Heated bed maxing out at 90 degrees, blowing breakers, heating up everything

I recently installed a heated bed, regular sized PCB MK II from a good American seller, and as the title says it wold get to about 90 degrees and RAMPS would stop inputting power. My PSU is rated for 20 amps, and this is only the bed by the way. I'm not trying to heat up or power anything else. The MOSFET terminals on RAMPS are very very hot as well, and when I went to feel them I burnt my finger on one of the fuses. So it's clear that they are blowing, but why? How do I fix it? My multimeter won't measure current that high.

Thanks!

what voltage is the power supply?

---

-=( blog )=- -=( thingiverse )=- -=( 3Dindustries )=- -=( Aluhotend - mostly metal hotend)=--=( Facebook )=-

I've seen this happen with overheating MOSFETs. Try sticking a tiny heatsink on the MOSFET, or put a small fan blowing towards the controller, see if that helps. Alternatively, try replacing the MOSFET with one with lower RDS on, like the IRLB8748. The RAMPS board I got from China had a cheap mosfet, which was one of the problems.

Thanks. The power supply is 12 volts.

I feel though that the MOSFET was only warm while the fuse was the thing that was very hot. I feel like too much current is going through ramps or that it's resisting it somewhere. Any other ideas?

Did you measure the resistance of your PCB heated bed? It should be around 1.1 - 1.2 ohms.

Hi, its a proven design in hardware and thermistor monitoring your temp, and the simplest item of a 3d printer to get right, position of thermistor and appropriate rated 12v supply, you may need to revise ohms law?