So a fatter wire has a higher heat capacity, and a thinner wire has a lower heat capacity. And a fatter wire heats slower and a thinner wire heats faster for the same amount of power.

But our power also varies with the thickness of the wire.

And at that point it gets so complicated I doubt that anyone is still reading. :-) Really, for various reasons, as long as we don't melt the wire too soon, I think we want to maximize the power delivery.

[tbzep]

Jeff's evidently got some time on his hands, pulling up dead threads and doing simple physics. If I didn't know you were from Austin, I'd think you were snowed in somewhere. :p

[Jeff Walther]

Jeff's evidently got some time on his hands, pulling up dead threads and doing simple physics. If I didn't know you were from Austin, I'd think you were snowed in somewhere. :p

<chuckle> I was actually searching through old threads for the one about how much it would cost to set up a BP engine making business. I don't have the cash or ambition for it, but I just read David Sleeter's book and was curious. Anyway, while I was hunting for that old thread I found these two old ones...

Interestingly...actually, if I'm going to discuss Sleeter's book I should probably start a different thread.

Oh, as to being snowed in, as you say, no. But I am sort of between projects at work. I got pulled off one to work on an urgent-ish one. But I've done the background research and can't start the actual work until the software guys finish developing this other thing. So I'm at a bit of a stand still.

I should probably just go back to the first project, but I was afraid it would look like I was ignoring them if I was seen working on the first one. Although, thinking about it, it would probably look worse if I'm seen just posting to YORF...

[tbzep]

<chuckle> I was actually searching through old threads for the one about how much it would cost to set up a BP engine making business. I don't have the cash or ambition for it, but I just read David Sleeter's book and was curious. Anyway, while I was hunting for that old thread I found these two old ones...

Interestingly...actually, if I'm going to discuss Sleeter's book I should probably start a different thread.

I think I remember either Carl, Bill Stine, or Gary Rosenfield making a fairly extensive post about starting up a BP business here on YORF. Several others posted with guesses/estimates, but one of these guys pretty much laid it out and made it obvious why there's only one USA BP motor/engine :p manufacturer now.

[Mark II]

Words of wisdom that should be carried in the heart of every sport rocketry enthusiast! :D

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Actually, I just barely caught a glimpse of that message, barely discernible way up there in the sky, as it sailed over my head. :eek:

What's an ohmmeter? :confused:

Sorry - I'm E.E.-impaired (actually, more like deprived, but that's another story...).

Seriously, you completely lost me after the words "do the following:". :(

Mark \\.

[tbzep]

Words of wisdom that should be carried in the heart of every sport rocketry enthusiast! :D

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Actually, I just barely caught a glimpse of that message, barely discernible way up there in the sky, as it sailed over my head. :eek:

What's an ohmmeter? :confused:

Sorry - I'm E.E.-impaired (actually, more like deprived, but that's another story...).

Seriously, you completely lost me after the words "do the following:". :(

Mark \\.

It measures resistance in ohms. If you look at a multimeter, it's the setting with a little horseshoe looking greek letter, Omega.

[Rocketflite]

Just be aware that, this small a diameter wire will not yield much thermal energy. The pyrogen your using must be very sensative in order to reliably ignite. We sell such a pyrogen called MF-KIT. It incorperates magnesium metal so that it burns very hot.

Greg Dyben
Rocketflite

[micromeister]

AGW 40 Nichrome wire can be used for Bridge wire but is WAY to thin for use as a single wire igniter. it's very thin stuff, personally I've only used AGW 38ga and that was enough of a pain to only use it as a bridge wire on Larger motor D12 cluster iginters.


Generally 34 to 30 gauge is about right for plain nichrome igniters. the folded micro igniters in the pic are 30ga:)
Hope this helps