Bed begins to heat, then stops

After a short hiatus in usage of my printer, my bed does not heat.
Stats:

• Prusa Mendel i2
• RAMPS 1.4
• Marlin firmware
• ATX psu

My extruder heats up normally. When I set a temperature for my bed, it begins heating but around 70C, the MOSFET LED goes out and the temperature plummets. If I disconnect the one or both of the bed wires, LED will come back on until I reconnect the wires. The only way to reset it seems to be to unplug USB and turn off the PSU.

My bed has a resistance of about 1.1 ohms, which seems within normal limits (although confirmation would be great). Any help or ideas will be greatly appreciated.

Maybe your 11A resettable fuse is shutting down. Do you have a fan to cool it?

Thanks!! Just checked with my mutimeter and theres no continuity through the fuse when the bed turns off. Its also too hot touch. I could probably jury rig a fan setup to cool it, but now I'm wondering if the fuse cutting out could be indicating a bigger problem. Also, my PSU can go up to 26 amps; is there anything wrong with just shorting over the fuse?

Edited 1 time(s). Last edit at 05/02/2013 10:00PM by thedingwing.

If the bed resistance is marginal the fuse will cut off without cooling on the fuse, I have one heated bed that's like this, what's the resistance of the bed?

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The resistance is around 1.8 ohms according to my meter. This would put the current at around 6.6 amps.

A cooling fan is usually needed on that fuse so just stick one on it - it will cool the stepper drivers at the same time.

Edited 1 time(s). Last edit at 05/03/2013 04:45AM by waitaki.

Quote
thedingwing

now I'm wondering if the fuse cutting out could be indicating a bigger problem.............is there anything wrong with just shorting over the fuse?

Well, just shorting over the fuse would eliminate this issue but, if there is a bigger problem then it will probably display itself as a nice smoldering pile of electronics if your lucky or a fire if your not!


Please DO NOT bypass it. Even with 12 or 24 volts there is enough amperage around to easily start a fire. The fuse is there for a reason.


Regards

Andy Hodges