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@@ -5830,6 +5830,58 @@ NAR #88757, TRA #12200
+
+ Igniter Current
+
+ The question "how much igniter current can Altus Metrum products
+ handle?" comes up fairly frequently. The short answer is "more than
+ you're likely to need", the remainder of this appendix provides a
+ longer answer.
+
+
+ Current Products
+
+ The FET switches we're using on all of our current products that
+ have pyro channels are the Vishay Siliconix Si7232DN. These parts
+ have exceptionally low Rds(on) values, better than 0.02 ohms! That
+ means they aren't making a lot of heat... and the limit on current
+ is "package limited", meaning it's all about how much you can heat
+ the die before something breaks.
+
+
+ Cutting to the chase, the Si7232DN specs are 25 amps *continuous* at
+ 20V at a temperature of 25C. In pulsed mode, they're rated for 40A.
+ However, those specs are a little mis-leading because it really is
+ all about the heat generated... you can get something like 85A
+ through one briefly. Note that a typical commercial e-match only
+ needed about 13 microseconds to fire in tests on my bench a couple
+ years ago!
+
+
+ So a great plan is to use something like an e-match as the initiator
+ and build up pyrogen(s) as required to actually light what you're
+ trying to light... But if you want to use a high-current igniter,
+ we can probably handle it!
+
+
+
+ Version 1 Products
+
+ The FET switches used on TeleMetrum v1 and TeleMini v1 products
+ were Fairchild FDS9926A. The Rds(on) values under our operating
+ conditions are on the order of 0.04 ohms. These parts were rated
+ for a continuous current-carrying capacity of 6.5A, and a pulsed
+ current capacity of 20A.
+
+
+ As with the more modern parts, the real limit is based on the heat
+ generated in the part during the firing interval. So, while the
+ specs on these parts aren't as good as the ones we use on current
+ products, they were still great, and we never had a complaint about
+ current carrying capacity with any of our v1 boards.
+
+
+
Release Notes