I took a different if less elegant approach to get the fans to start every time. I use a relay for each fan with the common side to the fan, n.o. side to a pot to adjust the slow speed voltage and the n.c. side to 12V directly. The relay is triggered by a make-on-delay timer that energizes the relay 2 seconds after power up. ( the timer is adjustable but 2 seconds is enough to be sure they are spinning ) So on start the thing whirs to life in a huge rush of air, then CLICK! and it quiets down. Sounds kinda cool... it may be crude but its been in service that way for about a year with no problems. If the relay ever fails to fire, the fans will just stay on 12V.
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