Heat Bed Temperature not regulated

I am not sure how this happened, but after some usage suddenly the settings for the temperature regulating the heat bed does not turn off the current. I have tried problem solving this for three days now and thought I would see if anyone had a similar problem.

Turning the machine on and connecting Repetier Host, without turning on the heat bed the temperature starts to rise. as if I had turned on the heat bed to preheat. It is as if now the connection is unregulated and is always in the on state. When I do turn on the heat bed to see if the set temperature 60 degree will allow for the printer to regulate the temperature, all it does is continually keep heating. The Thermistor is registering the current/active temperature, but the controller card is not turning of the electricity when the temperature is reached. I don't know if that helps in describing the issue that I am having, but any help would be appreciated.

The board I am using is the arduino 1.4 ramp (single extruder, board config set to 33)

I also tried using gcode M140 S60 - and nothing happens

I re-uploaded the Marlin Firmware and tried resetting the board with no success..

Edited 3 time(s). Last edit at 04/19/2014 04:56PM by bishupz1.

Check the Mosfet for the bed. Put one meter lead on the negative power input terminal and the other on the Mosfet legs in turn. If the left leg is showing 5v, then the controller is switching the bed on erroneously (or something is telling it to). If no 5v then check the middle leg - should be around 10-12v in the off state. So, if there is no 5v on pin1 and no 12v on pin 2 then the Mosfet may be faulty. The 3rd leg is connected permanently to ground. Give that a try first.

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Waitaki 3D Printer

So I had to separate the shield to test the mosfet, upon looking at the bottom of the board my guess is that the Mosfet did burn out... I am going through tutorials on how to measure the mosfet with a multimeter, from what I saw in the attached image my best guess is that this Mosfet burnt out...

You can still test the mosfet on them solder tabs. And, this mosfet does get hot so no surprise its discoloured - that's why a fan and heat sink is mandatory.

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Waitaki 3D Printer

Do you know of any US based company that sells the P55NF06 Mosfet? I have not had any luck with local companies.

Do you know of any US based company that sells the P55NF06 Mosfet? I have not had any luck with local companies, I also don't know enough to compare an equal product if there is any..

I'm not in the US but you could try Mouser?

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Waitaki 3D Printer

get an alternative. All are drop in replacements, and all better than the original

mosfet is Best mosfets for driving your heated bed: IRLB3813, IRLB8743, FDP8870, AOT240L, CSD18502KCS. These mosfets will run merely warm with NO heatsink and a load current of 10A. They should be good to over 15A with no heatsink according to the worst-case numbers in their datasheets and is also see also [reprap.org]