Heated bed keeps getting hotter while printing

Hi!

For some reason, since a few days I have a big issue with my RAMPS board. I don't know why, but when I print, after a while, the heated bed becomes hotter and hotter. At first, setpoint is reached nicely, and things settle down. However, after a while, the readings get hotter, and keep increasing. I once reached 110degC with my setpoint at 60degC. Obviously, I shut things down, as I do not trust this behaviour.

I tried multiple prints, and all now have this issue. The thermistor seems to be working, as it is reading the temp, and indeed the heated bed gets abnormally hot. What I did notice a while back is that for some reason, the readings of my thermistors (bot bed as nozzle) all are quite jitterish when temperatures are low (ie heating is shut off).

These are the readings I get while printing:

1. Start 1st print (this is a larger print)
2. Without any interaction of myself, the bed temp starts to increase (setpoint did not change)
3. I shut off printing, and put on a new 3D model
4. I started printing the 2nd print (smaller different print)
5. Temperature again starts rising at about layer 3. Temp rise is much faster, maybe because it's a smaller print.
6. I shut off the printing, and all heating. As you can see, for some reason the temp readings are very jitterish.

This is a detail of the last minutes when the temperature control was shut off:


Could it be that the jitterish readings are messing with the PID controller? What might be the reason for this malfunction?

Edited 1 time(s). Last edit at 04/25/2015 07:56AM by Stephanel.

What is powering your RAMPS board? I experienced something similar when I (under)powered my RAMPS by way of a Raspberry Pi that didn't have a large enough power supply.

I have a separate 12V power supply... I have exchanged the RAMPS board and still I have the same problems. The temperature reading seems OK. Indeed, it might be that the power supply is the cause, but then I would expect the bed not to heat up at all, instead of keeping heating?... I set the setpoint and somehow my RAMPS board does not seem to recognize this and switch off the heating power?...

Looking at the "Output Bed" graph, it seems that the software is not turning on the heater between B and C, so I'd say the software is working properly, and you have some kind of hardware fault. I had something similar on a PrintrBoard output that was controlling nozzle temperature... it wouldn't turn off, and kept getting hotter and hotter until I pulled the plug. I tracked the problem back to the CPU pin which was at 5V all the time, despite the software telling it to turn off. I don't know whether the problem was in the CPU itself or the MOSFET.

It's also interesting that stopping the printing at C turned off the bed heat problem, that things seem to be working properly (more or less, there seems to be a self-corrected error at 10:00) between D and E, and that the rate at E is much higher than at B, so somehow the power supplied to the bed does vary, and is in some way related to what you're doing. It would be interesting to know what changed at E... was there a change in the position of the head? Maybe there's a short somewhere between your nozzle heater and bed heater? Or between one of your steppers and the bed? When you have another of these uncommanded temperature increases, I suggest checking the voltage at the bed and work backwards to the RAMPS terminals, MOSFET output pin, MOSFET input pin, etc.

Another diagnostic would be to swap the heater connections to the nozzle and bed at the RAMPS board, also swap the extruder and bed heater pin settings in the software, and reflash the controller. If the problem now moves to the nozzle, then the problem is on the RAMPS or controller board. If the problem stays with the bed, then its in the wiring between RAMPS and bed.

Hi there! Thanks for your thorough analysis! I am abroad now so not able to work on the printer right now...

At E I changed the 3d model. The first one had a larger base while the second had a smaller one. As you say, I also think it's a hardware problem. I will definitely try to do what you suggested!

Thank again and I let you know my findings (probably next week only...)

The only thing I see which makes me wonder, is that if it truly were a hardware issue, then why does it stop overheating when the job is terminated? Heater off is heater off to the hardware . . . why is one working and not the other? Unless we can verify your posted traces with actual MOSFET gate voltages that match, I would not take the traces as 100% accurate . . .

- Tim

I had a similar issue when the heatsink for the heated-bed was touching the heatsink for the extruder. See if the two mosfets backsides are maybe touching each other? It would Murphy law type of thing because you tried two RAMPS but worth checking anyways...


(there is no need for heatsink on extruder so I removed that and problem solved)

Quote
tadawson
The only thing I see which makes me wonder, is that if it truly were a hardware issue, then why does it stop overheating when the job is terminated?

Yeah, I wondered too... my thoughts on that are that maybe
(a) when the job terminated, the head perhaps homed and disconnected a short.
(b) there was a short between extruder heater and bed heater... when both are turned off when the job terminated, everything cooled down.
(c) short between stepper and bed? Once the stepper is unpowered when the job is terminated, the bed cools down.

Quote
LarsK
I had a similar issue when the heatsink for the heated-bed was touching the heatsink for the extruder. See if the two mosfets backsides are maybe touching each other? It would Murphy law type of thing because you tried two RAMPS but worth checking anyways...

The reason why this happened as it did is the metal tab is the same as the drain pin on most MOSFETs. When they touched you were allowing either or both MOSFETs to serve as the switch to turn on and off both circuits involved.

Hey all!

You are great remote trouble-shooters! It seems indeed that the reason was that the MOSFETs were touching eachother. I carefully moved them a bit apart, and now all is working perfectly!

Fantastic!! Thanks a lot to you all!

Ran into the same problem. MOSFETs were indeed touching. Thanks!