RAMPS shuts down heatbed 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.

I would check the fuse, the big yellow one above the power inputs. It may be tripping. Also check the connectors for power both input and connector for the bed, the may be starting to melt, once they get hot enough one of them maybe breaking contact.

JSLuke
You have cross posted this. See your other post, I'm not answering again!

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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.