The following circuit has been designed to drive a contactor. In addition to the primary relay the contactor has a secondary auxiliary (AUX) relay that can be used to latch the contactor on.
The circuit is operated by microcontroller that drives the gate of the MOSFET for 500 ms. This powers the contactor coil which closes both relays of the contactor. Once the AUX relay closes the MOSFET is bypassed and the contactor is latched on. A normally closed button connected in series with the AUX relay can be used to de-latch the contactor.
A failure regularly occurs where the MOSFET fails short preventing the contactor from ever being disengaged. The MOSFET has been replaced several times, so this is not an isolated issue.
What could be causing this?