So I have just finished the build of my Prusa i3 running Ramps 1.4 with a 12v 20amp 240watt PSU, and when I wire in the 3 pins for the heatbed it starts getting hot and reaches about 40 degrees Celsius before cutting out because the yellow resistor/diode (not sure what it is) gets too hot. If I wire it up as a 24v using just pins 2 and 3 it will reach 72 degrees Celsius but nowhere near the 110c I need. Please help.
It looks like its resistance is too low, so it is taking more than 10A and the 11A polyfuse (yellow thing) is tripping.
The easiest solution is probably to short out the polyfuse, or cool it with a fan. It is worth measure the bed's resistance first as it might be far too low. Don't forget to subtract the resistance of your meter leads.
If you are using 12v, make sure you have 2 and 3 running to earth. I just joined 2 and 3 with solder so I just had a single earth and a positive running from the board to my ramps.
Adding a fan for cooling your polyfuse may help, but make sure that all your parts are sound. I had the exact same problem, and it turned out the underside of the polyfuse had a crack running down it. Replacing the polyfuse fixed it, so maybe check that.
