Replicator 2 tuning: improve main PCB & motor drivers cooling

The bottom of the printer holds main PCB + 4 motor drivers. Air flow is not designed in a best way. This tunning will use a fan to improve cooling. A special electronic circuit is used to sense temperature and to drive fan in a smart way. Circuit is also limits voltage by 12v to use a regular 12v 80mm fan (from any PC store) by sourcing from 24V. Power source 24V is available on replicator main PCB (you can find marked hole to be soldered + you will need similar hole marked as GND).
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