Heated Bed Stops Heating At 45 Degrees

I have Prusa Mendel i2 with a heated bed, RAMPS board and Marlin firmware.

I've been printing successfully for several weeks without any issues.

Suddenly, a problem has developed with the heated bed. When I set the bed temperature to 110 degrees, the temperature rises to approximately 45 degrees and then starts falling again all the way back to ambient temperature. Once the temperature starts falling it continues to do so - setting the temperature to 110 again makes no difference.

I've measured the voltage on the bed and it's 12V when the bed is heating and 0V when it stops. It's as if the firmware is switching the bed off at 45 degrees.

The temperature monitor in pronterface shows the temperature throughout so I don't see how it can be a thermistor problem.

Watching the temperature rise, I have noticed that it rises at a normal gradual rate up until approximately 40 degrees and then the temperature suddenly jumps by 5 degrees to approximately 45 degrees - it then rises slowly to 45.6-45.9 and then starts falling.

Any suggestions on what could be the fault?

Thanks.

James.

Edited 1 time(s). Last edit at 03/15/2014 06:31AM by jsluke.

This could be the yellow polyfuse cutting out. It could also be the Mosfet shutting down.. Yes, it can work fine for weeks then start giving trouble.
So, aim a constantly running fan at the ramps board and fit a heat sink to the bed Mosfet if you haven't already done so.

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Waitaki 3D Printer

Many thanks - fitting a fan to the RAMPS board solved the problem and it doesn't look like any of the components are damaged.

Thanks again.

James.