So this just happened.
While busy with one of the initial prints I noticed the heatbed temp reading dropped down to ambient. The print carried on and completed but the neg connection on the ramps melted. The mosfet was also black so must have got a lot of heat through it.
The heatbed is 300x200mm at 140w 12v or 24v but wired as 12v and running of the ramps.
My plan is to replace the Mosfet and the terminal connection from the other ramps that is now spare parts.
I will also try source a relay to run direct off the power supply.
Does anyone have any thoughts or ideas what is going on with this???