Happy new year everybody!

Before and after excessively celebrating a new, awesome year with the people of the Attraktor Makerspace, I had time to setup a small testing rig for more thorough testing of the heated beds. @Alex Rich suggested recording a temperature graph to show the performance in comparison to the regular MK2 heated bed, which will be covered in this log, however, this is not meant as a competition. Josef Prusa, who has done a great job designing the MK2 in a way so it would under no circumstances experience a thermal runaway, which is very much the harder thing to do without employing additional fuses etc.

Comparison with MK2 at 12V DC

The setup:

Contestants: MK2 (12 V version) and mains voltage heated bed

PSU: modified 12 V AC 500W ATX, dedicated to heating the MK2

Controller: Arduino 2560 clone and RAMPS 1.4

Wiring: MK2 attached heated bed port of RAMPS / 230 V AC heated bed attached to extruder port of RAMPS via FOTEK SSR-40 DA

Temperature sensors: EPCOS 100k

Starting temperature: 21 °C

Target temperature: 110 °C

Inital temperature difference (max): 1 °C

Additional info: PID autotune with 8 cycles performed before on both heaters, heaters placed on cork mat offset by M3x50mm screws, plain PCB with no printing plate attached to heated beds

With this setup, I ran this short GCODE snippet from Repetier host, which starts the heating for both heated beds simultaneously:

M104 S110
M140 S110

Repetier Host was then used to graph the temperature curves from both heaters, rendering the following curve:

RED: mains heated bed at 230 V AC

BLUE: MK2 heated bed at 12 V DC

The mains voltage heated bed reaches stable printing temperature after about 1:40 minutes while the MK2 almost reaches 110 °C after about 10 minutes but then struggles to hold the temperature. So, in terms of heating performance, this is a win for the mains voltage heated bed, but it also becomes clear why it really needs a thermal fuse: Whereas the MK2 is designed to passively max out at 110 °C, the mains voltage heated bed is just powerful enough to run away and shoot through the roof, so safe operation requires an active mechanism to let it max out at a certain temperature, which is the thermal fuse.

Comparison with MK2 at 24V AC "overdrive"

The setup:

Contestants: MK2 (12 V version) and mains voltage heated bed

PSU: modified 12 V AC 500W ATX, dedicated to Arduino and a 24 V 500 V AC transformer dedicated to heating the MK2

Controller: Arduino 2560 clone and RAMPS 1.4

Wiring: MK2 attached extruder port of RAMPS via FOTEK SSR-40 DA / 230 V AC heated bed attached to heated bed port of RAMPS via FOTEK SSR-40 DA

Temperature sensors: EPCOS 100k

Starting temperature: 21 °C

Target temperature: 110 °C

Inital temperature difference (max): 1 °C

Additional info: PID autotune with 8 cycles performed before on both heaters, heaters placed on cork mat offset by M3x50mm screws, plain PCB with no printing plate attached to heated beds

And again the GCODE snippet to initiate the race:

M104 S110
M140 S11

And Repetier Host renders the following curve:

RED: MK2 heated bed at 24 V AC

BLUE: mains heated bed at 230 V AC

As expected, in this "overdrive" mode, the performance of both heaters is quite similar, since both heaters are supposed to output about 500 watts of heat, they both finish heatup in about 1:40 minutes. The 20% manufacturing (aka planning) error in the mains powered heated bed noticeably decreases performance, but it also becomes clear that even though PID was properly tuned on both heaters, the MK2 overshoots a bit more. This might be due to the single sided heating traces, whereas the traces run on the top while the temperature sensor is attached to the bottom. The double sided layout of the mains voltage PCB provides some "symmetry" of temperature. Note that the MK2 is not designed to run at 500W, so it does not have pads for a thermal fuse.

Other observations: Breathing

One very noteable difference between the two beds is however totally unrelated to the involved voltages. It's the breathing of the heated bed, and I noticed how extreme it is when I went through the photos I had taken for this log:

The MK2 heated bed is a compound of one layer of FR4 and one layer of copper on top of it, so the thermal expansion of the copper causes the heated bed to bend. You might already have observed this when dealing with improperly calibrated PID settings or BANG BANG controlled heated beds where it causes prints to come out wavy.

Since the mains voltage heated bed is a sandwich with copper on both sides of the FR4, it also doesn't breathe, thus - besides faster heatup time - is expected to also contribute to the overall printing quality.

More testing

Since the testing setup is in place now, I'll also start the 130° C 30 days endurance test on the mains voltage heated bed. This is the next important step to ensure that this prototype is safe to be tested by betatesters, which may apply in the comments on this project.