By Joe Zastrow
03/25/2006

Purpose

To design a nichrome wire wrapped igniter to be used for electric fired fireworks shows.

Method A

After a trying a number of different methods and experiments, I came up with an effective igniter. I will describe how to make this one in great detail. At the end of this document, I will describe some of the other methods I experimented with.

Procedure:

Cut a 5” piece of the two conductor copper wire and strip all four ends. Strip about Ό” of the insulation from the two wires on one end and about ½” of the insulation from the other two ends.
Now take a 3”-4” piece of nichrome wire and stick it in the side slot of the wire wrapping tool. Push most of the nichrome wire through the side slot, leaving about Ύ” of nichrome wire sticking out of the side slot. Some wire wrapping tools have a small mark or slot that indicates how much wire should be left sticking out the side slot. Then place the tool on one of the ends of the copper wire that has Ό” of the insulation stripped off of it.  There is a center hole that copper wire slides into.
With your thumb, gently hold the nichrome in place and turn the tool clockwise until all the nichrome wire goes into the side slot.  Gently remove the tool from the copper wire.
Use the needle nose pliers to fold and crimp the end of the copper wire over the wrapped nichrome so it is secure. Cut a 2.25” piece of fuse with the knife. Place the fuse on top of the wrapped end and hold both between your finger and thumb. Take the nichrome wire and wrap it over the fuse and then under in a tight counter clockwise spiral. Wrap it 2-4 full turns. Give a tight pull at each turn. Bring the nichrome wire up behind the fuse and align the other end of copper wire next to it.  The distance between the two ends should be about ½”-Ύ”.
Now stick the nichrome wire into the half-moon shaped hole at the end of the tool and out through the side slot.
Slide the copper wire into the center hole at the end of the tool. Pull the nichrome wire firmly. Turn the tool clockwise until the nichrome wire disappears into the slot and it is all wrapped around the wire. Gently remove the tool from the nichrome wire. Use the needle nose pliers to fold and crimp copper wire over the wrapped nichrome wire.
Now cut a piece of tape (about 1.25”) that is long enough to cover the two ends of the copper wrapped wire. Place the fuse on the sticky side of the tape. It should go lengthwise in the very center of the tape. Line it up so there will be about 1.5” of exposed fuse and it covers both ends of wrapped wire. Fold the tape evenly over the fuse and the ends of wire. Press sticky sides the tape firmly together. The igniter is now finished. It should be stored in an air and water tight container until is ready to use. Then the exposed piece of fuse can be attached to the fuse of a firework and a shooting wire can be attached to the other end of the igniter.

Options & Details

In the supplies section, I said four different kinds of tape (cellophane, masking, duct and black electrical tape) could be used. There are advantages and disadvantages to each. The cellophane tape is water resistant and usually leaves very little residue when the igniter burns. But it is not very spark resistant.  Stray sparks could cause premature ignition during a show. Also it is not as rigid and secure as the other tapes. If the igniter gets mishandled it may come apart or the nichrome wire may break. Masking tape is less water resistant, but more spark resistant. It is also more rigid. Though after ignition, the masking tape tends to burn or glow longer than any other tape. A small persistent flame may ignite the side of a firework. This does not occur with cellophane tape. Duct tape is both water and spark resistant and more rigid than masking tape. Though, it is possible for it to burn after ignition, it is much less likely than masking tape. It leaves a grey blob of gunk after ignition. Black electrical tape is more water resistant, spark resistant, rigid than duct tape. Good quality electrical tape stays stuck together better in colder or hotter weather than any other tape. It rarely burns, but leaves a black tube or blob of gunk. It is usually the most expensive. The total cost of the materials for an igniter is about $0.06.

Also in the supplies section, I said 31-38 gage could be used. 38 gage wire needs the least amount of amperage to heat up and break (0.53-1.09 amps). The low end of this range is close to what is required for an e-match. But it can be tricky to work with because it is so thin. The wire is more likely to break or tangle during construction of the igniter. Unless you have very skillful fingers you will probably have more defective igniters. If you measure a 38 gage igniter with an ohmmeter, it should have 4 ohms or less resistance. 31 or 32 gage wire is thicker and easier to use, and less likely to break during construction of the igniter. It requires more amps to heat up and break (2-3.2 amps or more).  If you measure a 31-32 gage igniter with an ohmmeter, it should have 2 ohms or less of resistance. The 35-36 gage wire is a compromise. It requires 0.75-1.48 amps to heat up and break. If measured with an ohmmeter, it should have 3 or less ohms of resistance.

Important Notes About Connecting the Igniter

The fuse portion of the igniter needs to be firmly attached to the fuse of the firework. Either black electrical tape, duct tape or zip ties works well for this. Both the fuse portion and the copper wire of the igniter should be taped securely to the firework or some other support. More tape should NOT be put over the nichrome wire. Obviously, this will prevent the igniter from becoming accidentally detached. But there are some other important issues as well. If the fuse portion is not attached, the fuse will immediately be propelled away from the nichrome wire when the igniter is fired. In one test, the igniter fuse flew off with so much force that it pulled the fuse out of the firework. Also the fuse is propelled away so quickly that it either straightens out the nichrome spiral or it unwraps more nichrome wire off the ends of the copper wire. In either case, this increases the amperage to break the wire or increases resistance of the wire. It greatly increases the chances that the nichrome wire will not break. Also, since the fuse flies away, there is less heat generated by the fuse to melt or break the nichrome wire. So the firing panel will indicate that the igniter has not fired. Holding down the fire button again or longer might break the wire, but it can also cause the tape around the nichrome wire to ignite into a small torch. This happened in most of the tests where the fuse portion of the igniter was not secured. If the copper wire is not secured, it can be propelled away from the fuse. The nichrome wire may not break and the tape might burn in this situation as well.

Time

I have been able to make about one igniter in 3-4 minutes. Some people can probably do this much faster than me. Creating an assembly line could decrease this time. Get a piece of wood and use it as a template to quickly cut many pieces of the copper wire to the same length. Do the same for the nichrome wire and the fuse. You might even precut the tape. Once you have many pieces, construct many igniters.

Method B

This method was based on a wire wrapped rocket igniter that I found on a web page. You use copper wire as in the above example, but you cut one end ½”- Ύ” short. Strip insulation off these two ends. Use the wire wrapping tool to wrap nichrome wire around the shorter end, crimp the end of the nichrome wire as before. For this method and methods C & D, make sure you do not crimp the wire over or near the other wire. Otherwise, the wires might short out if the insulation burns up. Then spiral the nichrome wire a few times around the longer copper wire. Finally, use the wire wrap tool to wrap wire around the end of the copper wire. Crimp the end over the wrapped nichrome. This igniter can be made very quickly. The igniter is then attached to the fuse or the extended fuse of a firework.

Method C

This method was similar to method B. But instead of just spiraling nichrome wire around the copper wire, the nichrome wire was spiraled around a piece of fuse and the copper wire. This was then wrapped with tape (cellophane, masking, duct or black electrical).  Here is an example, without tape, so you can see how it was constructed.

Method D

This method was similar to the method C. But the nichrome wire was spiraled only around the fuse. It was covered with tape  (cellophane, masking, duct or black electrical). Here is an example without the tape.

The only real difference between method A and method D is that the tape goes around the fuse and one of the wires in method D, but in method A it goes around the fuse and only the ends of the copper wire.

Another variation of each of the above methods was to wrap some black electrical tape around the copper wire before the nichrome wire is spiraled around or near it. This would add some extra insulation to the copper wire. It would also take longer to make. 

Methods B-D had the advantage of being very sturdy units. They were also much smaller packages than any other method. The one problem with these methods is that there was a higher possibility of the insulation on the copper wire burning up. Even if the nichrome wire would break, another portion of the nichrome wire would then touch the exposed copper wire. The firing panel would report that the igniter had not fired. If the fire button was pushed again, the nichrome might break again in another spot, but then it would touch the copper wire in another spot. Every once in a while continuity was never broken. Sometimes the insulation would burn up where the two copper wires where touching, which would also cause a dead short. Other times, the tape would heat up and ignite into a small torch. That small torch might ignite the side of the firework.

Method B had the worst problems because the nichrome was entirely wrapped around the copper wire. Also, more heat would be on the copper wire than the fuse next to it. Method C was an improvement as the nichrome wire was around both the wire and the fuse. Method D was an improvement on method C because the nichrome was just around the fuse. But because the nichrome wire is not wrapped around the copper wire, it is not quite as sturdy as method C. Also, method D usually takes a little longer to make than method C. I think method A is probably the best, but it is probably the slowest to make. Methods C and D are acceptable because problems occur infrequently. I would not recommend method B for use for fireworks in a display. As a rocket igniter is probably okay. Whether the nichrome wire breaks or not is not an issue, because the rockets are fired one at a time and it will be obvious if it is successfully ignited or not.