17 replies to this topic

[Chyros]

Hi guys,

I've heard magnesium powder ignites when a current passes through it. Now that sounds like a fun experiment. So, I'm trying to make an ignition device for it. I kinda suck at physics though, so I need your help.

I realise this is not an everyday request for you guys, but any help would be appreciated. I'm trying to make a very simple e-match device which I can use to get a current to run through the powder. So far, I've bought a long electrical cable with a lamp on/off switch in between, which has a plug so I can plug it right into a socket at home. I thought that fastening the cables onto a small wooden board, with the wire ends stripped and spaced apart, and piling a small amount of magnesium between the wires would make a good firing mechanism.

I'm afraid I might short circuit something though. So, I thought, maybe I should a resistance in the circuit, to reduce the current flowing, or replace the almost-too-simplistic switch with a pulse swith that would minimize firing time.

Anybody have any suggestions?

Edited by Chyros, 31 July 2006 - 05:15 PM.

[Arthur Brown]

mains live wires and pyro together seem too close to ordnance disposal by trial and error. DON'T do it.

A UK standard ignitor passes 20ma for testing circuit continuity without firing, and takes 500ma for a certain fire, Most UK firing controls run off batteries and some have a capacitor discharge cct to put out a pulse of about 30 - 36 volts, -- sufficient to fire 25+ igniters in series, or use long wires for fewer igniters.

[Anders Greenman]

Regarding the short circuit, a cheap fuse wich is easy to replace should do the trick. They can be bought for a good price at any hardware store carrying automotive stuff. You can try to mix the Mg with some NC laq. and paste it between the two cables so that it makes a "conductive bridge" wich (if mg has the right resistance) will heat up and hopefully catch fire. This is very similar to a graphite type igniter. I prefer a rechargeable lead-battery instead of the socket (less danger).

Mg resistance: (20 ?C) 43.9 nΩ?m This one may be possible.

Edited by Anders Greenman, 30 July 2006 - 10:07 PM.

[Chyros]

OK, so a battery might do? Would it suffice to take a 9V battery and just connect wires to the powder (with a switch in between, naturally)? Or do I need two batteries? Or some kind of transformer? I can make some simple electrical circuits (learned at least THAT much at school ), but I don't know much about them anymore.

[karlfoxman]

Hi guys,

I've heard magnesium powder ignites when a current passes through it. Now that sounds like a fun experiment. So, I'm trying to make a detonator for it. I kinda suck at physics though, so I need your help.

I realise this is not an everyday request for you guys, but any help would be appreciated. I'm trying to make a very simple detonator device which I can use to get a current to run through the powder. So far, I've bought a long electrical cable with a lamp on/off switch in between, which has a plug so I can plug it right into a socket at home. I thought that fastening the cables onto a small wooden board, with the wire ends stripped and spaced apart, and piling a small amount of magnesium between the wires would make a good firing mechanism.

I'm afraid I might short circuit something though. So, I thought, maybe I should a resistance in the circuit, to reduce the current flowing, or replace the almost-too-simplistic switch with a pulse swith that would minimize firing time.

Anybody have any suggestions?

Are you planning on using mains??! That is a very foolish thing to do, if your wanting to make electrical ignitors then I must stress to do a search on this forum for the correct ways on doing it. Using mains will most likely lead to disaster! Also the term 'detonator' is used in HE, it creates a detonation wave. Please use the correct terms, you are trying to make electrical ignitors or e-matches.

[pyromaniac303]

I know this is going back to basics a bit for some of you but hopefully it will help understanding what sort of power you are dealing with, and why you should not use high voltages from continous sources such as mains.

A 9V battery has a very limited current for the sort of drain you will put on it. A standard 9V (assuming your on about the small rectangular 'PP3' type) contains about 150mAh. A standard car battery contains approximately 150Ah, which is 1000x the amount of current in the smaller one, also because a car battery is 12V it has a greater chance of being able to ignite the mixture.

mAh and Ah are milliamps per hour and amps per hour, which means that it will take an hour to go flat if you take that much current from it. Due to the huge amount of power stored in a car battery or other large lead acid battery, I would suggest you use a 5A fuse in the circuit so that your wires don't melt if you get a short somewhere.

When you connect the circuit you are almost getting a short, the resistance of the magnesium is very low, so use some thick wires. If you have a resistance meter you can test this (if you dont already have one they are only about ?5-?10 for a cheap one), the formula to work out how much current you get is

Voltage / Resistance = Current

So if you have a 1 ohm circuit then from your car battery you will take 12A, but if you were using mains you would be taking a very dangerous 240A! This is why it is not a good idea to use mains as there is not much that would withstand that sort of current, and as your circuit is going to be probably in the 1 to 20 ohms region its going to take quite a lot, even if it is just for a split second while the ignitor goes off.

The reason the magnesium would ignite is because the power from the battery causes it to heat up rapidly, which means there needs to be quite a lot of power used before the magnesium reaches ignition point. Using thinner pieces of magnesium would reduce the heat taken for ignition therefore you will not need as much power before it. Instead of using a large pile of magnesium powder, use a very thin trail and it should heat up faster. Also you might want to try using a piece of magnesium ribbon cut in half, like the bridge wire method, though do not try to solder to magnesium as it will just oxidise the surface and make a mess of it.

Good luck with it anyway.

[Anders Greenman]

Since mains have a great hazard, I reccomend using a small motorcycle, or something similar, battery with a capacity of about 2-10 Ah. My 12V rechargeable lead-battery with 4.5 Ah capacity works great. It can deliver enough amps for the most stubborn igniters and it doesn't cost that much either.
I payed about 14? for the battery and a 500mAh charger.
Here is the link (Norwegian, stick to the pictures ).
And this is the charger.

[Chyros]

Thanks guys, I'll use a lead battery then. How much current would it take to ignite a very small pile or line (I'm able to work out the resistance of the circuit myself if I know how much)?

Edited by Chyros, 31 July 2006 - 05:14 PM.

[portfire]

hope this link will help Chyros, look down the right hand collum it give's you all mg's properties including Electrical resistivity.and as karlfoxman said, 'detonator' is used in HE, it creates a detonation wave. Please use the correct terms, you are trying to make electrical ignitors or e-matches'

http://en.wikipedia.org/wiki/Magnesium

Edited by portfire, 31 July 2006 - 11:23 AM.

[Chyros]

OK, that's quite a low resistance. The page doesn't list at what current it ignites though, so would putting in a resistance of about 4 ohm (plus the resistance of the wires) do? I'd say at that current the wires wouldn't be put at that big a stress...

I also corrected the word "detonator" in my previous posts .

Edited by Chyros, 31 July 2006 - 05:23 PM.

[alany]

Magnesium is just a metal, it won't magically ignite from a current passed through it unless the current heats it to the point it will react with whatever environment it is in. Magnesium in air actually needs to be pretty damn hot before it ignites.

Of course the Wikipedia page won't list an "ignition current" because there are so many variables as to make such a "property" of a metal completely useless. Even ignition temperature in air of the bulk metal is highly variable and depends on its surface condition.

[Chyros]

Magnesium is just a metal, it won't magically ignite from a current passed through it unless the current heats it to the point it will react with whatever environment it is in.

Well I never thought it would, I just don't know enough about physics to know at what current it will ignite. I'll make a circuit in which I can put a variable resistance, then. I'll try a few things out. Thanks!

[adamw]

I'm shuddering for so many reasons....

[Chyros]

I'm shuddering it won't set the powder off... Anyways I'm going on a holiday to good ole London tomorrow; when I come back I'll see how it works and report back.

[shadowpyro]

I'd quite like to know whether if this works, the magnesium would be perfect for high ignition temperature compositions. e.g thermite

Adam, lol why are you shuddering for so many reasons?