The attached is a schematic for the add on MOSFET upgrade to an Anet A8 3D printer.
The hot bed connector on the main board can melt, so this board bypasses the board by using the signal and powering the hot bed directly from the AC/DC supply.
It seems this board is overly engineered. Although it shows AC on the input, the input voltage is DC (even if it were AC, the circuit doesn't have a filtering capacitor). Also, why use an opto since the entire printer is referenced to the same ground?
I'm guessing the bridge rectifier eliminates wiring the DC input backwards (it also wastes voltage/power), but why an opto?