heatbed won't stay on

I've. been printing on a cold bed in PLA because I didn't have a spare thermistor for the heatbed. I finally got a thermistor and got it wired up, but I can't get the bed to stay on! In pronterface, if I manually set the bed temp, the mosfet comes on, the light lights up, and the bed starts to warm, and I can see the temperature climb in pronterface, but after a few seconds it switches back off and won't come on again for awhile. Temperature got up as high as 59C the first time, but now it usually switches off in the 40s.


What am I doing wrong?

Measure the resistance of your heatbed, there have been a couple of reports recently of heatbed PCBs with slightly under spec resistance, meaning they draw too much power and trip the thermal fuse. I think theres a bad batch of PCBs out there, but they do work perfectly well if you can keep the fuse from tripping, pointing a fan at it should do the trick. Or, as a last resort, bypass the larger of the two fuses on the RAMPS board.

Position a fan to blow over the resettable fuse on the RAMPS board.
My MK2 board measures ~1.1 OHMS and that's enough without cooling such that the fuse overheats and cuts out power. Once the fuse trips, you pretty much have to power off and give it a few seconds to cool.
With the fan I have no issues, and it has the nice advantage of also blowing over the stepper heat sinks making the pot settings a bit less critical.

It was the fuse... it's getting VERY hot... however, my heatbed measures 1.3 ohms, which is what I expect!

I mounted a fan blowing across the fuse, and that helps, but it's still not perfect yet. I'd like to design a nice ducted case to pull air through.

Perhaps the fuse is the wrong value. A common mistake is to confuse holding current with fusing current, there is a 1:2 ratio.

What are the markings on it?

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