Heated bed cooling off mid print

This happens inconsistently and I am at a loss as to why. Because the bed takes so long to heat up, I typically just set the temp manually before starting my print and don't have the Chamber module in skeinforge activated at all. However a few times I've started my print only to find halfway through its completely cool and my parts are peeling off. At first I assumed there must be a loose connection, but I paused the print, manually set the temp again, and resumed and as it went along it started heating again. So something during the print must have turned off heat.

I looked through my gcode and the only M140 commands I see are in my start.gcode and end.gcode (turn on and turn off, respectively). I'm using the latest Sprinter master from github, Repsnapper, and RAMPS 1.3. I'm powering my bed directly off of RAMPS. Anyone have any ideas as to what might be going wrong?

It could be the watchdog in the firmware if you have that activated, or maybe your power supply is maxed out and is cutting out, causing the software to reset? That's less likely.

I don't think the software is resetting because the nozzle heater stays on. I don't know about the watchdog, what is that?

Something similar happened to me when the MOSFET controlling the heated bed got too hot that it melted the solder. Check your MOSFET. It might be getting soft and then losing contact when it gets too hot. Then after it cools down, the solder hardens again and you're able to heat the bed again. If you don't have a heatsink on the MOSFET, you should add one.

I don't have a heatsink on it. I'll definitely put one on and take a look at the solder connections.

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Had a very similar issue

I figured out a semi-nasty workaround for this, but at least it works. I put an M140 command in my homing.gcode, so the bed gets switched back on after each layer. A single layer isn't long enough for it to cool off (I guess unless I'm doing a full bed print), so overall it stays relatively stable.