Rocketry Planet

Making reliable low current igniters isn't as hard as you might think. With a little planning, a couple of calculations and the right components, you can make igniters to fire your air-starts. The following is how I make a very reliable "low current" air-start igniter.

Links have been provided to the products and manufacturers that were used for these igniters. Typically a signed liability release form is required to purchase some of the products listed. The e-match and igniter pyrogen kits from QuickBurst are very low cost and will support making dozens of e-matches and igniters from the pyrogens included. You will need to provide additional bridge chips and shooter wire.

I highly recommend that you make and ground test several igniters until you are also confident with the reliability and performance. When necessary, you will probably want to modify the construction method to accommodate larger motors.

1. Wire: I use a 20ga solid core two strand wire. Pictured is a section of "shooters" wire with insulation removed from the end and a second section of insulation removed about 1-1/4" from the end. Also in the picture are some of the e-match heads that solder to the igniter.

Additional bridge chips and shooters' wire (pictured above) are available from the following sources:

• http://www.oda-ent.com/
• http://www.skylighter.com/mall/ignition.asp
• http://www.aeroconsystems.com/electronics/nichrome.htm
• (There are probably many other sources)

2. E-Match Heads: I use pre-made e-match heads that have a very small (50ga) bridge wire. You can see the bridge wire on the small end of the head. For some smaller motors (29mm–38mm), these e-match heads are often larger than what I would like to have passing thru the nozzle. So I trim the chip shorter and narrower if possible using small diagonal cutters.

3. Resistance Testing: After trimming the e-match head, measure and record the resistance of each chip. I use two e-match heads for each igniter — the heads are connected in parallel. It is not necessary that each e-match head have the same resistance, however knowing the resistance of each chip will ensure a consistent check after the igniter is fully assembled.

4. Solder Match Heads: I first solder the end e-match head, then gently fold the wire and solder a second e-match head to the folded exposed wire. This may take some practice, I often use a clothes-pin to clamp the wire and e-match head together while soldering.

5. Second Resistance Testing: The igniter assembly is wired in parallel and the expected resistance will drop. Using the formula for parallel resistance and the recorded values from previously testing the bridge chips, we can determine an expected resistance value for the igniter.

Using e-match head #1 restistance (R1 = 0.90ohms) and e-match head #2 restistance (R2 = 0.85ohms):

Then the total resistance should be 0.44 ohms for this igniter. Record the total resistance for later use.

NOTE: If the actual resistance is not close to the expected value determined by these calculations, do not use this igniter as it may very likely fail.

6. Pyrogen Dip: The e-match heads are dipped into a low current pyrogen — be sure to fully coat the end of the head and bridge wire per the manufacturers instructions. I use the Quickburst e-match kit pyrogen dip. This product is available on the Quickburst website at http://www.quickburst.net/e_matchkit.htm.

NOTE: I do not coat the pyrogen with NC Lacquer or other sealers. Additionally, let the pyrogen fully dry before proceeding.

7. Wire Wrap: Often I find the wire will not stay in a smooth folded position, so I use a very fine wire and wrap the end to hold the sections together. The wire is trimmed flush with the insulation. I typically use a single strand of wire found in a multi-stranded wire or lamp cord.

8. Igniter Dip: The igniter is now ready for the outer pyrogen. Dip the igniter into a pyrogen used for igniters — it is important to fully coat both ends of the e-match heads and the dark low-current pyrogen. Let dry completely before use or testing.

I use the QuickBurst Quick Dip igniter pyrogen for the igniter outer pyrogen dip. This product is available at http://www.quickburst.net/quick_dip.htm

NOTE: I have also used other igniter pyrogens with success.

9. Ground Testing: I connect the leads to an extended wire (about 12') and ground test using a single AA 1.5v battery. The e-match bridge chips should fire simultaneously and burn the igniter from both ends. I suggest that you make several igniters and know that you have consistent results. Then ground test several more igniters with the flight controller or timer that you intend to fly.