@Waitaki, heated bed MK2a, which I had once, have the same problem...
At that time when I measure the resistance between the two lines it is 0.02 ohm, then I was advised to remove the copper layer by reprap members via IRC, and so I milled the backside copper and checked the resistance between the two lines, it is measured as 1.2 ohm, then I connected the power supply it worked fine :)
"The problem is just when the board is connected to the ramps. I did today connect the heat bed to the psu by its self and it heated up fast and seemed to be fine. I did not leave it on there for a long time though as I could not control the temp. "
I suspect Here he is getting 0.9 ohm it should be working fine, it may be due to some other faults (may be the fuse, Mosfet, wires?)!
To solve this use Double wires so current will be separated and the wires will get less hot, and more current you get
Mosfet, you need to heat sink it! (normal TO-220 heatsink will be available in electronic store, you can attach the heatsink upside down)
Attach a cooling fan to cool down the Mosfet and stepper drivers
if it is okay with you, use a Glass fuse, with glass fuse holder by bypassing the yellow fuse!
At that time when I measure the resistance between the two lines it is 0.02 ohm, then I was advised to remove the copper layer by reprap members via IRC, and so I milled the backside copper and checked the resistance between the two lines, it is measured as 1.2 ohm, then I connected the power supply it worked fine :)
"The problem is just when the board is connected to the ramps. I did today connect the heat bed to the psu by its self and it heated up fast and seemed to be fine. I did not leave it on there for a long time though as I could not control the temp. "
I suspect Here he is getting 0.9 ohm it should be working fine, it may be due to some other faults (may be the fuse, Mosfet, wires?)!
To solve this use Double wires so current will be separated and the wires will get less hot, and more current you get
Mosfet, you need to heat sink it! (normal TO-220 heatsink will be available in electronic store, you can attach the heatsink upside down)
Attach a cooling fan to cool down the Mosfet and stepper drivers
if it is okay with you, use a Glass fuse, with glass fuse holder by bypassing the yellow fuse!