Heated bed issues

When heating my MK2 heated bed, it will reach approx 50deg and then shut itself off. After this it refuse to accept any more commands to turn on the heat unless I restart the board. After a restart the bed will only heat for several seconds before dying. The extruder is still heating fine. I've tried different hosts and firmware. The MK2 heats fine if I hook up 12V direct. I'm thinking its got to be the Arduino or RAMPS. Any ideas on or experience with what might be causing the problem?

Prusa
RAMPS 1.4
Budaschnozzle w/ MK2

There is a 11 amp resettable fuse that is used for the heated bed. If you are tripping that fuse then that may be the problem. How many amps is your bed pulling?

Also, if your thermistor connection gets screwed up and the RAMPS can't calculate the temp anymore then the bed may shut off, I think that depends on the settings in your firmware. I know Marlin has that protection built in. (atleast for hot end temps).

This sounds like a fuse issue.

Hi Entropy. Thanks for responding. The thermistor seems to be functioning. I tried running 12v directly to the bed and the thermistor continued to read temps accurately well beyond the usual failure point.
I have a cheap meter rated 10A so the readings may not be realistic. Amperage starts out at around 11.8, then starts slowly dropping to about 10 at which point it dies off quickly to about 0.25.

Hi,

This might not help,must I have observed that when the cabling I used for my heated bed wasn't ideal (I had a block connector in line) that I would only reach 90 degrees no matter what. Rewriring with the right stuff solved the problem.

The replacement fuse I ordered just arrived. I switched it out with the one on the RAMPS and all is well. Thanks

I was printing without problems for a few weeks but the problem has returned. The 11A fuse has once again failed. Is this a common problem? I was heating the bed up to 110 for ABS. Is the temp too much?

Molotov Wrote:
-------------------------------------------------------
> I was printing without problems for a few weeks
> but the problem has returned. The 11A fuse has
> once again failed. Is this a common problem? I
> was heating the bed up to 110 for ABS. Is the
> temp too much?

Is the fuse rated for 11A holding or tripping current?

Also, they are thermal fuses, which means they may fail at less than the rated current if they get too hot. Do you have a fan blowing over your electronics?

I personally have not had one of those fail, but I've only used two sets of RAMPS to power a heated bed directly.

---

Help improve the RepRap wiki! Just click "Edit" in the top-right corner of the page and start typing. Anyone can edit the wiki!

Mine does exactly this, with 2 ramps boards, put a small fan in a position to blow air over the larger fuse. The fuse will go from being finger burning hot to tepidly warm and the problem will go away. I have a 80mm fan positioned to blow air over the electronics and just directly wired to 12V.

I suspect the issue is that the Mk2 bed I have has marginally low resistance, it measures 1.3 Ohms, but that's not taking into account the resistance of the test leads.

That makes sense. Other than stepper heatsinks, I have been neglecting the heat generated by the electronics. Luckily I ordered extra fuses last time. I popped in a new fuse and my next print is going to be a fan mount.