Heat Bed Turning Off

I have been having some issues with the heatbed on my first reprap, and was hoping to get a little bit of help. When i turn my heat bed on (set to the default ABS of 110deg), it starts to heat up, but then turns off when it reaches about 60deg, and doesn't turn back on. I've tried it with and without the hotend turned on, but it behaves the same either way, so i don't think its that my RAMPS isn't receiving enough power to support both. I measured the resistance of the board after it turns off, and its still normal (~2ohms), and the voltage across it is about 0.25V. The hotend works just fine, and so do all the motors, end stops, thermistors, etc.

Any further ideas of what might be wrong, or steps i can use to debug it? The major components I'm using are:

Heatbed: MK2a
Controller: RAMPS 1.4 + SainSmart Mega
Power Supply: XFX Pro 550W

Have you got a fan blowing across your yellow fuses and the D8 mosfet (which should also have a heatsink)?

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Waitaki 3D Printer

I don't currently, no. I had seen some stuff about fans, but the guide i was following didn't mention them, so i haven't put one in yet. And i actually didn't even know they made mosfet heat sinks, i always wondered what those bolt holes were for :-)

I found this thread that you posted in which was interesting, although my issue is kind of different. After the heatbed turns off, if i unplug everything and let it sit for a while, the heatbed will turn back on. Im thinking this means nothing is totally blown. The D8 mosfet also doesn't heat up immediately when turned on (my first pre-assembled RAMPS board did this when i first got it, but i replaced it). It does heat up quite a bit over time though.

Are there any components that temporarily burn out with heat, but somehow reset? Or is there something in the software that powers itself off in order to protect the components?

Those yellow fuses can shut down with heat - that's what their supposed to do but they will normally reset when cooled. If the current draw of the bed is a little high, they will overheat and shut down, which is why the bed switches itself back on after a few minutes. Stick a fan on there - and don't forget the heatsink. Keeping the fuses cooled will allow it to work effectively.

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Waitaki 3D Printer

I fitted a solid state relay to my Ramps board to control the bed, following a suggestion on here. It works well and takes all the heat away from it.
Reprap beds can also crack, mine did across a corner, causing a short. Just a suggestion but maybe a track in your bed opens up as it gets hot and closes as it cools down. As soon as it cuts out, you could immediately disconnect the bed wires before it cools and check for continuity.

Waitaki, the heatsink only goes on the mosfet, correct? The fuses don't get cooled other than by the airflow?

Dave, interesting i hadn't heard of that yet. I'll look into that if I keep having issues. My heatbed did actually have shorting issues at first, when my wires were touching the exposed copper plate on the through-holes. My first though was that this was happening again, so i checked the resistance when it first turned off, but everything looked fine

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peterno3
Waitaki, the heatsink only goes on the mosfet, correct? The fuses don't get cooled other than by the airflow?

Dave, interesting i hadn't heard of that yet. I'll look into that if I keep having issues. My heatbed did actually have shorting issues at first, when my wires were touching the exposed copper plate on the through-holes. My first though was that this was happening again, so i checked the resistance when it first turned off, but everything looked fine

Yes, its usually only the D8 that requires a heatsink but it is essential! And yes, fuses are only cooled by the surrounding air. My Ramps boards have been cooled since day 1 and never had a fuse trip heating the bed.

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Waitaki 3D Printer

It was actually one of my corner through holes that cracked causing a short. I found it when I noticed the corner supporting bolt & spring too hot to touch. I put a fibre washer top and bottom of the through hole to electrically insulate it.

Thank you for your help, I finally got my heat sinks in, and adding one seems to let it heat up to full temp.

Do you happen to know, does the mosfet generate the most heat when the bed is heating to max temp? Would adding a fan during warm up, then removing it during the actual build help, or would it need a fan through the whole build? My electronics are mounted in a place that won't easily allow a fan while building, and I'd rather not move them at this point if i can avoid it