Mk2 Heatbed + Ramps not working

Understanding of electronics 5/10
Im using a mk2 heatbed
Ramps 1.4
Several different power supplys
Purchased
[www.ebay.com]
Mirlin
Set headbed in config to 1? just says a 100k resistor same as the epoc 100k? i set both temps to 1
Im using Pronterface

Also these big guys get really really warm is this normal?
[reprap.org]

Issue:
Mk2 heat bed heats fine up until 50c mark
It heats then dies off and cools down

Im assuming Im using a good power supply Pulled 2 out of 2 working computers to test both with same results
I have also hacked a xbox 360 power supply to supply just the volatage to 10a line while using pc power supply to power 5a line
xbox 360 power supply is rated at
203w 12v - 16.5A or another at 150 watt 12v 12.1A

MK2 bed Tests at .8-.9 ohms think this is my problem but how do i fix? how did i cause?
Mk2 head bed has small spot where coating is scratch off like maybe a small screw fell on it? very very small in size


Any suggestions? Any way to test heat bed (Like hooking directly to 12v line) Am I missing something small in config?

Edited 3 time(s). Last edit at 07/15/2012 04:41PM by bladez.

0.8R is a bit lower than normal. The usual resistance is 1.2R so that it takes 10A from 12V. With 0.8R you may be tripping the 11A thermal fuse (the larger of the two yellow squares).

I don't think you caused it. It is just out of spec as it is hard to control the resistance of a PCB. I actually prefer them a bit on the low side as 1.2R ones don't get hot enough for my liking.

You could try cooling the board with a fan.

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[www.hydraraptor.blogspot.com]

Ty for the info

I ended up rewiring the mk2 board and adding the fan
Seems those Yellow square boxes are staying a lot cooler, before was to hot to touch almost
so board went up to full temp

Thanks again for the info