Nessalco Posted August 31, 2011 Posted August 31, 2011 I'm working towards building an electronic firing system. This system will only be used by me, 2-3 times a year. It will be based on DB25 cables, and the longest cable run will be 200'. There will be no more than 4 ematches on a cue. I'm currently using a direct-wired system I hacked together. I works OK, but the resistance of the cables makes firing 2 e-matches at the end of 200' of cable 'iffy'. In my work with rocketry, we rely heavily on relay-based systems, because of the high current drain of the igniters and solenoids we use. I've designed and built several simple systems. I've read about the concerns with pyrotechnic systems based on mechanical relays, and want to make sure I'm not missing something in my considerations. Currently I'm leaning towards a relay based system, 24 cue, with a 3v firing voltage at the slats. Is it correct that the concern with mechanical relays concerns their failure modes? I only see two of immediate consequence.... 1) The contacts weld shut, causing a device to fire instantly when power is applied (even if the relay coil is not energized) 2) Impact forces from devices firing could cause the contacts to 'jump', firing another device prematurely. The relays I'm considering are dual contact, so I was going to hang an error-check circuit on one side, which would light an LED if a set of contacts was welded closed. This circuit would be energized before any firing voltage was present in the slats. The relays can withstand a 50G jolt without disrupting functionality. I'm hard pressed to see how that kind of force could be applied unless the slat was mechanically attached to the pod. Are there other issues I should consider? Thanks in advance. Kevin
Peret Posted September 1, 2011 Posted September 1, 2011 Contact welding with cradle relays is not usually a problem, except with highly inductive loads, which igniters are not. Similarly, they need a hell of a jolt to bounce closed. If it's a concern, consider solid state switching with thyristors. Apart from the increased reliability, thyristors are cheaper than relays.
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