Heated bed no control.

After two months of running very nice prints on my Mendelmax 1.5 my heated bed is out of control.
When I start a print it looks all fine, but when I return after some time the heated bed heats up without any reason.

I am using a arduino mega with ramps 1.4 running Cura on a macbook pro.

If any one has good suggestions they are welcome!

Dop 13

Assuming the software and firmware is fine my best guess would be a failing thermistor on the bed or a failing mosfet on the Ramps board.

Hello Downunder35m,

I replaced the thermistor and set my bed temp to zero. After about 20 minutes I could bake an egg next to my print.. I think a faulty mosfet as well now. Replacing the ramps is the easiest option.

Thanks for your reply

Dop13

Not really, you can check the mosfets with a multimeter and if one is faulty replace it for less then 2 bucks...
But if the mosfet is working fine it means the stuff driving it has failed, then of course a replacement of the entire board is more effective.
It might pay off to add a SSR on the output to drive the bed, this way the mosfet has no work to do and if something fails it will be the cheap SSR.
Also makes it really easy to use a seperate power supply just for the bed.

Thanks for the help, I will test the mosfets tomorrow. If it is faulty I am going to replace it and add a ssr to the circuit. It is just as using relays for my lghts in my series landrover. All the current goes through the lightswitch. (Lucas prince of darkness)

Dop13