Finally the gate pull down and the -12V to the FET source.
Can the +12 to optocoupler collector be avoided? Because +12 has 30 A.
is it save to solder the 4n35.
Does the high temp destory the IC?
The fact that the 12V supply can provide 30A should be of no concern to you nor the transistor. The current drawn will only be in accordance with Ohm's law, and if that doesn't dictate 30A then 30A will not flow. The important thing is that the 12V supply is totally isolated from the arduino circuit.
But what is the use of the FET at this setup?
Do I need Inductors?