Quote Originally Posted by kd7eir View Post
He is bypassing the FET on the RAMPS board with the MOSFET he ordered. The silicone heating pad is 200 watts, and the MOSFET he ordered is rated at 25 amps, so that is fine. The NTC3950 thermistor that is shipped with the 200 watt silicone heater is a type 11. The five volt control signal from the RAMPS connects to the MOSFET he ordered, the 12 volts for the silicone heater connects straight to the MOSFET, as does the silicone heater, so he is only using the 5 volt control voltage from the RAMPS. This is actually a very nice setup - similar to my setup except I am using an SSR.
Incidently... I killed my RAMPS board on my Folger Tech i3-2020 printer. I'm not sure how it happened, but I might have shorted something on the Bed Heater circuit. I had a previously killed RAMPS board from another printer in my spare parts pile. I de-soldered two of the MOS-FET's from the other RAMPS board and transplanted them on the freshly killed board. I put one MOS-FET where it normally goes. And then the other MOS-FET got soldered on the bottom of the board to the same pads. So now I have two MOS-FET's switching the bed current.

I know some people are cringing. But I did look at the current / voltage curves for those MOS-FET's. And when the bed is heating up, I can feel each of the doubled up MOS-FET's getting warm. But they are much less warm than they used to get. Just doubling up the MOS-FET's does seem to work and not stress them as hard.