Comments: How to Build a Remote Kill Switch
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You might want to put a flyback diode on the main relay; when I built the circuit I had a lot of problems with the input pins on my microcontroller getting flipped by the switching noise from the relay. Moving the microcontroller further away from the relay also helps.
The rating on the main relay there seems a little low (28VDC for a 48V supply, and that's before talking about back-EMF from the motor...). You may want to try testing its ability to break the motor's operating current safely - a safety system that welds its contacts closed at the worst possible moment is no good to anyone.
It is indeed a 24V (contacts rated to 28V) relay but after significant testing it's done the job well. Worked great when our motor controller failed unsafe (100% throttle). That was exciting. Luckily, Alicia was watching and noticed something was wrong so hit the remote kill and I was able to brake to a stop.
I had a hard time finding a solid, reasonably priced, readily available 48V relay. Please feel free to post other options/solutions.