Minimum All Fire Energy of MJG Technologies Firewire Initiator

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Jun 26, 2015
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This information may be of value to someone wanting to use a MJG Firewire Initiator electric match with a low voltage single cell LiPo battery. While the minimum all fire energy is published for the J-Tek igniter, this data is not available for the Firewire Initiator.

We tested a small number of the Firewire initiators. They all ignited at 15 mJ or less with a 3.7V power supply. The lowest energy we saw ignition was 7 mJ. These tests were done with a battery at 3.7V and a variable pulse width. For short pulse widths (low energy tests) we got no ignition so we can see that the threshold is somewhere near the 15 mJ. Of course I would want to test 1000 of these to know what the actual minimum firing energy is but our system should always send at least 800 mJ so we will always have a large safety margin I think.

Note that the minimum all fire current of 0.6 Amps is required based upon data found on the MJG website.
I use the MJG FireWire Initiators with my StratoLoggers (100 and CG), and a 1S 25C LiPo.

I have not had a single failure, in either bench, or ground testing but more importantly in flight.

However 3.7 volts for a 1S LiPo is somewhere around a 50% charge so my results have all been on a fully charged LiPo @ 4.2 v to 4ish v.

None the less it is good data to know, so thank you for posting this.
Measuring the all fire energy requires testing a largish number (20 or so at a bare minimum) so that you can calculate the Maximum Likelihood Estimate to get the estimated mean and variance. The all fire energy would then be the energy that would ignite 99.9%.

Determining the test levels is tricky. You could go with the simple up/down (Bruceton) test but that requires a reasonably good guess of the results before you start. I like the Neyer D-optimal method and have used it on the Estes Solar igniters and Quest Q2G2. I would be happy to assist anyone who wants to do that with the Firewire. When I sent an email to MJG asking about the all fire energy in 2014 the response strongly suggested that they didn't use a valid method of estimating the all fire current.

I suspect that you have underestimated the energy applied. I tried firing the Firewire using an RDAS Compact and failed. (Which is why I asked MJG about the all fire energy.) That uses a 2,200uF capacitor charged to 9V or about 89mJ. Not all of that would be delivered to the load but certainly far more than 15mJ. How did you determine energy?
The setup was very simple. A lipo battery charged to our lowest value we could imagine using (3.7V) and power FET with the gate connected to an Arduino and the ignitor (1 ohm) in series with those components (On resistance of the FET was less than 100 mOhm). The FET was pulsed with pulses down to 0.01ms and up to 1.06ms. 9 ignitors were used in the test. We never saw ignition less than 0.5 ms and always saw ignition above 1 ms. So if we are delivering the energy suggested by these component values then the minimum all fire energy is probably somewhere above the 15 mJ. The actual all fire energy depends on the variability as you talked about. Regardless, we are not going to be operating down to those levels so this experiment was really about making sure we were not close to the limits.
In your case, I wonder about some non-linearities that might show up with current / voltage applied to the ignitor. When I think about it, it seems like it should work.