Well for now I came up with a solution for my 12x12 heated bed issues. For those that weren’t in the thread my RAMPS board couldn’t deal with the amperage draw and was shutting down so I tried to use a solid state relay to by pass.
After one burning up and the next coming locked in the closed position I’ve given up on them until I can build one of the MOSFET circuits I’ve been given diagrams of (thanks guys!)
I remembered I was able to use this bed with a RAMPS board from Ultimachine and the problems showed up with I switched to a board from RepRapDiscount. I swapped the board from my old Prusa build that was from Ultimachine and BOOM bed works.
Lesson learned cheap electronics from China only save money. Time and effort just end up being wasted.
WRT to poly fuse replacement, I’ve seen folk using replaceable car fuses. That way you can spec the fuse for your application.
Replacing mosfet is a little more tricky as the pcb will be designed for a particular heat disipation threshold, a beefier mosfet may get hot enough to exceed that threshold. Heatsinks and fans could alleviate this but theres a risk associated to these cooling methods failing. Personally, I’d create a standalone sheild, that way you can add the freewheel diode, which the ramps board doesn’t (but should) have.