heat bed problems

Hi there,
i have just put together my rep rap huxly with Melzi board which i actually purchased last august but only just got round to putting together, all seems to be working correctly other than the heat bed ,it just heats up on its own even when switched off on the pronterface and the temperature reading is -14. i have done all the usual things checked voltage , continuity etc all seems to be fine but the MOSFET on the bed is getting very hot can anyone please advise. all so my x axis is homing the wrong way even though it operates correctly manually.
thank you John-Paul

Edited 1 time(s). Last edit at 10/15/2013 09:32AM by jphayes.

Hmm apparently no-one has an idea.. Maybe if it is 'off' the value internally is zero, and the value below it triggers it to turn on.

The -14 temperature is presumably wrong! Maybe that is just too far off. Are you using the right thermistor or thermistor table? (FTR: my bed gives an incorrect reading too, says it is 7.7C, and my hot end 22C)

The hot bed FET on my RAMPS gets hot. I think this is normal, hence the common use of a heat sink on this FET and generally fan cooling the whole of the RAMPS

However the hot bed should not be on at all, unless you set a temperature in Pronterface

If you have a multimeter check the gate voltage on all the FETs, I suspect that your Arduino firmware is somehow incompatible with your RAMPS

If your temperature reading is -14 C, either you are in a *very* cold room, or your thermistor is wired wrong. I'd fix that before you go any further. It's also possible that you have a thermistor setting in your .h file that's not correct for your thermistor. Since they mostly are 100K parts, a .h setting for a 4.7K resistor with a 1K resistor on your Ramps board is the most likey error.