I had a heated bed failure after I cleaned off the PEI sheet. It was an add-on that I put on but it never worked and was really glued down. I was liberal with the alcohol while I was rubbing it off and I guess that bit of solvent must have changed things with the heating element. A few hours later my printer had thermal runaway. The heated bed failed to heat anymore.
The problem was there was no current to the heat bed. The MOFET or something must have blown. I was planning on changing out the controller and had the replacement on-hand. Despite having an external MOSFET laying around I wired the replacement controller up running the heated bed through it and and after a few hours experienced the same thing. I knew I should have used the external MOSFET!
Solution
Losing two boards because their heated bed failed in the same way motivated me to research options. First, I don’t need to replace the controller because I figured out how to use a different pin for the heated bed.
I obviously need to replace the heated bed but I couldn’t find the bed for my FLSun Cube anywhere. From measurements I determined it is the same as the Creality CR10, 310×310 mm. Furthermore in “Everything about heated beds” Thomas made a comment that totally made sense to me. Why are we converting 110V to 12V and then trying to pump 5 or 6 Amps through the control board to the heat bed? It would be more efficient to run 110V to the heated bed and pumping 5 Amps at 110 V produces serious heat and there is a silicone heated pad made for the Creality CR10. I believe it even has holes in the right places and should work with my FLSun.
Conversion
In my case my the Rumba Plus controller board has a problem with the heated bed circuit. Using Marlin I changed the heat bed pin to one that is exposed. I could see that PWM2, which is PE3 (maps to -> Arduino Digital Pin 5) is available in EXP3 shown in this diagram which can be deciphered to Digital Pin via Arduino Pin mapping. PWM1 and PWM 2 are used for servos and it is useful to know where they are.
With a pin identified I ordered the 110 V silicone heat pad for the Creality CR10 and happen to have a Solid State Relay that can handle the load. My experience an SSR with a good heat-sink is not a bad idea so I ordered this one online.
Typically you would just use the Heated Bed Output to the SSR signal but as I said, I am using my controller from the heated bed failure.
The safety ground has to be connected to the metal frame because the heat pad will now be using 110 but that is always a good idea anyway because it is good to have a shorter path than through the controller to ground for static electrical shocks.
Summary
Some mistakes lead to discovery. In my case experience to avoid a heated bed failure don’t pump the high current for a heated bed through the controller board. If you blow an external MOSFET or SSR (unlikely) it is a lot less expensive to replace.
I have to make note that offloading the heated bed was necessary with my ANet A8 for safety reasons from what I read. From my heated bed failure experience I believe it is a really good idea.
An SSR can handle 110V and removes the entire load from the heated bed. I’ll will print an enclosure to assure it isn’t possible to touch the leads or get burned by the heat-sink and I’ll cool it with a fan. This posting was about how I use my ‘Project’ printer. It is the place where I blow up control boards trying out new ideas and learning about 3D-Printing.
