RAMPS Heater Bed Issue

Hi All,

Hopefully this is the correct place to be posting this.


Setup:

- Prusa i3 Rework.
- RAMPS 1.4
- MK2a PCB heatbed


Issue:

Heater bed only heats up to a 50 degrees before it cuts out.


Troubleshooting steps taken:

Tested the voltage on the heater bed output while the heater bed is heating up. Observed the voltage slowly dropping until it quickly drops to 0 volts. Upon further investigation, I noticed that the MOSFET (IRF3205) for the heater bed was overheating and shutting down.

I purchased a new RAMPS 1.4 board and tested heating up the heater bed but I let the magic smoke out on the MOSFET (P55NF06L). Just goes to show that having a MOSFET with a thermal shutdown is crucial in this type of application.

I swapped around the D8 and D9 MOSFETs and still having the same issue.


Thoughts:

This started to happen over a month ago and I don't know why. I've been running this set up for about 4 years with no issues. Recently I upgraded from Marlin 1.0.2 to 1.1.8. There were no issues at all and I have had many successful prints. So my thoughts went to the firmware in that I switched from bang-bang to PID control for the heater bed. I upload my 1.0.2 code again and I'm still noticing the MOSFET overheating and shutting down.

At this moment I am thinking of swapping out the 460 Watt PSU, but I am seeing a constant 12.3 volts on the RAMPS input.

Maybe I need to look into purchasing another control board besides the RAMPS.

Maybe your 4 year old MK2b has a shortcut somewhere?

I thought it was the thermistor or the PCB heater bed so I ordered a new thermistor and a MK3 Alu heater bed. Still having the same issue.

Maybe the MK3 Alu bed pulls too much for the MOSFET to handle.

On the cheap Chinese RAMPS I have always changed the heat bed MOSFET to a IRLB8743PBF

My last couple of builds I have been using a RAMPS from StaticBoards. Much better quality.
[www.tindie.com]

---

My updated Instructable on our Prusa i3 Build
[www.instructables.com]

Quote
SteveRoy
On the cheap Chinese RAMPS I have always changed the heat bed MOSFET to a IRLB8743PBF

My last couple of builds I have been using a RAMPS from StaticBoards. Much better quality.
[www.tindie.com]

Thanks Steve, I will try get a hold of some IRLB8743PBF MOSFETs. I will also try the RAMPS board from StaticBoards.


On another note, I switched to using #define BED_LIMIT_SWITCHING with #define MAX_BED_POWER 204 (80% duty cycle) and it seems to heat up past 50 degrees with no issues. May need to add a heat sink or a fan.

Just an update:

I swapped the IRF3205 for the IRLB8743PBF MOSFET that SteveRoy mentioned above and It was warm to the touch after the heatbed was heating up.

Next problem... The fuse!

When I was heating up the heatbed with the new MOSFET installed, I was still seeing the same problem as before but this time the fuse was over heating.

Any suggestions on a replacement for the XX30 UF900 bed fuse?

Edited 1 time(s). Last edit at 06/15/2018 12:35AM by minor59er.

Replace fuse on ramps with thick wire then install inline fuse for heated bed.

Thanks Dust, I will take that into consideration as a final option.



I don't know if anyone has experienced this before, but I just replace my server PSU with a MeanWell PSU, and I think the heatbed and fuse are doing better.

Could the server PSU start to have malfunctioned and somehow let more current through?

Current is pulled

Its possible the meanwell can supply less current and is maxing out.

The heat bed can pull 9 amps
I = V/R so 12V with a 1.3 ohm head bed is 9.2A

---

My updated Instructable on our Prusa i3 Build
[www.instructables.com]