Heatbed melted connection on Ramps


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???


This is the reason I bumped up to 24V (kind of counter-intuitive, but higher voltage means lower amperage.)
One option would be to get one of these (NOT necessarily THIS one);



Same has happened to mine. That’s one of the shortcomings of the RAMPS board that Marty has addressed with his PICA board so I’ve opted to replace it with a PICA instead of another RAMPS.

The screw terminals used on RAMPS are probably the biggest source of failure on the design (just google “RAMPS power connector melted”). They are barely rated to handle the kind of current we put through them, and it doesn’t take much to result in a meltdown.


So today I replaced the Mosfet with a higher rated one and also removed the terminals for the heatbed off the ramps replacing them with soldered on fly leads.

I will be getting a heatsink for the Mosfet and the Solid State DC Relay tomorrow.

When testing just now heating the bed up to 100 degrees C went quicker that before plus the Mosfet is hardly warm to the touch.

I am thinking it must have been a poor connection and a rubbish Mosfet.


I am using this heater controller from Reprap.me http://www.reprap.me/power-expander.html with a 500x500 heated bed @ 24v on 12v Ramps. at 60deg it doesn’t even get warm.