Bridge Wire:  High-Current Ignitor Construction

The "exploding bridge wire" is commonly used as the core of an ignitor.  Electric current is passed through a thin wire, making it get really hot.  Thus it ignites whatever pyrogen is placed next to it.

The following bridge wires can be fired with a 12 volt car battery, such as is commonly used at rocket-club launches. 
At home I use 110volt AC house current through long drop cords, lending credence to the term "exploding bridge wire."  They burst with enthusiasm.

These are for ground-based ignition only, where lots of current is available.
They will NOT fire from a 9-volt battery, so DO NOT attempt to use them with your altimeter or staging device!

Materials and equipment:
Well, I don't really need all of this stuff but I am a junk freak, and most of it will be used before this project is finised.
The bridge wire is the interior of a lamp cord.  Stripping the insulation from 6 inches or so to reveals 40 or 50 very thin strands.  These measure .006 inch on my caliper.  Separate one strand from the bundle and draw it through the superfine sandpaper until it gleams.  This will make soldering much easier.  Sand gently, especially if using the "coarser" 320 grit sandpaper, as these strands break very easily.  600 grit is ideal, if you have it.

The computer geeks at my workplace are always throwing out sections of network cable.  I lurk around the corner, sneak up to the trash pile, grab a cable, wrestle it to the ground and subdue it with a knot.  Whew!

A sharp knife is used to slit the covering two or three inches down, revealing four pairs of wires, neatly twisted together.  Hence the name "twisted pair" cable.  They are nicely color-coded, and are ideal for making these ignitors.
Then I pull and pull to get more of the covering off.  This is a lot of work, partly because there isn't much to grip, and the covering tends to break if you go at it too hard.
Once a foot or two is stripped, I tie the covering to something sturdy and use both hands.  This makes it easier to apply just the right pull.
Here I have cut 2-foot sections for 8 potential ignitors.  I like to start with long wires.  They can be re-used by cutting off any damaged sections after use.  Long, virgin ignitors are good for launches.  Short, experienced ones are fine for static tests and ignitor experiments.  This saves a bit of work.
A knot is tied 4 to 6 inches from one end.  The wires are untwisted up to the knot, knot keeps them from separating any further.  About 1 inch of insulation is stripped from each wire.  Sometimes I wonder if I should wait on this, for safety purposes to prevent accidental ignition, and to keep the wire clean until launch time.  Then I get paranoid about coming up to the launch stand without a wire-stripper, so I go ahead an strip them.

Now to the other end, where the bridge wire will go.  The wires are untwisted about 2 inches, and one of them is cut 1/2 inch shorter than the other.  About 1/8 inch of insulation is stripped from the end of each wire.  If a stripped end is not shiny, sand it or scrape it clean!
The fine bridge wire is wrapped 3 or 4 times around the stripped end of the short wire.  Apologies for the last picture, the wire had slipped partly off.  I corrected this error before soldering, but forgot to get a photo.
These thin wires are pretty squirrely to handle, so I find it better to clamp down the soldering iron and bring the wires to it.

Tug gently on the bridge wire after soldering.  If it slips off, it didn't get soldered.  Do it again.
Now twist the wires back together, at least as tightly as they were before.  This is to reduce the strain on the thin bridge wire.  Wrap the bridge wire strand around the longer wire 4 or 5 times on the insulation, then 3 or 4 times on the stripped end.  Solder the end.
Now I like to trim the loose ends, usually cutting off a bit of the network wire at the far end, and getting rid of any frills at the shorter end.  Isn't it pretty!
Not a bad idea to test these with the voltmeter for continuity.  But I usually don't.  They invariably work at this point.  More important is to test one just before an important firing, to be sure the slings and arrows of time and travel have not broken the bridge or compromised a solder-joint.
Longer ignitors are wrapped around three fingers.  Shortish ones around two.  This makes it easy to find the right length on the first pick.
So here is today's production.  Took me about an hour to do this.  Some folks prefer to buy ignitors.  That is fine with me.  I prefer to make everything I can.

To be an effective ignitor, these will need some pyrogen.  That web page is coming, but the short of it is:  wrap the bridge-wire end in masking tape with 1/4 gram of black powder.  Add a few magnesium to titanium turnings for more heat.  These work very well with my candy motors.   I am testing fuse paper for use as the ignitor pyrogen.  It is exceedingly cheap and easy, and in my tests so far has worked quite well.

Question:  Why aren't you using Nichrome?  Isn't it better?

Answer:  It isn't needed when plenty of current is available.  And copper is virtually free, pretty, and solders easily. 

These ignitors have proven very reliable for me.  In several hundred tests over the last three years, I have had a few ignition failures.  All of them have occurred either because the pyrogen did not ignite the propellant, or because the power did not get delivered to the ignitor.  Not once has the bridge wire failed to ignite the pyrogen.  It's hard to imagine how nichrome could be better in that regard.

Nichrome is better is when current is limited, such as in flight.  Using very fine nichrome wire, one should be able to make ignitors like this which could fire from a small battery.  Thus they could be used with battery-powered altimeters, timers, or staging devices. 

But I have another cheap and easy trick for that...

Jimmy Yawn
rev. 4/13/05
Recrystallized Rocketry