The pad side safe/arm switch is useful, but is not as good as it could be. The schematic doesn't include this switch but it appears to switch +12V to v_pyro. Which means that when this switch is closed, many single point failure modes come into play. If a MOSFET has failed that output will fire. If the driver to a MOSFET has failed it will fire. Low probability events but the consequences could be serious. If you are lucky the motor doesn't CATO and you just have to change your armor.
Use a relay instead to provide v_pyro. The relay is controlled by the system and closed when an arm command is received. This way everyone is at a safe distance when those single point failure modes come into play. The single pad side safe/arm switch is placed in series with the relay coil so it can be disabled. In series with the relay output is another option. The alarm outputs are then not controlled by a GPIO output but by the presence of 12V on the output of this relay. You get a warning when the system arms or when the relay fails. You will need another source (3.3V) of power to drive the continuity check. A diode and a resistor does the job. (A PFET with its source to v_pyro and gate to 3.3V provides the alarm output.) A 40A automotive relay should be adequate here since it would not be switched under load. It is better than the rocker switch. (Relays plus a socket can be purchased for $5 a set at All Electronics.) The DARS launch controllers have used this idea since 2004. No relays have died yet. http://home.earthlink.net/~schultdw/controller/controller.html The Earthlink servers have been down for over a week now so you will probably have to use the Wayback Machine version: https://web.archive.org/web/20180426172954/http://home.earthlink.net/~schultdw/controller/controller.html The gate drive (3.3V) to the output FETs is a little weak. I like the MIC5018 driver in situations like this to make sure that they are fully enhanced. Not so much for extra output current but to reduce power dissipation. -- http://home.earthlink.net/~david.schultz The cheaper the crook, the gaudier the patter. - Sam Spade _______________________________________________ altusmetrum mailing list [email protected] http://lists.gag.com/mailman/listinfo/altusmetrum
